draft-ietf-openpgp-rfc2440bis-22.txt   rfc4880.txt 
Network Working Group Jon Callas
Internet-Draft PGP Corporation
Intended status: Standards Track
Expires October 2007 Lutz Donnerhacke
Apr 2007
Obsoletes: 1991, 2440 Hal Finney Network Working Group J. Callas
Request for Comments: 4880 PGP Corporation
Obsoletes: 1991, 2440 L. Donnerhacke
Category: Standards Track IKS GmbH
H. Finney
PGP Corporation PGP Corporation
D. Shaw
R. Thayer
November 2007
David Shaw OpenPGP Message Format
Rodney Thayer
OpenPGP Message Format
draft-ietf-openpgp-rfc2440bis-22
Status of this Memo
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Copyright Notice Status of This Memo
Copyright (C) The IETF Trust (2007). This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract Abstract
This document is maintained in order to publish all necessary This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on information needed to develop interoperable applications based on the
the OpenPGP format. It is not a step-by-step cookbook for writing an OpenPGP format. It is not a step-by-step cookbook for writing an
application. It describes only the format and methods needed to application. It describes only the format and methods needed to
read, check, generate, and write conforming packets crossing any read, check, generate, and write conforming packets crossing any
network. It does not deal with storage and implementation questions. network. It does not deal with storage and implementation questions.
It does, however, discuss implementation issues necessary to avoid It does, however, discuss implementation issues necessary to avoid
security flaws. security flaws.
OpenPGP software uses a combination of strong public-key and OpenPGP software uses a combination of strong public-key and
symmetric cryptography to provide security services for electronic symmetric cryptography to provide security services for electronic
communications and data storage. These services include communications and data storage. These services include
confidentiality, key management, authentication, and digital confidentiality, key management, authentication, and digital
signatures. This document specifies the message formats used in signatures. This document specifies the message formats used in
OpenPGP. OpenPGP.
Table of Contents Table of Contents
Status of this Memo 1 1. Introduction ....................................................5
Copyright Notice 1 1.1. Terms ......................................................5
Abstract 1 2. General functions ...............................................6
Table of Contents 3 2.1. Confidentiality via Encryption .............................6
1. Introduction 7 2.2. Authentication via Digital Signature .......................7
1.1. Terms 7 2.3. Compression ................................................7
2. General functions 7 2.4. Conversion to Radix-64 .....................................8
2.1. Confidentiality via Encryption 8 2.5. Signature-Only Applications ................................8
2.2. Authentication via Digital signature 9 3. Data Element Formats ............................................8
2.3. Compression 9 3.1. Scalar Numbers .............................................8
2.4. Conversion to Radix-64 9 3.2. Multiprecision Integers ....................................9
2.5. Signature-Only Applications 10 3.3. Key IDs ....................................................9
3. Data Element Formats 10 3.4. Text .......................................................9
3.1. Scalar numbers 10 3.5. Time Fields ...............................................10
3.2. Multiprecision Integers 10 3.6. Keyrings ..................................................10
3.3. Key IDs 11 3.7. String-to-Key (S2K) Specifiers ............................10
3.4. Text 11 3.7.1. String-to-Key (S2K) Specifier Types ................10
3.5. Time fields 11 3.7.1.1. Simple S2K ................................10
3.6. Keyrings 11 3.7.1.2. Salted S2K ................................11
3.7. String-to-key (S2K) specifiers 11 3.7.1.3. Iterated and Salted S2K ...................11
3.7.1. String-to-key (S2K) specifier types 11 3.7.2. String-to-Key Usage ................................12
3.7.1.1. Simple S2K 12 3.7.2.1. Secret-Key Encryption .....................12
3.7.1.2. Salted S2K 12 3.7.2.2. Symmetric-Key Message Encryption ..........13
3.7.1.3. Iterated and Salted S2K 12 4. Packet Syntax ..................................................13
3.7.2. String-to-key usage 13 4.1. Overview ..................................................13
3.7.2.1. Secret key encryption 13 4.2. Packet Headers ............................................13
3.7.2.2. Symmetric-key message encryption 14 4.2.1. Old Format Packet Lengths ..........................14
4. Packet Syntax 14 4.2.2. New Format Packet Lengths ..........................15
4.1. Overview 14 4.2.2.1. One-Octet Lengths .........................15
4.2. Packet Headers 14 4.2.2.2. Two-Octet Lengths .........................15
4.2.1. Old-Format Packet Lengths 15 4.2.2.3. Five-Octet Lengths ........................15
4.2.2. New-Format Packet Lengths 15 4.2.2.4. Partial Body Lengths ......................16
4.2.2.1. One-Octet Lengths 16 4.2.3. Packet Length Examples .............................16
4.2.2.2. Two-Octet Lengths 16 4.3. Packet Tags ...............................................17
4.2.2.3. Five-Octet Lengths 16 5. Packet Types ...................................................17
4.2.2.4. Partial Body Lengths 16 5.1. Public-Key Encrypted Session Key Packets (Tag 1) ..........17
4.2.3. Packet Length Examples 17 5.2. Signature Packet (Tag 2) ..................................19
4.3. Packet Tags 17 5.2.1. Signature Types ....................................19
5. Packet Types 18 5.2.2. Version 3 Signature Packet Format ..................21
5.1. Public-Key Encrypted Session Key Packets (Tag 1) 18 5.2.3. Version 4 Signature Packet Format ..................24
5.2. Signature Packet (Tag 2) 19 5.2.3.1. Signature Subpacket Specification .........25
5.2.1. Signature Types 20 5.2.3.2. Signature Subpacket Types .................27
5.2.2. Version 3 Signature Packet Format 22 5.2.3.3. Notes on Self-Signatures ..................27
5.2.3. Version 4 Signature Packet Format 24 5.2.3.4. Signature Creation Time ...................28
5.2.3.1. Signature Subpacket Specification 25 5.2.3.5. Issuer ....................................28
5.2.3.2. Signature Subpacket Types 27 5.2.3.6. Key Expiration Time .......................28
5.2.3.3. Notes on Self-Signatures 27 5.2.3.7. Preferred Symmetric Algorithms ............28
5.2.3.4. Signature creation time 28 5.2.3.8. Preferred Hash Algorithms .................29
5.2.3.5. Issuer 28 5.2.3.9. Preferred Compression Algorithms ..........29
5.2.3.6. Key expiration time 28 5.2.3.10. Signature Expiration Time ................29
5.2.3.7. Preferred symmetric algorithms 28 5.2.3.11. Exportable Certification .................29
5.2.3.8. Preferred hash algorithms 29 5.2.3.12. Revocable ................................30
5.2.3.9. Preferred compression algorithms 29 5.2.3.13. Trust Signature ..........................30
5.2.3.10.Signature expiration time 29 5.2.3.14. Regular Expression .......................31
5.2.3.11.Exportable Certification 29 5.2.3.15. Revocation Key ...........................31
5.2.3.12.Revocable 30 5.2.3.16. Notation Data ............................31
5.2.3.13.Trust signature 30 5.2.3.17. Key Server Preferences ...................32
5.2.3.14.Regular expression 30 5.2.3.18. Preferred Key Server .....................33
5.2.3.15.Revocation key 31 5.2.3.19. Primary User ID ..........................33
5.2.3.16.Notation Data 31 5.2.3.20. Policy URI ...............................33
5.2.3.17.Key server preferences 32 5.2.3.21. Key Flags ................................33
5.2.3.18.Preferred key server 32 5.2.3.22. Signer's User ID .........................34
5.2.3.19.Primary User ID 32 5.2.3.23. Reason for Revocation ....................35
5.2.3.20.Policy URI 33 5.2.3.24. Features .................................36
5.2.3.21.Key Flags 33 5.2.3.25. Signature Target .........................36
5.2.3.22.Signer's User ID 34 5.2.3.26. Embedded Signature .......................37
5.2.3.23.Reason for Revocation 34 5.2.4. Computing Signatures ...............................37
5.2.3.24.Features 35 5.2.4.1. Subpacket Hints ...........................38
5.2.3.25.Signature Target 35 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) .......38
5.2.3.26.Embedded Signature 36 5.4. One-Pass Signature Packets (Tag 4) ........................39
5.2.4. Computing Signatures 36 5.5. Key Material Packet .......................................40
5.2.4.1. Subpacket Hints 37 5.5.1. Key Packet Variants ................................40
5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 37 5.5.1.1. Public-Key Packet (Tag 6) .................40
5.4. One-Pass Signature Packets (Tag 4) 38 5.5.1.2. Public-Subkey Packet (Tag 14) .............40
5.5. Key Material Packet 39 5.5.1.3. Secret-Key Packet (Tag 5) .................41
5.5.1. Key Packet Variants 39 5.5.1.4. Secret-Subkey Packet (Tag 7) ..............41
5.5.1.1. Public Key Packet (Tag 6) 39 5.5.2. Public-Key Packet Formats ..........................41
5.5.1.2. Public Subkey Packet (Tag 14) 39 5.5.3. Secret-Key Packet Formats ..........................43
5.5.1.3. Secret Key Packet (Tag 5) 39 5.6. Compressed Data Packet (Tag 8) ............................45
5.5.1.4. Secret Subkey Packet (Tag 7) 40 5.7. Symmetrically Encrypted Data Packet (Tag 9) ...............45
5.5.2. Public Key Packet Formats 40 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) ..........46
5.5.3. Secret Key Packet Formats 41 5.9. Literal Data Packet (Tag 11) ..............................46
5.6. Compressed Data Packet (Tag 8) 43 5.10. Trust Packet (Tag 12) ....................................47
5.7. Symmetrically Encrypted Data Packet (Tag 9) 44 5.11. User ID Packet (Tag 13) ..................................48
5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 44 5.12. User Attribute Packet (Tag 17) ...........................48
5.9. Literal Data Packet (Tag 11) 45 5.12.1. The Image Attribute Subpacket .....................48
5.10. Trust Packet (Tag 12) 46 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) ..49
5.11. User ID Packet (Tag 13) 46 5.14. Modification Detection Code Packet (Tag 19) ..............52
5.12. User Attribute Packet (Tag 17) 46 6. Radix-64 Conversions ...........................................53
5.12.1. The Image Attribute Subpacket 47 6.1. An Implementation of the CRC-24 in "C" ....................54
5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 47 6.2. Forming ASCII Armor .......................................54
5.14. Modification Detection Code Packet (Tag 19) 50 6.3. Encoding Binary in Radix-64 ...............................57
6. Radix-64 Conversions 51 6.4. Decoding Radix-64 .........................................58
6.1. An Implementation of the CRC-24 in "C" 51 6.5. Examples of Radix-64 ......................................59
6.2. Forming ASCII Armor 52 6.6. Example of an ASCII Armored Message .......................59
6.3. Encoding Binary in Radix-64 54 7. Cleartext Signature Framework ..................................59
6.4. Decoding Radix-64 55 7.1. Dash-Escaped Text .........................................60
6.5. Examples of Radix-64 56 8. Regular Expressions ............................................61
6.6. Example of an ASCII Armored Message 56 9. Constants ......................................................61
7. Cleartext signature framework 56 9.1. Public-Key Algorithms .....................................62
7.1. Dash-Escaped Text 57 9.2. Symmetric-Key Algorithms ..................................62
8. Regular Expressions 58 9.3. Compression Algorithms ....................................63
9. Constants 58 9.4. Hash Algorithms ...........................................63
9.1. Public Key Algorithms 59 10. IANA Considerations ...........................................63
9.2. Symmetric Key Algorithms 59 10.1. New String-to-Key Specifier Types ........................64
9.3. Compression Algorithms 60 10.2. New Packets ..............................................64
9.4. Hash Algorithms 60 10.2.1. User Attribute Types ..............................64
10. IANA Considerations 60 10.2.1.1. Image Format Subpacket Types .............64
10.1. New String-to-Key specifier types 60 10.2.2. New Signature Subpackets ..........................64
10.2. New Packets 61 10.2.2.1. Signature Notation Data Subpackets .......65
10.2.1. User Attribute Types 61 10.2.2.2. Key Server Preference Extensions .........65
10.2.1.1.Image Format Subpacket Types 61 10.2.2.3. Key Flags Extensions .....................65
10.2.2. New Signature Subpackets 61 10.2.2.4. Reason For Revocation Extensions .........65
10.2.2.1.Signature Notation Data Subpackets 61 10.2.2.5. Implementation Features ..................66
10.2.2.2.Key Server Preference Extensions 62 10.2.3. New Packet Versions ...............................66
10.2.2.3.Key Flags Extensions 62 10.3. New Algorithms ...........................................66
10.2.2.4.Reason For Revocation Extensions 62 10.3.1. Public-Key Algorithms .............................66
10.2.2.5.Implementation Features 62 10.3.2. Symmetric-Key Algorithms ..........................67
10.2.3. New Packet Versions 62 10.3.3. Hash Algorithms ...................................67
10.3. New Algorithms 63 10.3.4. Compression Algorithms ............................67
10.3.1. Public Key Algorithms 63 11. Packet Composition ............................................67
10.3.2. Symmetric Key Algorithms 63 11.1. Transferable Public Keys .................................67
10.3.3. Hash Algorithms 63 11.2. Transferable Secret Keys .................................69
10.3.4. Compression Algorithms 64 11.3. OpenPGP Messages .........................................69
11. Packet Composition 64 11.4. Detached Signatures ......................................70
11.1. Transferable Public Keys 64 12. Enhanced Key Formats ..........................................70
11.2. Transferable Secret Keys 65 12.1. Key Structures ...........................................70
11.3. OpenPGP Messages 65 12.2. Key IDs and Fingerprints .................................71
11.4. Detached Signatures 66 13. Notes on Algorithms ...........................................72
12. Enhanced Key Formats 66 13.1. PKCS#1 Encoding in OpenPGP ...............................72
12.1. Key Structures 66 13.1.1. EME-PKCS1-v1_5-ENCODE .............................73
12.2. Key IDs and Fingerprints 67 13.1.2. EME-PKCS1-v1_5-DECODE .............................73
13. Notes on Algorithms 68 13.1.3. EMSA-PKCS1-v1_5 ...................................74
13.1. PKCS#1 Encoding In OpenPGP 68 13.2. Symmetric Algorithm Preferences ..........................75
13.1.1. EME-PKCS1-v1_5-ENCODE 69 13.3. Other Algorithm Preferences ..............................76
13.1.2. EME-PKCS1-v1_5-DECODE 69 13.3.1. Compression Preferences ...........................76
13.1.3. EMSA-PKCS1-v1_5 70 13.3.2. Hash Algorithm Preferences ........................76
13.2. Symmetric Algorithm Preferences 71 13.4. Plaintext ................................................77
13.3. Other Algorithm Preferences 71 13.5. RSA ......................................................77
13.3.1. Compression Preferences 71 13.6. DSA ......................................................77
13.3.2. Hash Algorithm Preferences 72 13.7. Elgamal ..................................................78
13.4. Plaintext 72 13.8. Reserved Algorithm Numbers ...............................78
13.5. RSA 72 13.9. OpenPGP CFB Mode .........................................78
13.6. DSA 73 13.10. Private or Experimental Parameters ......................79
13.7. Elgamal 73 13.11. Extension of the MDC System .............................80
13.8. Reserved Algorithm Numbers 73 13.12. Meta-Considerations for Expansion .......................80
13.9. OpenPGP CFB mode 74 14. Security Considerations .......................................81
13.10. Private or Experimental Parameters 75 15. Implementation Nits ...........................................84
13.11. Extension of the MDC System 75 16. References ....................................................86
13.12. Meta-Considerations for Expansion 76 16.1. Normative References .....................................86
14. Security Considerations 76 16.2. Informative References ...................................88
15. Implementation Nits 79
16. Authors' Addresses 80
17. References (Normative) 81
18. References (Informative) 83
19. Full Copyright Statement 84
20. Intellectual Property 84
1. Introduction 1. Introduction
This document provides information on the message-exchange packet This document provides information on the message-exchange packet
formats used by OpenPGP to provide encryption, decryption, signing, formats used by OpenPGP to provide encryption, decryption, signing,
and key management functions. It is a revision of RFC 2440, "OpenPGP and key management functions. It is a revision of RFC 2440, "OpenPGP
Message Format", which itself replaces RFC 1991, "PGP Message Message Format", which itself replaces RFC 1991, "PGP Message
Exchange Formats." [RFC1991] [RFC2440] Exchange Formats" [RFC1991] [RFC2440].
1.1. Terms 1.1. Terms
* OpenPGP - This is a definition for security software that uses * OpenPGP - This is a term for security software that uses PGP 5.x
PGP 5.x as a basis, formalized in RFC 2440 and this document. as a basis, formalized in RFC 2440 and this document.
* PGP - Pretty Good Privacy. PGP is a family of software systems * PGP - Pretty Good Privacy. PGP is a family of software systems
developed by Philip R. Zimmermann from which OpenPGP is based. developed by Philip R. Zimmermann from which OpenPGP is based.
* PGP 2.6.x - This version of PGP has many variants, hence the * PGP 2.6.x - This version of PGP has many variants, hence the term
term PGP 2.6.x. It used only RSA, MD5, and IDEA for its PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic
cryptographic transforms. An informational RFC, RFC 1991, was transforms. An informational RFC, RFC 1991, was written
written describing this version of PGP. describing this version of PGP.
* PGP 5.x - This version of PGP is formerly known as "PGP 3" in * PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
the community and also in the predecessor of this document, RFC community and also in the predecessor of this document, RFC 1991.
1991. It has new formats and corrects a number of problems in It has new formats and corrects a number of problems in the PGP
the PGP 2.6.x design. It is referred to here as PGP 5.x because 2.6.x design. It is referred to here as PGP 5.x because that
that software was the first release of the "PGP 3" code base. software was the first release of the "PGP 3" code base.
* GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP * GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP
implementation that avoids all encumbered algorithms. implementation that avoids all encumbered algorithms.
Consequently, early versions of GnuPG did not include RSA public Consequently, early versions of GnuPG did not include RSA public
keys. GnuPG may or may not have (depending on version) support keys. GnuPG may or may not have (depending on version) support
for IDEA or other encumbered algorithms. for IDEA or other encumbered algorithms.
"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP
PGP Corporation and are used with permission. The term "OpenPGP" Corporation and are used with permission. The term "OpenPGP" refers
refers to the protocol described in this and related documents. to the protocol described in this and related documents.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. document are to be interpreted as described in [RFC2119].
The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
this document when used to describe namespace allocation are to be this document when used to describe namespace allocation are to be
interpreted as described in RFC 2434. interpreted as described in [RFC2434].
2. General functions 2. General functions
OpenPGP provides data integrity services for messages and data files OpenPGP provides data integrity services for messages and data files
by using these core technologies: by using these core technologies:
- digital signatures - digital signatures
- encryption - encryption
- compression - compression
- radix-64 conversion - Radix-64 conversion
In addition, OpenPGP provides key management and certificate In addition, OpenPGP provides key management and certificate
services, but many of these are beyond the scope of this document. services, but many of these are beyond the scope of this document.
2.1. Confidentiality via Encryption 2.1. Confidentiality via Encryption
OpenPGP combines symmetric-key encryption and public key encryption OpenPGP combines symmetric-key encryption and public-key encryption
to provide confidentiality. When made confidential, first the object to provide confidentiality. When made confidential, first the object
is encrypted using a symmetric encryption algorithm. Each symmetric is encrypted using a symmetric encryption algorithm. Each symmetric
key is used only once, for a single object. A new "session key" is key is used only once, for a single object. A new "session key" is
generated as a random number for each object (sometimes referred to generated as a random number for each object (sometimes referred to
as a session). Since it is used only once, the session key is bound as a session). Since it is used only once, the session key is bound
to the message and transmitted with it. To protect the key, it is to the message and transmitted with it. To protect the key, it is
encrypted with the receiver's public key. The sequence is as encrypted with the receiver's public key. The sequence is as
follows: follows:
1. The sender creates a message. 1. The sender creates a message.
2. The sending OpenPGP generates a random number to be used as a 2. The sending OpenPGP generates a random number to be used as a
session key for this message only. session key for this message only.
3. The session key is encrypted using each recipient's public key. 3. The session key is encrypted using each recipient's public key.
These "encrypted session keys" start the message. These "encrypted session keys" start the message.
4. The sending OpenPGP encrypts the message using the session key, 4. The sending OpenPGP encrypts the message using the session key,
which forms the remainder of the message. Note that the message which forms the remainder of the message. Note that the message
is also usually compressed. is also usually compressed.
5. The receiving OpenPGP decrypts the session key using the 5. The receiving OpenPGP decrypts the session key using the
recipient's private key. recipient's private key.
6. The receiving OpenPGP decrypts the message using the session 6. The receiving OpenPGP decrypts the message using the session key.
key. If the message was compressed, it will be decompressed. If the message was compressed, it will be decompressed.
With symmetric-key encryption, an object may be encrypted with a With symmetric-key encryption, an object may be encrypted with a
symmetric key derived from a passphrase (or other shared secret), or symmetric key derived from a passphrase (or other shared secret), or
a two-stage mechanism similar to the public-key method described a two-stage mechanism similar to the public-key method described
above in which a session key is itself encrypted with a symmetric above in which a session key is itself encrypted with a symmetric
algorithm keyed from a shared secret. algorithm keyed from a shared secret.
Both digital signature and confidentiality services may be applied Both digital signature and confidentiality services may be applied to
to the same message. First, a signature is generated for the message the same message. First, a signature is generated for the message
and attached to the message. Then, the message plus signature is and attached to the message. Then the message plus signature is
encrypted using a symmetric session key. Finally, the session key is encrypted using a symmetric session key. Finally, the session key is
encrypted using public-key encryption and prefixed to the encrypted encrypted using public-key encryption and prefixed to the encrypted
block. block.
2.2. Authentication via Digital signature 2.2. Authentication via Digital Signature
The digital signature uses a hash code or message digest algorithm, The digital signature uses a hash code or message digest algorithm,
and a public-key signature algorithm. The sequence is as follows: and a public-key signature algorithm. The sequence is as follows:
1. The sender creates a message. 1. The sender creates a message.
2. The sending software generates a hash code of the message. 2. The sending software generates a hash code of the message.
3. The sending software generates a signature from the hash code 3. The sending software generates a signature from the hash code
using the sender's private key. using the sender's private key.
4. The binary signature is attached to the message. 4. The binary signature is attached to the message.
5. The receiving software keeps a copy of the message signature. 5. The receiving software keeps a copy of the message signature.
6. The receiving software generates a new hash code for the 6. The receiving software generates a new hash code for the received
received message and verifies it using the message's signature. message and verifies it using the message's signature. If the
If the verification is successful, the message is accepted as verification is successful, the message is accepted as authentic.
authentic.
2.3. Compression 2.3. Compression
OpenPGP implementations SHOULD compress the message after applying OpenPGP implementations SHOULD compress the message after applying
the signature but before encryption. the signature but before encryption.
If an implementation does not implement compression, its authors If an implementation does not implement compression, its authors
should be aware that most OpenPGP messages in the world are should be aware that most OpenPGP messages in the world are
compressed. Thus, it may even be wise for a space-constrained compressed. Thus, it may even be wise for a space-constrained
implementation to implement decompression, but not compression. implementation to implement decompression, but not compression.
Furthermore, compression has the added side-effect that some types Furthermore, compression has the added side effect that some types of
of attacks can be thwarted by the fact that slightly altered, attacks can be thwarted by the fact that slightly altered, compressed
compressed data rarely uncompresses without severe errors. This is data rarely uncompresses without severe errors. This is hardly
hardly rigorous, but it is operationally useful. These attacks can rigorous, but it is operationally useful. These attacks can be
be rigorously prevented by implementing and using Modification rigorously prevented by implementing and using Modification Detection
Detection Codes as described in sections following. Codes as described in sections following.
2.4. Conversion to Radix-64 2.4. Conversion to Radix-64
OpenPGP's underlying native representation for encrypted messages, OpenPGP's underlying native representation for encrypted messages,
signature certificates, and keys is a stream of arbitrary octets. signature certificates, and keys is a stream of arbitrary octets.
Some systems only permit the use of blocks consisting of seven-bit, Some systems only permit the use of blocks consisting of seven-bit,
printable text. For transporting OpenPGP's native raw binary octets printable text. For transporting OpenPGP's native raw binary octets
through channels that are not safe to raw binary data, a printable through channels that are not safe to raw binary data, a printable
encoding of these binary octets is needed. OpenPGP provides the encoding of these binary octets is needed. OpenPGP provides the
service of converting the raw 8-bit binary octet stream to a stream service of converting the raw 8-bit binary octet stream to a stream
of printable ASCII characters, called Radix-64 encoding or ASCII of printable ASCII characters, called Radix-64 encoding or ASCII
Armor. Armor.
Implementations SHOULD provide Radix-64 conversions. Implementations SHOULD provide Radix-64 conversions.
2.5. Signature-Only Applications 2.5. Signature-Only Applications
OpenPGP is designed for applications that use both encryption and OpenPGP is designed for applications that use both encryption and
signatures, but there are a number of problems that are solved by a signatures, but there are a number of problems that are solved by a
signature-only implementation. Although this specification requires signature-only implementation. Although this specification requires
both encryption and signatures, it is reasonable for there to be both encryption and signatures, it is reasonable for there to be
subset implementations that are non-conformant only in that they subset implementations that are non-conformant only in that they omit
omit encryption. encryption.
3. Data Element Formats 3. Data Element Formats
This section describes the data elements used by OpenPGP. This section describes the data elements used by OpenPGP.
3.1. Scalar numbers 3.1. Scalar Numbers
Scalar numbers are unsigned, and are always stored in big-endian Scalar numbers are unsigned and are always stored in big-endian
format. Using n[k] to refer to the kth octet being interpreted, the format. Using n[k] to refer to the kth octet being interpreted, the
value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) + four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
n[3]). n[3]).
3.2. Multiprecision Integers 3.2. Multiprecision Integers
Multiprecision Integers (also called MPIs) are unsigned integers Multiprecision integers (also called MPIs) are unsigned integers used
used to hold large integers such as the ones used in cryptographic to hold large integers such as the ones used in cryptographic
calculations. calculations.
An MPI consists of two pieces: a two-octet scalar that is the length An MPI consists of two pieces: a two-octet scalar that is the length
of the MPI in bits followed by a string of octets that contain the of the MPI in bits followed by a string of octets that contain the
actual integer. actual integer.
These octets form a big-endian number; a big-endian number can be These octets form a big-endian number; a big-endian number can be
made into an MPI by prefixing it with the appropriate length. made into an MPI by prefixing it with the appropriate length.
Examples: Examples:
(all numbers are in hexadecimal) (all numbers are in hexadecimal)
The string of octets [00 01 01] forms an MPI with the value 1. The The string of octets [00 01 01] forms an MPI with the value 1. The
string [00 09 01 FF] forms an MPI with the value of 511. string [00 09 01 FF] forms an MPI with the value of 511.
Additional rules: Additional rules:
The size of an MPI is ((MPI.length + 7) / 8) + 2 octets. The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
The length field of an MPI describes the length starting from its The length field of an MPI describes the length starting from its
most significant non-zero bit. Thus, the MPI [00 02 01] is not most significant non-zero bit. Thus, the MPI [00 02 01] is not
formed correctly. It should be [00 01 01]. formed correctly. It should be [00 01 01].
Unused bits of an MPI MUST be zero. Unused bits of an MPI MUST be zero.
Also note that when an MPI is encrypted, the length refers to the Also note that when an MPI is encrypted, the length refers to the
plaintext MPI. It may be ill-formed in its ciphertext. plaintext MPI. It may be ill-formed in its ciphertext.
3.3. Key IDs 3.3. Key IDs
A Key ID is an eight-octet scalar that identifies a key. A Key ID is an eight-octet scalar that identifies a key.
Implementations SHOULD NOT assume that Key IDs are unique. The Implementations SHOULD NOT assume that Key IDs are unique. The
section, "Enhanced Key Formats" below describes how Key IDs are section "Enhanced Key Formats" below describes how Key IDs are
formed. formed.
3.4. Text 3.4. Text
Unless otherwise specified, the character set for text is the UTF-8 Unless otherwise specified, the character set for text is the UTF-8
[RFC3629] encoding of Unicode [ISO10646]. [RFC3629] encoding of Unicode [ISO10646].
3.5. Time fields 3.5. Time Fields
A time field is an unsigned four-octet number containing the number A time field is an unsigned four-octet number containing the number
of seconds elapsed since midnight, 1 January 1970 UTC. of seconds elapsed since midnight, 1 January 1970 UTC.
3.6. Keyrings 3.6. Keyrings
A keyring is a collection of one or more keys in a file or database. A keyring is a collection of one or more keys in a file or database.
Traditionally, a keyring is simply a sequential list of keys, but Traditionally, a keyring is simply a sequential list of keys, but may
may be any suitable database. It is beyond the scope of this be any suitable database. It is beyond the scope of this standard to
standard to discuss the details of keyrings or other databases. discuss the details of keyrings or other databases.
3.7. String-to-key (S2K) specifiers 3.7. String-to-Key (S2K) Specifiers
String-to-key (S2K) specifiers are used to convert passphrase String-to-key (S2K) specifiers are used to convert passphrase strings
strings into symmetric-key encryption/decryption keys. They are used into symmetric-key encryption/decryption keys. They are used in two
in two places, currently: to encrypt the secret part of private keys places, currently: to encrypt the secret part of private keys in the
in the private keyring, and to convert passphrases to encryption private keyring, and to convert passphrases to encryption keys for
keys for symmetrically encrypted messages. symmetrically encrypted messages.
3.7.1. String-to-key (S2K) specifier types 3.7.1. String-to-Key (S2K) Specifier Types
There are three types of S2K specifiers currently supported, and There are three types of S2K specifiers currently supported, and
some reserved values: some reserved values:
ID S2K Type ID S2K Type
-- --- ---- -- --------
0 Simple S2K 0 Simple S2K
1 Salted S2K 1 Salted S2K
2 Reserved value 2 Reserved value
3 Iterated and Salted S2K 3 Iterated and Salted S2K
100 to 110 Private/Experimental S2K 100 to 110 Private/Experimental S2K
These are described as follows: These are described in Sections 3.7.1.1 - 3.7.1.3.
3.7.1.1. Simple S2K 3.7.1.1. Simple S2K
This directly hashes the string to produce the key data. See below This directly hashes the string to produce the key data. See below
for how this hashing is done. for how this hashing is done.
Octet 0: 0x00 Octet 0: 0x00
Octet 1: hash algorithm Octet 1: hash algorithm
Simple S2K hashes the passphrase to produce the session key. The Simple S2K hashes the passphrase to produce the session key. The
manner in which this is done depends on the size of the session key manner in which this is done depends on the size of the session key
(which will depend on the cipher used) and the size of the hash (which will depend on the cipher used) and the size of the hash
algorithm's output. If the hash size is greater than the session key algorithm's output. If the hash size is greater than the session key
size, the high-order (leftmost) octets of the hash are used as the size, the high-order (leftmost) octets of the hash are used as the
key. key.
If the hash size is less than the key size, multiple instances of If the hash size is less than the key size, multiple instances of the
the hash context are created -- enough to produce the required key hash context are created -- enough to produce the required key data.
data. These instances are preloaded with 0, 1, 2, ... octets of These instances are preloaded with 0, 1, 2, ... octets of zeros (that
zeros (that is to say, the first instance has no preloading, the is to say, the first instance has no preloading, the second gets
second gets preloaded with 1 octet of zero, the third is preloaded preloaded with 1 octet of zero, the third is preloaded with two
with two octets of zeros, and so forth). octets of zeros, and so forth).
As the data is hashed, it is given independently to each hash As the data is hashed, it is given independently to each hash
context. Since the contexts have been initialized differently, they context. Since the contexts have been initialized differently, they
will each produce different hash output. Once the passphrase is will each produce different hash output. Once the passphrase is
hashed, the output data from the multiple hashes is concatenated, hashed, the output data from the multiple hashes is concatenated,
first hash leftmost, to produce the key data, with any excess octets first hash leftmost, to produce the key data, with any excess octets
on the right discarded. on the right discarded.
3.7.1.2. Salted S2K 3.7.1.2. Salted S2K
This includes a "salt" value in the S2K specifier -- some arbitrary This includes a "salt" value in the S2K specifier -- some arbitrary
data -- that gets hashed along with the passphrase string, to help data -- that gets hashed along with the passphrase string, to help
prevent dictionary attacks. prevent dictionary attacks.
Octet 0: 0x01 Octet 0: 0x01
Octet 1: hash algorithm Octet 1: hash algorithm
Octets 2-9: 8-octet salt value Octets 2-9: 8-octet salt value
Salted S2K is exactly like Simple S2K, except that the input to the Salted S2K is exactly like Simple S2K, except that the input to the
hash function(s) consists of the 8 octets of salt from the S2K hash function(s) consists of the 8 octets of salt from the S2K
specifier, followed by the passphrase. specifier, followed by the passphrase.
3.7.1.3. Iterated and Salted S2K 3.7.1.3. Iterated and Salted S2K
This includes both a salt and an octet count. The salt is combined This includes both a salt and an octet count. The salt is combined
with the passphrase and the resulting value is hashed repeatedly. with the passphrase and the resulting value is hashed repeatedly.
This further increases the amount of work an attacker must do to try This further increases the amount of work an attacker must do to try
dictionary attacks. dictionary attacks.
Octet 0: 0x03 Octet 0: 0x03
Octet 1: hash algorithm Octet 1: hash algorithm
Octets 2-9: 8-octet salt value Octets 2-9: 8-octet salt value
Octet 10: count, a one-octet, coded value Octet 10: count, a one-octet, coded value
The count is coded into a one-octet number using the following The count is coded into a one-octet number using the following
formula: formula:
#define EXPBIAS 6 #define EXPBIAS 6
count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS); count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
The above formula is in C, where "Int32" is a type for a 32-bit The above formula is in C, where "Int32" is a type for a 32-bit
integer, and the variable "c" is the coded count, Octet 10. integer, and the variable "c" is the coded count, Octet 10.
Iterated-Salted S2K hashes the passphrase and salt data multiple Iterated-Salted S2K hashes the passphrase and salt data multiple
times. The total number of octets to be hashed is specified in the times. The total number of octets to be hashed is specified in the
encoded count in the S2K specifier. Note that the resulting count encoded count in the S2K specifier. Note that the resulting count
value is an octet count of how many octets will be hashed, not an value is an octet count of how many octets will be hashed, not an
iteration count. iteration count.
Initially, one or more hash contexts are set up as with the other Initially, one or more hash contexts are set up as with the other S2K
S2K algorithms, depending on how many octets of key data are needed. algorithms, depending on how many octets of key data are needed.
Then the salt, followed by the passphrase data is repeatedly hashed Then the salt, followed by the passphrase data, is repeatedly hashed
until the number of octets specified by the octet count has been until the number of octets specified by the octet count has been
hashed. The one exception is that if the octet count is less than hashed. The one exception is that if the octet count is less than
the size of the salt plus passphrase, the full salt plus passphrase the size of the salt plus passphrase, the full salt plus passphrase
will be hashed even though that is greater than the octet count. will be hashed even though that is greater than the octet count.
After the hashing is done the data is unloaded from the hash After the hashing is done, the data is unloaded from the hash
context(s) as with the other S2K algorithms. context(s) as with the other S2K algorithms.
3.7.2. String-to-key usage 3.7.2. String-to-Key Usage
Implementations SHOULD use salted or iterated-and-salted S2K Implementations SHOULD use salted or iterated-and-salted S2K
specifiers, as simple S2K specifiers are more vulnerable to specifiers, as simple S2K specifiers are more vulnerable to
dictionary attacks. dictionary attacks.
3.7.2.1. Secret key encryption 3.7.2.1. Secret-Key Encryption
An S2K specifier can be stored in the secret keyring to specify how An S2K specifier can be stored in the secret keyring to specify how
to convert the passphrase to a key that unlocks the secret data. to convert the passphrase to a key that unlocks the secret data.
Older versions of PGP just stored a cipher algorithm octet preceding Older versions of PGP just stored a cipher algorithm octet preceding
the secret data or a zero to indicate that the secret data was the secret data or a zero to indicate that the secret data was
unencrypted. The MD5 hash function was always used to convert the unencrypted. The MD5 hash function was always used to convert the
passphrase to a key for the specified cipher algorithm. passphrase to a key for the specified cipher algorithm.
For compatibility, when an S2K specifier is used, the special value For compatibility, when an S2K specifier is used, the special value
254 or 255 is stored in the position where the hash algorithm octet 254 or 255 is stored in the position where the hash algorithm octet
would have been in the old data structure. This is then followed would have been in the old data structure. This is then followed
immediately by a one-octet algorithm identifier, and then by the S2K immediately by a one-octet algorithm identifier, and then by the S2K
specifier as encoded above. specifier as encoded above.
Therefore, preceding the secret data there will be one of these Therefore, preceding the secret data there will be one of these
possibilities: possibilities:
0: secret data is unencrypted (no passphrase) 0: secret data is unencrypted (no passphrase)
255 or 254: followed by algorithm octet and S2K specifier 255 or 254: followed by algorithm octet and S2K specifier
Cipher alg: use Simple S2K algorithm using MD5 hash Cipher alg: use Simple S2K algorithm using MD5 hash
This last possibility, the cipher algorithm number with an implicit This last possibility, the cipher algorithm number with an implicit
use of MD5 and IDEA, is provided for backward compatibility; it MAY use of MD5 and IDEA, is provided for backward compatibility; it MAY
be understood, but SHOULD NOT be generated, and is deprecated. be understood, but SHOULD NOT be generated, and is deprecated.
These are followed by an Initial Vector of the same length as the These are followed by an Initial Vector of the same length as the
block size of the cipher for the decryption of the secret values, if block size of the cipher for the decryption of the secret values, if
they are encrypted, and then the secret key values themselves. they are encrypted, and then the secret-key values themselves.
3.7.2.2. Symmetric-key message encryption 3.7.2.2. Symmetric-Key Message Encryption
OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
packet at the front of a message. This is used to allow S2K at the front of a message. This is used to allow S2K specifiers to
specifiers to be used for the passphrase conversion or to create be used for the passphrase conversion or to create messages with a
messages with a mix of symmetric-key ESKs and public-key ESKs. This mix of symmetric-key ESKs and public-key ESKs. This allows a message
allows a message to be decrypted either with a passphrase or a to be decrypted either with a passphrase or a public-key pair.
public key pair.
PGP 2.X always used IDEA with Simple string-to-key conversion when PGP 2.X always used IDEA with Simple string-to-key conversion when
encrypting a message with a symmetric algorithm. This is deprecated, encrypting a message with a symmetric algorithm. This is deprecated,
but MAY be used for backward-compatibility. but MAY be used for backward-compatibility.
4. Packet Syntax 4. Packet Syntax
This section describes the packets used by OpenPGP. This section describes the packets used by OpenPGP.
4.1. Overview 4.1. Overview
An OpenPGP message is constructed from a number of records that are An OpenPGP message is constructed from a number of records that are
traditionally called packets. A packet is a chunk of data that has a traditionally called packets. A packet is a chunk of data that has a
tag specifying its meaning. An OpenPGP message, keyring, tag specifying its meaning. An OpenPGP message, keyring,
certificate, and so forth consists of a number of packets. Some of certificate, and so forth consists of a number of packets. Some of
those packets may contain other OpenPGP packets (for example, a those packets may contain other OpenPGP packets (for example, a
compressed data packet, when uncompressed, contains OpenPGP compressed data packet, when uncompressed, contains OpenPGP packets).
packets).
Each packet consists of a packet header, followed by the packet Each packet consists of a packet header, followed by the packet body.
body. The packet header is of variable length. The packet header is of variable length.
4.2. Packet Headers 4.2. Packet Headers
The first octet of the packet header is called the "Packet Tag." It The first octet of the packet header is called the "Packet Tag". It
determines the format of the header and denotes the packet contents. determines the format of the header and denotes the packet contents.
The remainder of the packet header is the length of the packet. The remainder of the packet header is the length of the packet.
Note that the most significant bit is the left-most bit, called bit Note that the most significant bit is the leftmost bit, called bit 7.
7. A mask for this bit is 0x80 in hexadecimal. A mask for this bit is 0x80 in hexadecimal.
+---------------+ +---------------+
PTag |7 6 5 4 3 2 1 0| PTag |7 6 5 4 3 2 1 0|
+---------------+ +---------------+
Bit 7 -- Always one Bit 7 -- Always one
Bit 6 -- New packet format if set Bit 6 -- New packet format if set
PGP 2.6.x only uses old format packets. Thus, software that PGP 2.6.x only uses old format packets. Thus, software that
interoperates with those versions of PGP must only use old format interoperates with those versions of PGP must only use old format
packets. If interoperability is not an issue, the new packet format packets. If interoperability is not an issue, the new packet format
is RECOMMENDED. Note that old format packets have four bits of is RECOMMENDED. Note that old format packets have four bits of
packet tags, and new format packets have six; some features cannot packet tags, and new format packets have six; some features cannot be
be used and still be backward-compatible. used and still be backward-compatible.
Also note that packets with a tag greater than or equal to 16 MUST Also note that packets with a tag greater than or equal to 16 MUST
use new format packets. The old format packets can only express tags use new format packets. The old format packets can only express tags
less than or equal to 15. less than or equal to 15.
Old format packets contain: Old format packets contain:
Bits 5-2 -- packet tag Bits 5-2 -- packet tag
Bits 1-0 - length-type Bits 1-0 -- length-type
New format packets contain: New format packets contain:
Bits 5-0 -- packet tag Bits 5-0 -- packet tag
4.2.1. Old-Format Packet Lengths 4.2.1. Old Format Packet Lengths
The meaning of the length-type in old-format packets is: The meaning of the length-type in old format packets is:
0 - The packet has a one-octet length. The header is 2 octets long. 0 - The packet has a one-octet length. The header is 2 octets long.
1 - The packet has a two-octet length. The header is 3 octets long. 1 - The packet has a two-octet length. The header is 3 octets long.
2 - The packet has a four-octet length. The header is 5 octets long. 2 - The packet has a four-octet length. The header is 5 octets long.
3 - The packet is of indeterminate length. The header is 1 octet 3 - The packet is of indeterminate length. The header is 1 octet
long, and the implementation must determine how long the packet long, and the implementation must determine how long the packet
is. If the packet is in a file, this means that the packet is. If the packet is in a file, this means that the packet
extends until the end of the file. In general, an implementation extends until the end of the file. In general, an implementation
SHOULD NOT use indeterminate length packets except where the end SHOULD NOT use indeterminate-length packets except where the end
of the data will be clear from the context, and even then it is of the data will be clear from the context, and even then it is
better to use a definite length, or a new-format header. The better to use a definite length, or a new format header. The new
new-format headers described below have a mechanism for format headers described below have a mechanism for precisely
precisely encoding data of indeterminate length. encoding data of indeterminate length.
4.2.2. New-Format Packet Lengths 4.2.2. New Format Packet Lengths
New format packets have four possible ways of encoding length: New format packets have four possible ways of encoding length:
1. A one-octet Body Length header encodes packet lengths of up to 1. A one-octet Body Length header encodes packet lengths of up to 191
191 octets. octets.
2. A two-octet Body Length header encodes packet lengths of 192 to 2. A two-octet Body Length header encodes packet lengths of 192 to
8383 octets. 8383 octets.
3. A five-octet Body Length header encodes packet lengths of up to 3. A five-octet Body Length header encodes packet lengths of up to
4,294,967,295 (0xFFFFFFFF) octets in length. (This actually 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
encodes a four-octet scalar number.) encodes a four-octet scalar number.)
4. When the length of the packet body is not known in advance by 4. When the length of the packet body is not known in advance by the
the issuer, Partial Body Length headers encode a packet of issuer, Partial Body Length headers encode a packet of
indeterminate length, effectively making it a stream. indeterminate length, effectively making it a stream.
4.2.2.1. One-Octet Lengths 4.2.2.1. One-Octet Lengths
A one-octet Body Length header encodes a length of from 0 to 191 A one-octet Body Length header encodes a length of 0 to 191 octets.
octets. This type of length header is recognized because the one This type of length header is recognized because the one octet value
octet value is less than 192. The body length is equal to: is less than 192. The body length is equal to:
bodyLen = 1st_octet; bodyLen = 1st_octet;
4.2.2.2. Two-Octet Lengths 4.2.2.2. Two-Octet Lengths
A two-octet Body Length header encodes a length of from 192 to 8383 A two-octet Body Length header encodes a length of 192 to 8383
octets. It is recognized because its first octet is in the range 192 octets. It is recognized because its first octet is in the range 192
to 223. The body length is equal to: to 223. The body length is equal to:
bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
4.2.2.3. Five-Octet Lengths 4.2.2.3. Five-Octet Lengths
A five-octet Body Length header consists of a single octet holding A five-octet Body Length header consists of a single octet holding
the value 255, followed by a four-octet scalar. The body length is the value 255, followed by a four-octet scalar. The body length is
equal to: equal to:
bodyLen = (2nd_octet << 24) | (3rd_octet << 16) | bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
(4th_octet << 8) | 5th_octet (4th_octet << 8) | 5th_octet
This basic set of one, two, and five-octet lengths is also used This basic set of one, two, and five-octet lengths is also used
internally to some packets. internally to some packets.
4.2.2.4. Partial Body Lengths 4.2.2.4. Partial Body Lengths
A Partial Body Length header is one octet long and encodes the A Partial Body Length header is one octet long and encodes the length
length of only part of the data packet. This length is a power of 2, of only part of the data packet. This length is a power of 2, from 1
from 1 to 1,073,741,824 (2 to the 30th power). It is recognized by to 1,073,741,824 (2 to the 30th power). It is recognized by its one
its one octet value that is greater than or equal to 224, and less octet value that is greater than or equal to 224, and less than 255.
than 255. The partial body length is equal to: The Partial Body Length is equal to:
partialBodyLen = 1 << (1st_octet & 0x1f); partialBodyLen = 1 << (1st_octet & 0x1F);
Each Partial Body Length header is followed by a portion of the Each Partial Body Length header is followed by a portion of the
packet body data. The Partial Body Length header specifies this packet body data. The Partial Body Length header specifies this
portion's length. Another length header (one octet, two-octet, portion's length. Another length header (one octet, two-octet,
five-octet, or partial) follows that portion. The last length header five-octet, or partial) follows that portion. The last length header
in the packet MUST NOT be a partial Body Length header. Partial Body in the packet MUST NOT be a Partial Body Length header. Partial Body
Length headers may only be used for the non-final parts of the Length headers may only be used for the non-final parts of the
packet. packet.
Note also that the last Body Length header can be a zero-length Note also that the last Body Length header can be a zero-length
header. header.
An implementation MAY use Partial Body Lengths for data packets, be An implementation MAY use Partial Body Lengths for data packets, be
they literal, compressed, or encrypted. The first partial length they literal, compressed, or encrypted. The first partial length
MUST be at least 512 octets long. Partial Body Lengths MUST NOT be MUST be at least 512 octets long. Partial Body Lengths MUST NOT be
used for any other packet types. used for any other packet types.
4.2.3. Packet Length Examples 4.2.3. Packet Length Examples
These examples show ways that new-format packets might encode the These examples show ways that new format packets might encode the
packet lengths. packet lengths.
A packet with length 100 may have its length encoded in one octet: A packet with length 100 may have its length encoded in one octet:
0x64. This is followed by 100 octets of data. 0x64. This is followed by 100 octets of data.
A packet with length 1723 may have its length coded in two octets: A packet with length 1723 may have its length encoded in two octets:
0xC5, 0xFB. This header is followed by the 1723 octets of data. 0xC5, 0xFB. This header is followed by the 1723 octets of data.
A packet with length 100000 may have its length encoded in five A packet with length 100000 may have its length encoded in five
octets: 0xFF, 0x00, 0x01, 0x86, 0xA0. octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
It might also be encoded in the following octet stream: 0xEF, first It might also be encoded in the following octet stream: 0xEF, first
32768 octets of data; 0xE1, next two octets of data; 0xE0, next one 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
1693 octets of data. This is just one possible encoding, and many octets of data. This is just one possible encoding, and many
variations are possible on the size of the Partial Body Length variations are possible on the size of the Partial Body Length
headers, as long as a regular Body Length header encodes the last headers, as long as a regular Body Length header encodes the last
portion of the data. portion of the data.
Please note that in all of these explanations, the total length of Please note that in all of these explanations, the total length of
the packet is the length of the header(s) plus the length of the the packet is the length of the header(s) plus the length of the
body. body.
4.3. Packet Tags 4.3. Packet Tags
The packet tag denotes what type of packet the body holds. Note that The packet tag denotes what type of packet the body holds. Note that
old format headers can only have tags less than 16, whereas new old format headers can only have tags less than 16, whereas new
format headers can have tags as great as 63. The defined tags (in format headers can have tags as great as 63. The defined tags (in
decimal) are: decimal) are as follows:
0 -- Reserved - a packet tag MUST NOT have this value 0 -- Reserved - a packet tag MUST NOT have this value
1 -- Public-Key Encrypted Session Key Packet 1 -- Public-Key Encrypted Session Key Packet
2 -- Signature Packet 2 -- Signature Packet
3 -- Symmetric-Key Encrypted Session Key Packet 3 -- Symmetric-Key Encrypted Session Key Packet
4 -- One-Pass Signature Packet 4 -- One-Pass Signature Packet
5 -- Secret Key Packet 5 -- Secret-Key Packet
6 -- Public Key Packet 6 -- Public-Key Packet
7 -- Secret Subkey Packet 7 -- Secret-Subkey Packet
8 -- Compressed Data Packet 8 -- Compressed Data Packet
9 -- Symmetrically Encrypted Data Packet 9 -- Symmetrically Encrypted Data Packet
10 -- Marker Packet 10 -- Marker Packet
11 -- Literal Data Packet 11 -- Literal Data Packet
12 -- Trust Packet 12 -- Trust Packet
13 -- User ID Packet 13 -- User ID Packet
14 -- Public Subkey Packet 14 -- Public-Subkey Packet
17 -- User Attribute Packet 17 -- User Attribute Packet
18 -- Sym. Encrypted and Integrity Protected Data Packet 18 -- Sym. Encrypted and Integrity Protected Data Packet
19 -- Modification Detection Code Packet 19 -- Modification Detection Code Packet
60 to 63 -- Private or Experimental Values 60 to 63 -- Private or Experimental Values
5. Packet Types 5. Packet Types
5.1. Public-Key Encrypted Session Key Packets (Tag 1) 5.1. Public-Key Encrypted Session Key Packets (Tag 1)
A Public-Key Encrypted Session Key packet holds the session key used A Public-Key Encrypted Session Key packet holds the session key used
to encrypt a message. Zero or more Public-Key Encrypted Session Key to encrypt a message. Zero or more Public-Key Encrypted Session Key
packets and/or Symmetric-Key Encrypted Session Key packets may packets and/or Symmetric-Key Encrypted Session Key packets may
precede a Symmetrically Encrypted Data Packet, which holds an precede a Symmetrically Encrypted Data Packet, which holds an
encrypted message. The message is encrypted with the session key, encrypted message. The message is encrypted with the session key,
and the session key is itself encrypted and stored in the Encrypted and the session key is itself encrypted and stored in the Encrypted
Session Key packet(s). The Symmetrically Encrypted Data Packet is Session Key packet(s). The Symmetrically Encrypted Data Packet is
preceded by one Public-Key Encrypted Session Key packet for each preceded by one Public-Key Encrypted Session Key packet for each
OpenPGP key to which the message is encrypted. The recipient of the OpenPGP key to which the message is encrypted. The recipient of the
message finds a session key that is encrypted to their public key, message finds a session key that is encrypted to their public key,
decrypts the session key, and then uses the session key to decrypt decrypts the session key, and then uses the session key to decrypt
the message. the message.
The body of this packet consists of: The body of this packet consists of:
- A one-octet number giving the version number of the packet type. - A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3. The currently defined value for packet version is 3.
- An eight-octet number that gives the key ID of the public key - An eight-octet number that gives the Key ID of the public key to
that the session key is encrypted to. If the session key is which the session key is encrypted. If the session key is
encrypted to a subkey then the key ID of this subkey is used encrypted to a subkey, then the Key ID of this subkey is used
here instead of the key ID of the primary key. here instead of the Key ID of the primary key.
- A one-octet number giving the public key algorithm used. - A one-octet number giving the public-key algorithm used.
- A string of octets that is the encrypted session key. This - A string of octets that is the encrypted session key. This
string takes up the remainder of the packet, and its contents string takes up the remainder of the packet, and its contents are
are dependent on the public key algorithm used. dependent on the public-key algorithm used.
Algorithm Specific Fields for RSA encryption Algorithm Specific Fields for RSA encryption
- multiprecision integer (MPI) of RSA encrypted value m**e mod n. - multiprecision integer (MPI) of RSA encrypted value m**e mod n.
Algorithm Specific Fields for Elgamal encryption: Algorithm Specific Fields for Elgamal encryption:
- MPI of Elgamal (Diffie-Hellman) value g**k mod p. - MPI of Elgamal (Diffie-Hellman) value g**k mod p.
- MPI of Elgamal (Diffie-Hellman) value m * y**k mod p. - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.
The value "m" in the above formulas is derived from the session key The value "m" in the above formulas is derived from the session key
as follows. First the session key is prefixed with a one-octet as follows. First, the session key is prefixed with a one-octet
algorithm identifier that specifies the symmetric encryption algorithm identifier that specifies the symmetric encryption
algorithm used to encrypt the following Symmetrically Encrypted Data algorithm used to encrypt the following Symmetrically Encrypted Data
Packet. Then a two-octet checksum is appended which is equal to the Packet. Then a two-octet checksum is appended, which is equal to the
sum of the preceding session key octets, not including the algorithm sum of the preceding session key octets, not including the algorithm
identifier, modulo 65536. This value is then encoded as described in identifier, modulo 65536. This value is then encoded as described in
PKCS#1 block encoding EME-PKCS1-v1_5 in Section 12.1 of RFC 3447 to PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
form the "m" value used in the formulas above. See Section 13.1 of form the "m" value used in the formulas above. See Section 13.1 of
this document for notes on OpenPGP's use of PKCS#1. this document for notes on OpenPGP's use of PKCS#1.
Note that when an implementation forms several PKESKs with one Note that when an implementation forms several PKESKs with one
session key, forming a message that can be decrypted by several session key, forming a message that can be decrypted by several keys,
keys, the implementation MUST make a new PKCS#1 encoding for each the implementation MUST make a new PKCS#1 encoding for each key.
key.
An implementation MAY accept or use a Key ID of zero as a "wild An implementation MAY accept or use a Key ID of zero as a "wild card"
card" or "speculative" Key ID. In this case, the receiving or "speculative" Key ID. In this case, the receiving implementation
implementation would try all available private keys, checking for a would try all available private keys, checking for a valid decrypted
valid decrypted session key. This format helps reduce traffic session key. This format helps reduce traffic analysis of messages.
analysis of messages.
5.2. Signature Packet (Tag 2) 5.2. Signature Packet (Tag 2)
A signature packet describes a binding between some public key and A Signature packet describes a binding between some public key and
some data. The most common signatures are a signature of a file or a some data. The most common signatures are a signature of a file or a
block of text, and a signature that is a certification of a User ID. block of text, and a signature that is a certification of a User ID.
Two versions of signature packets are defined. Version 3 provides Two versions of Signature packets are defined. Version 3 provides
basic signature information, while version 4 provides an expandable basic signature information, while version 4 provides an expandable
format with subpackets that can specify more information about the format with subpackets that can specify more information about the
signature. PGP 2.6.x only accepts version 3 signatures. signature. PGP 2.6.x only accepts version 3 signatures.
Implementations SHOULD accept V3 signatures. Implementations SHOULD Implementations SHOULD accept V3 signatures. Implementations SHOULD
generate V4 signatures. generate V4 signatures.
Note that if an implementation is creating an encrypted and signed Note that if an implementation is creating an encrypted and signed
message that is encrypted to a V3 key, it is reasonable to create a message that is encrypted to a V3 key, it is reasonable to create a
V3 signature. V3 signature.
5.2.1. Signature Types 5.2.1. Signature Types
There are a number of possible meanings for a signature, which are There are a number of possible meanings for a signature, which are
indicated in a signature type octet in any given signature. Please indicated in a signature type octet in any given signature. Please
note that the vagueness of these meanings is not a flaw, but a note that the vagueness of these meanings is not a flaw, but a
feature of the system. Because OpenPGP places final authority for feature of the system. Because OpenPGP places final authority for
validity upon the receiver of a signature, it may be that one validity upon the receiver of a signature, it may be that one
signer's casual act might be more rigorous than some other signer's casual act might be more rigorous than some other
authority's positive act. See section 5.2.4, "Computing Signatures," authority's positive act. See Section 5.2.4, "Computing Signatures",
for detailed information on how to compute and verify signatures of for detailed information on how to compute and verify signatures of
each type. each type.
These meanings are: These meanings are as follows:
0x00: Signature of a binary document. 0x00: Signature of a binary document.
This means the signer owns it, created it, or certifies that it This means the signer owns it, created it, or certifies that it
has not been modified. has not been modified.
0x01: Signature of a canonical text document. 0x01: Signature of a canonical text document.
This means the signer owns it, created it, or certifies that it This means the signer owns it, created it, or certifies that it
has not been modified. The signature is calculated over the text has not been modified. The signature is calculated over the text
data with its line endings converted to <CR><LF>. data with its line endings converted to <CR><LF>.
0x02: Standalone signature. 0x02: Standalone signature.
This signature is a signature of only its own subpacket This signature is a signature of only its own subpacket contents.
contents. It is calculated identically to a signature over a It is calculated identically to a signature over a zero-length
zero-length binary document. Note that it doesn't make sense to binary document. Note that it doesn't make sense to have a V3
have a V3 standalone signature. standalone signature.
0x10: Generic certification of a User ID and Public Key packet. 0x10: Generic certification of a User ID and Public-Key packet.
The issuer of this certification does not make any particular The issuer of this certification does not make any particular
assertion as to how well the certifier has checked that the assertion as to how well the certifier has checked that the owner
owner of the key is in fact the person described by the User ID. of the key is in fact the person described by the User ID.
0x11: Persona certification of a User ID and Public Key packet. 0x11: Persona certification of a User ID and Public-Key packet.
The issuer of this certification has not done any verification The issuer of this certification has not done any verification of
of the claim that the owner of this key is the User ID the claim that the owner of this key is the User ID specified.
specified.
0x12: Casual certification of a User ID and Public Key packet. 0x12: Casual certification of a User ID and Public-Key packet.
The issuer of this certification has done some casual The issuer of this certification has done some casual
verification of the claim of identity. verification of the claim of identity.
0x13: Positive certification of a User ID and Public Key packet. 0x13: Positive certification of a User ID and Public-Key packet.
The issuer of this certification has done substantial The issuer of this certification has done substantial
verification of the claim of identity. verification of the claim of identity.
Most OpenPGP implementations make their "key signatures" as 0x10 Most OpenPGP implementations make their "key signatures" as 0x10
certifications. Some implementations can issue 0x11-0x13 certifications. Some implementations can issue 0x11-0x13
certifications, but few differentiate between the types. certifications, but few differentiate between the types.
0x18: Subkey Binding Signature 0x18: Subkey Binding Signature
This signature is a statement by the top-level signing key that This signature is a statement by the top-level signing key that
indicates that it owns the subkey. This signature is calculated indicates that it owns the subkey. This signature is calculated
directly on the primary key and subkey, and not on any User ID directly on the primary key and subkey, and not on any User ID or
or other packets. A signature that binds a signing subkey MUST other packets. A signature that binds a signing subkey MUST have
have an embedded signature subpacket in this binding signature an Embedded Signature subpacket in this binding signature that
which contains a 0x19 signature made by the signing subkey on contains a 0x19 signature made by the signing subkey on the
the primary key and subkey. primary key and subkey.
0x19 Primary Key Binding Signature 0x19: Primary Key Binding Signature
This signature is a statement by a signing subkey, indicating This signature is a statement by a signing subkey, indicating
that it is owned by the primary key and subkey. This signature that it is owned by the primary key and subkey. This signature
is calculated the same way as a 0x18 signature: directly on the is calculated the same way as a 0x18 signature: directly on the
primary key and subkey, and not on any User ID or other packets. primary key and subkey, and not on any User ID or other packets.
0x1F: Signature directly on a key 0x1F: Signature directly on a key
This signature is calculated directly on a key. It binds the This signature is calculated directly on a key. It binds the
information in the signature subpackets to the key, and is information in the Signature subpackets to the key, and is
appropriate to be used for subpackets that provide information appropriate to be used for subpackets that provide information
about the key, such as the revocation key subpacket. It is also about the key, such as the Revocation Key subpacket. It is also
appropriate for statements that non-self certifiers want to make appropriate for statements that non-self certifiers want to make
about the key itself, rather than the binding between a key and about the key itself, rather than the binding between a key and a
a name. name.
0x20: Key revocation signature 0x20: Key revocation signature
The signature is calculated directly on the key being revoked. A The signature is calculated directly on the key being revoked. A
revoked key is not to be used. Only revocation signatures by the revoked key is not to be used. Only revocation signatures by the
key being revoked, or by an authorized revocation key, should be key being revoked, or by an authorized revocation key, should be
considered valid revocation signatures. considered valid revocation signatures.
0x28: Subkey revocation signature 0x28: Subkey revocation signature
The signature is calculated directly on the subkey being The signature is calculated directly on the subkey being revoked.
revoked. A revoked subkey is not to be used. Only revocation A revoked subkey is not to be used. Only revocation signatures
signatures by the top-level signature key that is bound to this by the top-level signature key that is bound to this subkey, or
subkey, or by an authorized revocation key, should be considered by an authorized revocation key, should be considered valid
valid revocation signatures. revocation signatures.
0x30: Certification revocation signature 0x30: Certification revocation signature
This signature revokes an earlier User ID certification This signature revokes an earlier User ID certification signature
signature (signature class 0x10 through 0x13) or direct-key (signature class 0x10 through 0x13) or direct-key signature
signature (0x1F). It should be issued by the same key that (0x1F). It should be issued by the same key that issued the
issued the revoked signature or an authorized revocation key. revoked signature or an authorized revocation key. The signature
The signature is computed over the same data as the certificate is computed over the same data as the certificate that it
that it revokes, and should have a later creation date than that revokes, and should have a later creation date than that
certificate. certificate.
0x40: Timestamp signature. 0x40: Timestamp signature.
This signature is only meaningful for the timestamp contained in This signature is only meaningful for the timestamp contained in
it. it.
0x50: Third-Party Confirmation signature. 0x50: Third-Party Confirmation signature.
This signature is a signature over some other OpenPGP signature This signature is a signature over some other OpenPGP Signature
packet(s). It is analogous to a notary seal on the signed data. packet(s). It is analogous to a notary seal on the signed data.
A third-party signature SHOULD include Signature Target A third-party signature SHOULD include Signature Target
subpacket(s) to give easy identification. Note that we really do subpacket(s) to give easy identification. Note that we really do
mean SHOULD. There are plausible uses for this (such as a blind mean SHOULD. There are plausible uses for this (such as a blind
party that only sees the signature, not the key nor source party that only sees the signature, not the key or source
document) that cannot include a target subpacket. document) that cannot include a target subpacket.
5.2.2. Version 3 Signature Packet Format 5.2.2. Version 3 Signature Packet Format
The body of a version 3 Signature Packet contains: The body of a version 3 Signature Packet contains:
- One-octet version number (3). - One-octet version number (3).
- One-octet length of following hashed material. MUST be 5. - One-octet length of following hashed material. MUST be 5.
- One-octet signature type. - One-octet signature type.
- Four-octet creation time. - Four-octet creation time.
- Eight-octet key ID of signer. - Eight-octet Key ID of signer.
- One-octet public key algorithm. - One-octet public-key algorithm.
- One-octet hash algorithm. - One-octet hash algorithm.
- Two-octet field holding left 16 bits of signed hash value. - Two-octet field holding left 16 bits of signed hash value.
- One or more multiprecision integers comprising the signature. - One or more multiprecision integers comprising the signature.
This portion is algorithm specific, as described below. This portion is algorithm specific, as described below.
The concatenation of the data to be signed, the signature type and The concatenation of the data to be signed, the signature type, and
creation time from the signature packet (5 additional octets) is creation time from the Signature packet (5 additional octets) is
hashed. The resulting hash value is used in the signature algorithm. hashed. The resulting hash value is used in the signature algorithm.
The high 16 bits (first two octets) of the hash are included in the The high 16 bits (first two octets) of the hash are included in the
signature packet to provide a quick test to reject some invalid Signature packet to provide a quick test to reject some invalid
signatures. signatures.
Algorithm Specific Fields for RSA signatures: Algorithm-Specific Fields for RSA signatures:
- multiprecision integer (MPI) of RSA signature value m**d mod n. - multiprecision integer (MPI) of RSA signature value m**d mod n.
Algorithm Specific Fields for DSA signatures: Algorithm-Specific Fields for DSA signatures:
- MPI of DSA value r. - MPI of DSA value r.
- MPI of DSA value s. - MPI of DSA value s.
The signature calculation is based on a hash of the signed data, as The signature calculation is based on a hash of the signed data, as
described above. The details of the calculation are different for described above. The details of the calculation are different for
DSA signatures than for RSA signatures. DSA signatures than for RSA signatures.
With RSA signatures, the hash value is encoded as described in With RSA signatures, the hash value is encoded using PKCS#1 encoding
PKCS#1 section 9.2.1 of RFC 3447 encoded using PKCS#1 encoding type type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447. This
EMSA-PKCS1-v1_5 as described in section 12.1 of RFC 3447. This requires inserting the hash value as an octet string into an ASN.1
requires inserting the hash value as an octet string into an ASN.1 structure. The object identifier for the type of hash being used is
structure. The object identifier for the type of hash being used is included in the structure. The hexadecimal representations for the
included in the structure. The hexadecimal representations for the currently defined hash algorithms are as follows:
currently defined hash algorithms are:
- MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05 - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
- RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01 - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
- SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
- SHA224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04 - SHA224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04
- SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01 - SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
- SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02 - SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
- SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
- SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03 The ASN.1 Object Identifiers (OIDs) are as follows:
The ASN.1 OIDs are: - MD5: 1.2.840.113549.2.5
- MD5: 1.2.840.113549.2.5 - RIPEMD-160: 1.3.36.3.2.1
- RIPEMD-160: 1.3.36.3.2.1 - SHA-1: 1.3.14.3.2.26
- SHA-1: 1.3.14.3.2.26 - SHA224: 2.16.840.1.101.3.4.2.4
- SHA224: 2.16.840.1.101.3.4.2.4 - SHA256: 2.16.840.1.101.3.4.2.1
- SHA256: 2.16.840.1.101.3.4.2.1 - SHA384: 2.16.840.1.101.3.4.2.2
- SHA384: 2.16.840.1.101.3.4.2.2 - SHA512: 2.16.840.1.101.3.4.2.3
- SHA512: 2.16.840.1.101.3.4.2.3 The full hash prefixes for these are as follows:
The full hash prefixes for these are: MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
0x04, 0x10
MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86, RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00, 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
0x04, 0x10
RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24, SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E, SHA224: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
0x00, 0x04, 0x1C
SHA224: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, SHA256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
0x00, 0x04, 0x1C 0x00, 0x04, 0x20
SHA256: 0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
0x00, 0x04, 0x20 0x00, 0x04, 0x30
SHA384: 0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
0x00, 0x04, 0x30 0x00, 0x04, 0x40
SHA512: 0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, DSA signatures MUST use hashes that are equal in size to the number
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, of bits of q, the group generated by the DSA key's generator value.
0x00, 0x04, 0x40
DSA signatures MUST use hashes that are equal in size to the number If the output size of the chosen hash is larger than the number of
of bits of q, the group generated by the DSA key's generator value. bits of q, the hash result is truncated to fit by taking the number
If the output size of the chosen hash is larger than the number of of leftmost bits equal to the number of bits of q. This (possibly
bits of q, the hash result is truncated to fit by taking the number truncated) hash function result is treated as a number and used
of leftmost bits equal to the number of bits of q. This (possibly directly in the DSA signature algorithm.
truncated) hash function result is treated as a number and used
directly in the DSA signature algorithm.
5.2.3. Version 4 Signature Packet Format 5.2.3. Version 4 Signature Packet Format
The body of a version 4 Signature Packet contains: The body of a version 4 Signature packet contains:
- One-octet version number (4). - One-octet version number (4).
- One-octet signature type. - One-octet signature type.
- One-octet public key algorithm. - One-octet public-key algorithm.
- One-octet hash algorithm. - One-octet hash algorithm.
- Two-octet scalar octet count for following hashed subpacket - Two-octet scalar octet count for following hashed subpacket data.
data. Note that this is the length in octets of all of the Note that this is the length in octets of all of the hashed
hashed subpackets; a pointer incremented by this number will subpackets; a pointer incremented by this number will skip over
skip over the hashed subpackets. the hashed subpackets.
- Hashed subpacket data set. (zero or more subpackets) - Hashed subpacket data set (zero or more subpackets).
- Two-octet scalar octet count for the following unhashed - Two-octet scalar octet count for the following unhashed subpacket
subpacket data. Note that this is the length in octets of all of data. Note that this is the length in octets of all of the
the unhashed subpackets; a pointer incremented by this number unhashed subpackets; a pointer incremented by this number will
will skip over the unhashed subpackets. skip over the unhashed subpackets.
- Unhashed subpacket data set. (zero or more subpackets) - Unhashed subpacket data set (zero or more subpackets).
- Two-octet field holding the left 16 bits of the signed hash
value.
- One or more multiprecision integers comprising the signature. - Two-octet field holding the left 16 bits of the signed hash
This portion is algorithm specific, as described above. value.
The concatenation of the data being signed and the signature data - One or more multiprecision integers comprising the signature.
from the version number through the hashed subpacket data This portion is algorithm specific, as described above.
(inclusive) is hashed. The resulting hash value is what is signed.
The left 16 bits of the hash are included in the signature packet to
provide a quick test to reject some invalid signatures.
There are two fields consisting of signature subpackets. The first The concatenation of the data being signed and the signature data
field is hashed with the rest of the signature data, while the from the version number through the hashed subpacket data (inclusive)
second is unhashed. The second set of subpackets is not is hashed. The resulting hash value is what is signed. The left 16
cryptographically protected by the signature and should include only bits of the hash are included in the Signature packet to provide a
advisory information. quick test to reject some invalid signatures.
The algorithms for converting the hash function result to a There are two fields consisting of Signature subpackets. The first
signature are described in a section below. field is hashed with the rest of the signature data, while the second
is unhashed. The second set of subpackets is not cryptographically
protected by the signature and should include only advisory
information.
5.2.3.1. Signature Subpacket Specification The algorithms for converting the hash function result to a signature
are described in a section below.
A subpacket data set consists of zero or more signature subpackets. 5.2.3.1. Signature Subpacket Specification
In signature packets the subpacket data set is preceded by a
two-octet scalar count of the length in octets of all the
subpackets. A pointer incremented by this number will skip over the
subpacket data set.
Each subpacket consists of a subpacket header and a body. The header A subpacket data set consists of zero or more Signature subpackets.
consists of: In Signature packets, the subpacket data set is preceded by a two-
octet scalar count of the length in octets of all the subpackets. A
pointer incremented by this number will skip over the subpacket data
set.
- the subpacket length (1, 2, or 5 octets) Each subpacket consists of a subpacket header and a body. The header
consists of:
- the subpacket type (1 octet) - the subpacket length (1, 2, or 5 octets),
and is followed by the subpacket specific data. - the subpacket type (1 octet),
The length includes the type octet but not this length. Its format and is followed by the subpacket-specific data.
is similar to the "new" format packet header lengths, but cannot
have partial body lengths. That is:
if the 1st octet < 192, then The length includes the type octet but not this length. Its format
lengthOfLength = 1 is similar to the "new" format packet header lengths, but cannot have
subpacketLen = 1st_octet Partial Body Lengths. That is:
if the 1st octet >= 192 and < 255, then if the 1st octet < 192, then
lengthOfLength = 2 lengthOfLength = 1
subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192 subpacketLen = 1st_octet
if the 1st octet = 255, then if the 1st octet >= 192 and < 255, then
lengthOfLength = 5 lengthOfLength = 2
subpacket length = [four-octet scalar starting at 2nd_octet] subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
The value of the subpacket type octet may be: if the 1st octet = 255, then
lengthOfLength = 5
subpacket length = [four-octet scalar starting at 2nd_octet]
0 = reserved The value of the subpacket type octet may be:
1 = reserved
2 = signature creation time
3 = signature expiration time
4 = exportable certification
5 = trust signature
6 = regular expression
7 = revocable
8 = reserved
9 = key expiration time
10 = placeholder for backward compatibility
11 = preferred symmetric algorithms
12 = revocation key
13 = reserved
14 = reserved
15 = reserved
16 = issuer key ID
17 = reserved
18 = reserved
19 = reserved
20 = notation data
21 = preferred hash algorithms
22 = preferred compression algorithms
23 = key server preferences
24 = preferred key server
25 = primary User ID
26 = policy URI
27 = key flags
28 = signer's User ID
29 = reason for revocation
30 = features
31 = signature target
32 = embedded signature
100 to 110 = private or experimental 0 = Reserved
1 = Reserved
2 = Signature Creation Time
3 = Signature Expiration Time
4 = Exportable Certification
5 = Trust Signature
6 = Regular Expression
7 = Revocable
8 = Reserved
9 = Key Expiration Time
10 = Placeholder for backward compatibility
11 = Preferred Symmetric Algorithms
12 = Revocation Key
13 = Reserved
14 = Reserved
15 = Reserved
16 = Issuer
17 = Reserved
18 = Reserved
19 = Reserved
20 = Notation Data
21 = Preferred Hash Algorithms
22 = Preferred Compression Algorithms
23 = Key Server Preferences
24 = Preferred Key Server
25 = Primary User ID
26 = Policy URI
27 = Key Flags
28 = Signer's User ID
29 = Reason for Revocation
30 = Features
31 = Signature Target
32 = Embedded Signature
100 To 110 = Private or experimental
An implementation SHOULD ignore any subpacket of a type that it does An implementation SHOULD ignore any subpacket of a type that it does
not recognize. not recognize.
Bit 7 of the subpacket type is the "critical" bit. If set, it Bit 7 of the subpacket type is the "critical" bit. If set, it
denotes that the subpacket is one that is critical for the evaluator denotes that the subpacket is one that is critical for the evaluator
of the signature to recognize. If a subpacket is encountered that is of the signature to recognize. If a subpacket is encountered that is
marked critical but is unknown to the evaluating software, the marked critical but is unknown to the evaluating software, the
evaluator SHOULD consider the signature to be in error. evaluator SHOULD consider the signature to be in error.
An evaluator may "recognize" a subpacket, but not implement it. The An evaluator may "recognize" a subpacket, but not implement it. The
purpose of the critical bit is to allow the signer to tell an purpose of the critical bit is to allow the signer to tell an
evaluator that it would prefer a new, unknown feature to generate an evaluator that it would prefer a new, unknown feature to generate an
error than be ignored. error than be ignored.
Implementations SHOULD implement "preferences" and the "reason for Implementations SHOULD implement the three preferred algorithm
revocation" subpackets. Note, however, that if an implementation subpackets (11, 21, and 22), as well as the "Reason for Revocation"
chooses not to implement some of the preferences, it is required to subpacket. Note, however, that if an implementation chooses not to
behave in a polite manner to respect the wishes of those users who implement some of the preferences, it is required to behave in a
do implement these preferences. polite manner to respect the wishes of those users who do implement
these preferences.
5.2.3.2. Signature Subpacket Types 5.2.3.2. Signature Subpacket Types
A number of subpackets are currently defined. Some subpackets apply A number of subpackets are currently defined. Some subpackets apply
to the signature itself and some are attributes of the key. to the signature itself and some are attributes of the key.
Subpackets that are found on a self-signature are placed on a Subpackets that are found on a self-signature are placed on a
certification made by the key itself. Note that a key may have more certification made by the key itself. Note that a key may have more
than one User ID, and thus may have more than one self-signature, than one User ID, and thus may have more than one self-signature, and
and differing subpackets. differing subpackets.
A subpacket may be found either in the hashed or unhashed subpacket A subpacket may be found either in the hashed or unhashed subpacket
sections of a signature. If a subpacket is not hashed, then the sections of a signature. If a subpacket is not hashed, then the
information in it cannot be considered definitive because it is not information in it cannot be considered definitive because it is not
part of the signature proper. part of the signature proper.
5.2.3.3. Notes on Self-Signatures 5.2.3.3. Notes on Self-Signatures
A self-signature is a binding signature made by the key the A self-signature is a binding signature made by the key to which the
signature refers to. There are three types of self-signatures, the signature refers. There are three types of self-signatures, the
certification signatures (types 0x10-0x13), the direct-key signature certification signatures (types 0x10-0x13), the direct-key signature
(type 0x1f), and the subkey binding signature (type 0x18). For (type 0x1F), and the subkey binding signature (type 0x18). For
certification self-signatures, each User ID may have a certification self-signatures, each User ID may have a self-
self-signature, and thus different subpackets in those signature, and thus different subpackets in those self-signatures.
self-signatures. For subkey binding signatures, each subkey in fact For subkey binding signatures, each subkey in fact has a self-
has a self-signature. Subpackets that appear in a certification signature. Subpackets that appear in a certification self-signature
self-signature apply to the username, and subpackets that appear in apply to the user name, and subpackets that appear in the subkey
the subkey self-signature apply to the subkey. Lastly, subpackets on self-signature apply to the subkey. Lastly, subpackets on the
the direct-key signature apply to the entire key. direct-key signature apply to the entire key.
Implementing software should interpret a self-signature's preference Implementing software should interpret a self-signature's preference
subpackets as narrowly as possible. For example, suppose a key has subpackets as narrowly as possible. For example, suppose a key has
two usernames, Alice and Bob. Suppose that Alice prefers the two user names, Alice and Bob. Suppose that Alice prefers the
symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES. If the
software locates this key via Alice's name, then the preferred software locates this key via Alice's name, then the preferred
algorithm is CAST5, if software locates the key via Bob's name, then algorithm is CAST5; if software locates the key via Bob's name, then
the preferred algorithm is IDEA. If the key is located by key ID, the preferred algorithm is IDEA. If the key is located by Key ID,
the algorithm of the primary User ID of the key provides the the algorithm of the primary User ID of the key provides the
preferred symmetric algorithm. preferred symmetric algorithm.
Revoking a self-signature or allowing it to expire has a semantic Revoking a self-signature or allowing it to expire has a semantic
meaning that varies with the signature type. Revoking the meaning that varies with the signature type. Revoking the self-
self-signature on a User ID effectively retires that user name. The signature on a User ID effectively retires that user name. The
self-signature is a statement, "My name X is tied to my signing key self-signature is a statement, "My name X is tied to my signing key
K" and is corroborated by other users' certifications. If another K" and is corroborated by other users' certifications. If another
user revokes their certification, they are effectively saying that user revokes their certification, they are effectively saying that
they no longer believe that name and that key are tied together. they no longer believe that name and that key are tied together.
Similarly, if the user themselves revokes their self-signature, it Similarly, if the users themselves revoke their self-signature, then
means the user no longer goes by that name, no longer has that email the users no longer go by that name, no longer have that email
address, etc. Revoking a binding signature effectively retires that address, etc. Revoking a binding signature effectively retires that
subkey. Revoking a direct-key signature cancels that signature. subkey. Revoking a direct-key signature cancels that signature.
Please see the "Reason for Revocation" subpacket below for more Please see the "Reason for Revocation" subpacket (Section 5.2.3.23)
relevant detail. for more relevant detail.
Since a self-signature contains important information about the Since a self-signature contains important information about the key's
key's use, an implementation SHOULD allow the user to rewrite the use, an implementation SHOULD allow the user to rewrite the self-
self-signature, and important information in it, such as preferences signature, and important information in it, such as preferences and
and key expiration. key expiration.
It is good practice to verify that a self-signature imported into an It is good practice to verify that a self-signature imported into an
implementation doesn't advertise features that the implementation implementation doesn't advertise features that the implementation
doesn't support, rewriting the signature as appropriate. doesn't support, rewriting the signature as appropriate.
An implementation that encounters multiple self-signatures on the An implementation that encounters multiple self-signatures on the
same object may resolve the ambiguity in any way it sees fit, but it same object may resolve the ambiguity in any way it sees fit, but it
is RECOMMENDED that priority be given to the most recent is RECOMMENDED that priority be given to the most recent self-
self-signature. signature.
5.2.3.4. Signature creation time 5.2.3.4. Signature Creation Time
(4 octet time field) (4-octet time field)
The time the signature was made. The time the signature was made.
MUST be present in the hashed area. MUST be present in the hashed area.
5.2.3.5. Issuer 5.2.3.5. Issuer
(8 octet key ID) (8-octet Key ID)
The OpenPGP key ID of the key issuing the signature. The OpenPGP Key ID of the key issuing the signature.
5.2.3.6. Key expiration time 5.2.3.6. Key Expiration Time
(4 octet time field) (4-octet time field)
The validity period of the key. This is the number of seconds after The validity period of the key. This is the number of seconds after
the key creation time that the key expires. If this is not present the key creation time that the key expires. If this is not present
or has a value of zero, the key never expires. This is found only on or has a value of zero, the key never expires. This is found only on
a self-signature. a self-signature.
5.2.3.7. Preferred symmetric algorithms 5.2.3.7. Preferred Symmetric Algorithms
(array of one-octet values) (array of one-octet values)
Symmetric algorithm numbers that indicate which algorithms the key
holder prefers to use. The subpacket body is an ordered list of
octets with the most preferred listed first. It is assumed that only
algorithms listed are supported by the recipient's software.
Algorithm numbers are in section 9. This is only found on a
self-signature.
5.2.3.8. Preferred hash algorithms Symmetric algorithm numbers that indicate which algorithms the key
holder prefers to use. The subpacket body is an ordered list of
octets with the most preferred listed first. It is assumed that only
algorithms listed are supported by the recipient's software.
Algorithm numbers are in Section 9. This is only found on a self-
signature.
(array of one-octet values) 5.2.3.8. Preferred Hash Algorithms
Message digest algorithm numbers that indicate which algorithms the (array of one-octet values)
key holder prefers to receive. Like the preferred symmetric
algorithms, the list is ordered. Algorithm numbers are in section 9.
This is only found on a self-signature.
5.2.3.9. Preferred compression algorithms Message digest algorithm numbers that indicate which algorithms the
key holder prefers to receive. Like the preferred symmetric
algorithms, the list is ordered. Algorithm numbers are in Section 9.
This is only found on a self-signature.
(array of one-octet values) 5.2.3.9. Preferred Compression Algorithms
Compression algorithm numbers that indicate which algorithms the key (array of one-octet values)
holder prefers to use. Like the preferred symmetric algorithms, the
list is ordered. Algorithm numbers are in section 9. If this
subpacket is not included, ZIP is preferred. A zero denotes that
uncompressed data is preferred; the key holder's software might have
no compression software in that implementation. This is only found
on a self-signature.
5.2.3.10. Signature expiration time Compression algorithm numbers that indicate which algorithms the key
holder prefers to use. Like the preferred symmetric algorithms, the
list is ordered. Algorithm numbers are in Section 9. If this
subpacket is not included, ZIP is preferred. A zero denotes that
uncompressed data is preferred; the key holder's software might have
no compression software in that implementation. This is only found
on a self-signature.
(4 octet time field) 5.2.3.10. Signature Expiration Time
The validity period of the signature. This is the number of seconds (4-octet time field)
after the signature creation time that the signature expires. If
this is not present or has a value of zero, it never expires.
5.2.3.11. Exportable Certification The validity period of the signature. This is the number of seconds
after the signature creation time that the signature expires. If
this is not present or has a value of zero, it never expires.
(1 octet of exportability, 0 for not, 1 for exportable) 5.2.3.11. Exportable Certification
This subpacket denotes whether a certification signature is (1 octet of exportability, 0 for not, 1 for exportable)
"exportable," to be used by other users than the signature's issuer.
The packet body contains a Boolean flag indicating whether the
signature is exportable. If this packet is not present, the
certification is exportable; it is equivalent to a flag containing a
1.
Non-exportable, or "local," certifications are signatures made by a This subpacket denotes whether a certification signature is
user to mark a key as valid within that user's implementation only. "exportable", to be used by other users than the signature's issuer.
Thus, when an implementation prepares a user's copy of a key for The packet body contains a Boolean flag indicating whether the
transport to another user (this is the process of "exporting" the signature is exportable. If this packet is not present, the
key), any local certification signatures are deleted from the key. certification is exportable; it is equivalent to a flag containing a
1.
The receiver of a transported key "imports" it, and likewise trims Non-exportable, or "local", certifications are signatures made by a
any local certifications. In normal operation, there won't be any, user to mark a key as valid within that user's implementation only.
assuming the import is performed on an exported key. However, there
are instances where this can reasonably happen. For example, if an
implementation allows keys to be imported from a key database in
addition to an exported key, then this situation can arise.
Some implementations do not represent the interest of a single user Thus, when an implementation prepares a user's copy of a key for
(for example, a key server). Such implementations always trim local transport to another user (this is the process of "exporting" the
certifications from any key they handle. key), any local certification signatures are deleted from the key.
5.2.3.12. Revocable The receiver of a transported key "imports" it, and likewise trims
any local certifications. In normal operation, there won't be any,
assuming the import is performed on an exported key. However, there
are instances where this can reasonably happen. For example, if an
implementation allows keys to be imported from a key database in
addition to an exported key, then this situation can arise.
(1 octet of revocability, 0 for not, 1 for revocable) Some implementations do not represent the interest of a single user
(for example, a key server). Such implementations always trim local
certifications from any key they handle.
Signature's revocability status. The packet body contains a Boolean 5.2.3.12. Revocable
flag indicating whether the signature is revocable. Signatures that
are not revocable have any later revocation signatures ignored. They
represent a commitment by the signer that he cannot revoke his
signature for the life of his key. If this packet is not present,
the signature is revocable.
5.2.3.13. Trust signature (1 octet of revocability, 0 for not, 1 for revocable)
(1 octet "level" (depth), 1 octet of trust amount) Signature's revocability status. The packet body contains a Boolean
flag indicating whether the signature is revocable. Signatures that
are not revocable have any later revocation signatures ignored. They
represent a commitment by the signer that he cannot revoke his
signature for the life of his key. If this packet is not present,
the signature is revocable.
Signer asserts that the key is not only valid, but also trustworthy, 5.2.3.13. Trust Signature
at the specified level. Level 0 has the same meaning as an ordinary
validity signature. Level 1 means that the signed key is asserted to
be a valid trusted introducer, with the 2nd octet of the body
specifying the degree of trust. Level 2 means that the signed key is
asserted to be trusted to issue level 1 trust signatures, i.e. that
it is a "meta introducer". Generally, a level n trust signature
asserts that a key is trusted to issue level n-1 trust signatures.
The trust amount is in a range from 0-255, interpreted such that
values less than 120 indicate partial trust and values of 120 or
greater indicate complete trust. Implementations SHOULD emit values
of 60 for partial trust and 120 for complete trust.
5.2.3.14. Regular expression (1 octet "level" (depth), 1 octet of trust amount)
(null-terminated regular expression) Signer asserts that the key is not only valid but also trustworthy at
the specified level. Level 0 has the same meaning as an ordinary
validity signature. Level 1 means that the signed key is asserted to
be a valid trusted introducer, with the 2nd octet of the body
specifying the degree of trust. Level 2 means that the signed key is
asserted to be trusted to issue level 1 trust signatures, i.e., that
it is a "meta introducer". Generally, a level n trust signature
asserts that a key is trusted to issue level n-1 trust signatures.
The trust amount is in a range from 0-255, interpreted such that
values less than 120 indicate partial trust and values of 120 or
greater indicate complete trust. Implementations SHOULD emit values
of 60 for partial trust and 120 for complete trust.
Used in conjunction with trust signature packets (of level > 0) to 5.2.3.14. Regular Expression
limit the scope of trust that is extended. Only signatures by the
target key on User IDs that match the regular expression in the body
of this packet have trust extended by the trust signature subpacket.
The regular expression uses the same syntax as the Henry Spencer's
"almost public domain" regular expression package. A description of
the syntax is found in a section below.
5.2.3.15. Revocation key (null-terminated regular expression)
(1 octet of class, 1 octet of PK algorithm ID, 20 octets of Used in conjunction with trust Signature packets (of level > 0) to
fingerprint) limit the scope of trust that is extended. Only signatures by the
target key on User IDs that match the regular expression in the body
of this packet have trust extended by the trust Signature subpacket.
The regular expression uses the same syntax as the Henry Spencer's
"almost public domain" regular expression [REGEX] package. A
description of the syntax is found in Section 8 below.
Authorizes the specified key to issue revocation signatures for this 5.2.3.15. Revocation Key
key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
then this means that the revocation information is sensitive. Other
bits are for future expansion to other kinds of authorizations. This
is found on a self-signature.
If the "sensitive" flag is set, the keyholder feels this subpacket (1 octet of class, 1 octet of public-key algorithm ID, 20 octets of
contains private trust information that describes a real-world fingerprint)
sensitive relationship. If this flag is set, implementations SHOULD
NOT export this signature to other users except in cases where the
data needs to be available: when the signature is being sent to the
designated revoker, or when it is accompanied by a revocation
signature from that revoker. Note that it may be appropriate to
isolate this subpacket within a separate signature so that it is not
combined with other subpackets that need to be exported.
5.2.3.16. Notation Data Authorizes the specified key to issue revocation signatures for this
key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
then this means that the revocation information is sensitive. Other
bits are for future expansion to other kinds of authorizations. This
is found on a self-signature.
(4 octets of flags, 2 octets of name length (M), If the "sensitive" flag is set, the keyholder feels this subpacket
2 octets of value length (N), contains private trust information that describes a real-world
M octets of name data, sensitive relationship. If this flag is set, implementations SHOULD
N octets of value data) NOT export this signature to other users except in cases where the
data needs to be available: when the signature is being sent to the
designated revoker, or when it is accompanied by a revocation
signature from that revoker. Note that it may be appropriate to
isolate this subpacket within a separate signature so that it is not
combined with other subpackets that need to be exported.
This subpacket describes a "notation" on the signature that the 5.2.3.16. Notation Data
issuer wishes to make. The notation has a name and a value, each of
which are strings of octets. There may be more than one notation in
a signature. Notations can be used for any extension the issuer of
the signature cares to make. The "flags" field holds four octets of
flags.
All undefined flags MUST be zero. Defined flags are: (4 octets of flags, 2 octets of name length (M),
2 octets of value length (N),
M octets of name data,
N octets of value data)
First octet: 0x80 = human-readable. This note value is text. This subpacket describes a "notation" on the signature that the
Other octets: none. issuer wishes to make. The notation has a name and a value, each of
which are strings of octets. There may be more than one notation in
a signature. Notations can be used for any extension the issuer of
the signature cares to make. The "flags" field holds four octets of
flags.
Notation names are arbitrary strings encoded in UTF-8. They reside All undefined flags MUST be zero. Defined flags are as follows:
two name spaces: The IETF name space and the user name space.
The IETF name space is registered with IANA. These names MUST NOT First octet: 0x80 = human-readable. This note value is text.
contain the "@" character (0x40). This this is a tag for the user Other octets: none.
name space.
Names in the user name space consist of a UTF-8 string tag followed Notation names are arbitrary strings encoded in UTF-8. They reside
by "@" followed by a DNS domain name. Note that the tag MUST NOT in two namespaces: The IETF namespace and the user namespace.
contain an "@" character. For example, the "sample" tag used by
Example Corporation could be "sample@example.com".
Names in a user space are owned and controlled by the owners of that The IETF namespace is registered with IANA. These names MUST NOT
domain. Obviously, it's of bad form to create a new name in a DNS contain the "@" character (0x40). This is a tag for the user
space that you don't own. namespace.
Since the user name space is in the form of an email address, Names in the user namespace consist of a UTF-8 string tag followed by
implementers MAY wish to arrange for that address to reach a person "@" followed by a DNS domain name. Note that the tag MUST NOT
who can be consulted about the use of the named tag. Note that due contain an "@" character. For example, the "sample" tag used by
to UTF-8 encoding, not all valid user space name tags are valid Example Corporation could be "sample@example.com".
email addresses.
If there is a critical notation, the criticality applies to that Names in a user space are owned and controlled by the owners of that
specific notation and not to notations in general. domain. Obviously, it's bad form to create a new name in a DNS space
that you don't own.
5.2.3.17. Key server preferences Since the user namespace is in the form of an email address,
implementers MAY wish to arrange for that address to reach a person
who can be consulted about the use of the named tag. Note that due
to UTF-8 encoding, not all valid user space name tags are valid email
addresses.
(N octets of flags) If there is a critical notation, the criticality applies to that
specific notation and not to notations in general.
This is a list of one-bit flags that indicate preferences that the 5.2.3.17. Key Server Preferences
key holder has about how the key is handled on a key server. All
undefined flags MUST be zero.
First octet: 0x80 = No-modify (N octets of flags)
the key holder requests that this key only be modified or
updated by the key holder or an administrator of the key server.
This is found only on a self-signature. This is a list of one-bit flags that indicate preferences that the
key holder has about how the key is handled on a key server. All
undefined flags MUST be zero.
5.2.3.18. Preferred key server First octet: 0x80 = No-modify
the key holder requests that this key only be modified or updated
by the key holder or an administrator of the key server.
(String) This is found only on a self-signature.
This is a URI of a key server that the key holder prefers be used 5.2.3.18. Preferred Key Server
for updates. Note that keys with multiple User IDs can have a
preferred key server for each User ID. Note also that since this is
a URI, the key server can actually be a copy of the key retrieved by
ftp, http, finger, etc.
5.2.3.19. Primary User ID (String)
(1 octet, Boolean) This is a URI of a key server that the key holder prefers be used for
updates. Note that keys with multiple User IDs can have a preferred
key server for each User ID. Note also that since this is a URI, the
key server can actually be a copy of the key retrieved by ftp, http,
finger, etc.
This is a flag in a User ID's self signature that states whether 5.2.3.19. Primary User ID
this User ID is the main User ID for this key. It is reasonable for
an implementation to resolve ambiguities in preferences, etc. by
referring to the primary User ID. If this flag is absent, its value
is zero. If more than one User ID in a key is marked as primary, the
implementation may resolve the ambiguity in any way it sees fit, but
it is RECOMMENDED that priority be given to the User ID with the
most recent self-signature.
When appearing on a self-signature on a User ID packet, this (1 octet, Boolean)
subpacket applies only to User ID packets. When appearing on a
self-signature on a User Attribute packet, this subpacket applies
only to User Attribute packets. That is to say, there are two
different and independent "primaries" - one for User IDs, and one
for User Attributes.
5.2.3.20. Policy URI This is a flag in a User ID's self-signature that states whether this
User ID is the main User ID for this key. It is reasonable for an
implementation to resolve ambiguities in preferences, etc. by
referring to the primary User ID. If this flag is absent, its value
is zero. If more than one User ID in a key is marked as primary, the
implementation may resolve the ambiguity in any way it sees fit, but
it is RECOMMENDED that priority be given to the User ID with the most
recent self-signature.
(String) When appearing on a self-signature on a User ID packet, this
subpacket applies only to User ID packets. When appearing on a
self-signature on a User Attribute packet, this subpacket applies
only to User Attribute packets. That is to say, there are two
different and independent "primaries" -- one for User IDs, and one
for User Attributes.
This subpacket contains a URI of a document that describes the 5.2.3.20. Policy URI
policy that the signature was issued under.
5.2.3.21. Key Flags (String)
(N octets of flags) This subpacket contains a URI of a document that describes the policy
under which the signature was issued.
This subpacket contains a list of binary flags that hold information 5.2.3.21. Key Flags
about a key. It is a string of octets, and an implementation MUST
NOT assume a fixed size. This is so it can grow over time. If a list
is shorter than an implementation expects, the unstated flags are
considered to be zero. The defined flags are:
First octet: (N octets of flags)
0x01 - This key may be used to certify other keys. This subpacket contains a list of binary flags that hold information
about a key. It is a string of octets, and an implementation MUST
NOT assume a fixed size. This is so it can grow over time. If a
list is shorter than an implementation expects, the unstated flags
are considered to be zero. The defined flags are as follows:
0x02 - This key may be used to sign data. First octet:
0x04 - This key may be used to encrypt communications. 0x01 - This key may be used to certify other keys.
0x08 - This key may be used to encrypt storage. 0x02 - This key may be used to sign data.
0x10 - The private component of this key may have been split by 0x04 - This key may be used to encrypt communications.
a secret-sharing mechanism.
0x20 - This key may be used for authentication. 0x08 - This key may be used to encrypt storage.
0x80 - The private component of this key may be in the 0x10 - The private component of this key may have been split
possession of more than one person. by a secret-sharing mechanism.
Usage notes: 0x20 - This key may be used for authentication.
The flags in this packet may appear in self-signatures or in 0x80 - The private component of this key may be in the
certification signatures. They mean different things depending on possession of more than one person.
who is making the statement -- for example, a certification
signature that has the "sign data" flag is stating that the
certification is for that use. On the other hand, the
"communications encryption" flag in a self-signature is stating a
preference that a given key be used for communications. Note
however, that it is a thorny issue to determine what is
"communications" and what is "storage." This decision is left wholly
up to the implementation; the authors of this document do not claim
any special wisdom on the issue, and realize that accepted opinion
may change.
The "split key" (0x10) and "group key" (0x80) flags are placed on a Usage notes:
self-signature only; they are meaningless on a certification
signature. They SHOULD be placed only on a direct-key signature
(type 0x1f) or a subkey signature (type 0x18), one that refers to
the key the flag applies to.
5.2.3.22. Signer's User ID The flags in this packet may appear in self-signatures or in
certification signatures. They mean different things depending on
who is making the statement -- for example, a certification signature
that has the "sign data" flag is stating that the certification is
for that use. On the other hand, the "communications encryption"
flag in a self-signature is stating a preference that a given key be
used for communications. Note however, that it is a thorny issue to
determine what is "communications" and what is "storage". This
decision is left wholly up to the implementation; the authors of this
document do not claim any special wisdom on the issue and realize
that accepted opinion may change.
(String) The "split key" (0x10) and "group key" (0x80) flags are placed on a
self-signature only; they are meaningless on a certification
signature. They SHOULD be placed only on a direct-key signature
(type 0x1F) or a subkey signature (type 0x18), one that refers to the
key the flag applies to.
This subpacket allows a keyholder to state which User ID is 5.2.3.22. Signer's User ID
responsible for the signing. Many keyholders use a single key for
different purposes, such as business communications as well as
personal communications. This subpacket allows such a keyholder to
state which of their roles is making a signature.
This subpacket is not appropriate to use to refer to a User (String)
Attribute packet.
5.2.3.23. Reason for Revocation This subpacket allows a keyholder to state which User ID is
responsible for the signing. Many keyholders use a single key for
different purposes, such as business communications as well as
personal communications. This subpacket allows such a keyholder to
state which of their roles is making a signature.
(1 octet of revocation code, N octets of reason string) This subpacket is not appropriate to use to refer to a User Attribute
packet.
This subpacket is used only in key revocation and certification 5.2.3.23. Reason for Revocation
revocation signatures. It describes the reason why the key or
certificate was revoked.
The first octet contains a machine-readable code that denotes the (1 octet of revocation code, N octets of reason string)
reason for the revocation:
This subpacket is used only in key revocation and certification
revocation signatures. It describes the reason why the key or
certificate was revoked.
The first octet contains a machine-readable code that denotes the
reason for the revocation:
0 - No reason specified (key revocations or cert revocations) 0 - No reason specified (key revocations or cert revocations)
1 - Key is superseded (key revocations) 1 - Key is superseded (key revocations)
2 - Key material has been compromised (key revocations) 2 - Key material has been compromised (key revocations)
3 - Key is retired and no longer used (key revocations) 3 - Key is retired and no longer used (key revocations)
32 - User ID information is no longer valid (cert revocations) 32 - User ID information is no longer valid (cert revocations)
100-110 - Private Use
Following the revocation code is a string of octets which gives Following the revocation code is a string of octets that gives
information about the reason for revocation in human-readable form information about the Reason for Revocation in human-readable form
(UTF-8). The string may be null, that is, of zero length. The length (UTF-8). The string may be null, that is, of zero length. The
of the subpacket is the length of the reason string plus one. length of the subpacket is the length of the reason string plus one.
An implementation SHOULD implement this subpacket, include it in all
An implementation SHOULD implement this subpacket, include it in all revocation signatures, and interpret revocations appropriately.
revocation signatures, and interpret revocations appropriately. There are important semantic differences between the reasons, and
There are important semantic differences between the reasons, and there are thus important reasons for revoking signatures.
there are thus important reasons for revoking signatures.
If a key has been revoked because of a compromise, all signatures If a key has been revoked because of a compromise, all signatures
created by that key are suspect. However, if it was merely created by that key are suspect. However, if it was merely
superseded or retired, old signatures are still valid. If the superseded or retired, old signatures are still valid. If the
revoked signature is the self-signature for certifying a User ID, a revoked signature is the self-signature for certifying a User ID, a
revocation denotes that that user name is no longer in use. Such a revocation denotes that that user name is no longer in use. Such a
revocation SHOULD include an 0x20 code. revocation SHOULD include a 0x20 code.
Note that any signature may be revoked, including a certification on Note that any signature may be revoked, including a certification on
some other person's key. There are many good reasons for revoking a some other person's key. There are many good reasons for revoking a
certification signature, such as the case where the keyholder leaves certification signature, such as the case where the keyholder leaves
the employ of a business with an email address. A revoked the employ of a business with an email address. A revoked
certification is no longer a part of validity calculations. certification is no longer a part of validity calculations.
5.2.3.24. Features 5.2.3.24. Features
(N octets of flags) (N octets of flags)
The features subpacket denotes which advanced OpenPGP features a The Features subpacket denotes which advanced OpenPGP features a
user's implementation supports. This is so that as features are user's implementation supports. This is so that as features are
added to OpenPGP that cannot be backwards-compatible, a user can added to OpenPGP that cannot be backwards-compatible, a user can
state that they can use that feature. The flags are single bits that state that they can use that feature. The flags are single bits that
indicate that a given feature is supported. indicate that a given feature is supported.
This subpacket is similar to a preferences subpacket, and only This subpacket is similar to a preferences subpacket, and only
appears in a self-signature. appears in a self-signature.
An implementation SHOULD NOT use a feature listed when sending to a An implementation SHOULD NOT use a feature listed when sending to a
user who does not state that they can use it. user who does not state that they can use it.
Defined features are: Defined features are as follows:
First octet: First octet:
0x01 - Modification Detection (packets 18 and 19) 0x01 - Modification Detection (packets 18 and 19)
If an implementation implements any of the defined features, it If an implementation implements any of the defined features, it
SHOULD implement the features subpacket, too. SHOULD implement the Features subpacket, too.
An implementation may freely infer features from other suitable An implementation may freely infer features from other suitable
implementation-dependent mechanisms. implementation-dependent mechanisms.
5.2.3.25. Signature Target 5.2.3.25. Signature Target
(1 octet PK algorithm, 1 octet hash algorithm, N octets hash) (1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)
This subpacket identifies a specific target signature that a This subpacket identifies a specific target signature to which a
signature refers to. For revocation signatures, this subpacket signature refers. For revocation signatures, this subpacket
provides explicit designation of which signature is being revoked. provides explicit designation of which signature is being revoked.
For a third-party or timestamp signature, this designates what For a third-party or timestamp signature, this designates what
signature is signed. All arguments are an identifier of that target signature is signed. All arguments are an identifier of that target
signature. signature.
The N octets of hash data MUST be the size of the hash of the The N octets of hash data MUST be the size of the hash of the
signature. For example, a target signature with a SHA-1 hash MUST signature. For example, a target signature with a SHA-1 hash MUST
have 20 octets of hash data. have 20 octets of hash data.
5.2.3.26. Embedded Signature 5.2.3.26. Embedded Signature
(1 signature packet body) (1 signature packet body)
This subpacket contains a complete signature packet body as This subpacket contains a complete Signature packet body as
specified in section 5.2 above. It is useful when one signature specified in Section 5.2 above. It is useful when one signature
needs to refer to, or be incorporated in, another signature. needs to refer to, or be incorporated in, another signature.
5.2.4. Computing Signatures 5.2.4. Computing Signatures
All signatures are formed by producing a hash over the signature All signatures are formed by producing a hash over the signature
data, and then using the resulting hash in the signature algorithm. data, and then using the resulting hash in the signature algorithm.
For binary document signatures (type 0x00), the document data is For binary document signatures (type 0x00), the document data is
hashed directly. For text document signatures (type 0x01), the hashed directly. For text document signatures (type 0x01), the
document is canonicalized by converting line endings to <CR><LF>, document is canonicalized by converting line endings to <CR><LF>,
and the resulting data is hashed. and the resulting data is hashed.
When a signature is made over a key, the hash data starts with the When a signature is made over a key, the hash data starts with the
octet 0x99, followed by a two-octet length of the key, and then body octet 0x99, followed by a two-octet length of the key, and then body
of the key packet. (Note that this is an old-style packet header for of the key packet. (Note that this is an old-style packet header for
a key packet with two-octet length.) A subkey binding signature a key packet with two-octet length.) A subkey binding signature
(type 0x18) or primary key binding signature (type 0x19) then hashes (type 0x18) or primary key binding signature (type 0x19) then hashes
the subkey using the same format as the main key (also using 0x99 as the subkey using the same format as the main key (also using 0x99 as
the first octet). Key revocation signatures (types 0x20 and 0x28) the first octet). Key revocation signatures (types 0x20 and 0x28)
hash only the key being revoked. hash only the key being revoked.
A certification signature (type 0x10 through 0x13) hashes the User A certification signature (type 0x10 through 0x13) hashes the User
ID being bound to the key into the hash context after the above ID being bound to the key into the hash context after the above
data. A V3 certification hashes the contents of the User ID or data. A V3 certification hashes the contents of the User ID or
attribute packet packet, without any header. A V4 certification attribute packet packet, without any header. A V4 certification
hashes the constant 0xb4 for User ID certifications or the constant hashes the constant 0xB4 for User ID certifications or the constant
0xd1 for User Attribute certifications, followed by a four-octet 0xD1 for User Attribute certifications, followed by a four-octet
number giving the length of the User ID or User Attribute data, and number giving the length of the User ID or User Attribute data, and
then the User ID or User Attribute data. then the User ID or User Attribute data.
When a signature is made over a signature packet (type 0x50), the When a signature is made over a Signature packet (type 0x50), the
hash data starts with the octet 0x88, followed by the four-octet hash data starts with the octet 0x88, followed by the four-octet
length of the signature, and then the body of the signature packet. length of the signature, and then the body of the Signature packet.
(Note that this is an old-style packet header for a signature packet (Note that this is an old-style packet header for a Signature packet
with the length-of-length set to zero). The unhashed subpacket data with the length-of-length set to zero.) The unhashed subpacket data
of the signature packet being hashed is not included in the hash and of the Signature packet being hashed is not included in the hash, and
the unhashed subpacket data length value is set to zero. the unhashed subpacket data length value is set to zero.
Once the data body is hashed, then a trailer is hashed. A V3 Once the data body is hashed, then a trailer is hashed. A V3
signature hashes five octets of the packet body, starting from the signature hashes five octets of the packet body, starting from the
signature type field. This data is the signature type, followed by signature type field. This data is the signature type, followed by
the four-octet signature time. A V4 signature hashes the packet body the four-octet signature time. A V4 signature hashes the packet body
starting from its first field, the version number, through the end starting from its first field, the version number, through the end
of the hashed subpacket data. Thus, the fields hashed are the of the hashed subpacket data. Thus, the fields hashed are the
signature version, the signature type, the public key algorithm, the signature version, the signature type, the public-key algorithm, the
hash algorithm, the hashed subpacket length, and the hashed hash algorithm, the hashed subpacket length, and the hashed
subpacket body. subpacket body.
V4 signatures also hash in a final trailer of six octets: the V4 signatures also hash in a final trailer of six octets: the
version of the signature packet, i.e. 0x04; 0xFF; a four-octet, version of the Signature packet, i.e., 0x04; 0xFF; and a four-octet,
big-endian number that is the length of the hashed data from the big-endian number that is the length of the hashed data from the
signature packet (note that this number does not include these final Signature packet (note that this number does not include these final
six octets. six octets).
After all this has been hashed in a single hash context the After all this has been hashed in a single hash context, the
resulting hash field is used in the signature algorithm, and placed resulting hash field is used in the signature algorithm and placed
at the end of the signature packet. at the end of the Signature packet.
5.2.4.1. Subpacket Hints 5.2.4.1. Subpacket Hints
It is certainly possible for a signature to contain conflicting It is certainly possible for a signature to contain conflicting
information in subpackets. For example, a signature may contain information in subpackets. For example, a signature may contain
multiple copies of a preference or multiple expiration times. In multiple copies of a preference or multiple expiration times. In
most cases, an implementation SHOULD use the last subpacket in the most cases, an implementation SHOULD use the last subpacket in the
signature, but MAY use any conflict resolution scheme that makes signature, but MAY use any conflict resolution scheme that makes
more sense. Please note that we are intentionally leaving conflict more sense. Please note that we are intentionally leaving conflict
resolution to the implementer; most conflicts are simply syntax resolution to the implementer; most conflicts are simply syntax
errors, and the wishy-washy language here allows a receiver to be errors, and the wishy-washy language here allows a receiver to be
generous in what they accept, while putting pressure on a creator to generous in what they accept, while putting pressure on a creator to
be stingy in what they generate. be stingy in what they generate.
Some apparent conflicts may actually make sense -- for example, Some apparent conflicts may actually make sense -- for example,
suppose a keyholder has an V3 key and a V4 key that share the same suppose a keyholder has a V3 key and a V4 key that share the same
RSA key material. Either of these keys can verify a signature RSA key material. Either of these keys can verify a signature
created by the other, and it may be reasonable for a signature to created by the other, and it may be reasonable for a signature to
contain an issuer subpacket for each key, as a way of explicitly contain an issuer subpacket for each key, as a way of explicitly
tying those keys to the signature. tying those keys to the signature.
5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) 5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3)
The Symmetric-Key Encrypted Session Key packet holds the The Symmetric-Key Encrypted Session Key packet holds the
symmetric-key encryption of a session key used to encrypt a message. symmetric-key encryption of a session key used to encrypt a message.
Zero or more Public-Key Encrypted Session Key packets and/or Zero or more Public-Key Encrypted Session Key packets and/or
Symmetric-Key Encrypted Session Key packets may precede a Symmetric-Key Encrypted Session Key packets may precede a
Symmetrically Encrypted Data Packet that holds an encrypted message. Symmetrically Encrypted Data packet that holds an encrypted message.
The message is encrypted with a session key, and the session key is The message is encrypted with a session key, and the session key is
itself encrypted and stored in the Encrypted Session Key packet or itself encrypted and stored in the Encrypted Session Key packet or
the Symmetric-Key Encrypted Session Key packet. the Symmetric-Key Encrypted Session Key packet.
If the Symmetrically Encrypted Data Packet is preceded by one or If the Symmetrically Encrypted Data packet is preceded by one or
more Symmetric-Key Encrypted Session Key packets, each specifies a more Symmetric-Key Encrypted Session Key packets, each specifies a
passphrase that may be used to decrypt the message. This allows a passphrase that may be used to decrypt the message. This allows a
message to be encrypted to a number of public keys, and also to one message to be encrypted to a number of public keys, and also to one
or more passphrases. This packet type is new, and is not generated or more passphrases. This packet type is new and is not generated
by PGP 2.x or PGP 5.0. by PGP 2.x or PGP 5.0.
The body of this packet consists of: The body of this packet consists of:
- A one-octet version number. The only currently defined version - A one-octet version number. The only currently defined version
is 4. is 4.
- A one-octet number describing the symmetric algorithm used. - A one-octet number describing the symmetric algorithm used.
- A string-to-key (S2K) specifier, length as defined above. - A string-to-key (S2K) specifier, length as defined above.
- Optionally, the encrypted session key itself, which is decrypted - Optionally, the encrypted session key itself, which is decrypted
with the string-to-key object. with the string-to-key object.
If the encrypted session key is not present (which can be detected If the encrypted session key is not present (which can be detected
on the basis of packet length and S2K specifier size), then the S2K on the basis of packet length and S2K specifier size), then the S2K
algorithm applied to the passphrase produces the session key for algorithm applied to the passphrase produces the session key for
decrypting the file, using the symmetric cipher algorithm from the decrypting the file, using the symmetric cipher algorithm from the
Symmetric-Key Encrypted Session Key packet. Symmetric-Key Encrypted Session Key packet.
If the encrypted session key is present, the result of applying the If the encrypted session key is present, the result of applying the
S2K algorithm to the passphrase is used to decrypt just that S2K algorithm to the passphrase is used to decrypt just that
encrypted session key field, using CFB mode with an IV of all zeros. encrypted session key field, using CFB mode with an IV of all zeros.
The decryption result consists of a one-octet algorithm identifier The decryption result consists of a one-octet algorithm identifier
that specifies the symmetric-key encryption algorithm used to that specifies the symmetric-key encryption algorithm used to
encrypt the following Symmetrically Encrypted Data Packet, followed encrypt the following Symmetrically Encrypted Data packet, followed
by the session key octets themselves. by the session key octets themselves.
Note: because an all-zero IV is used for this decryption, the S2K Note: because an all-zero IV is used for this decryption, the S2K
specifier MUST use a salt value, either a Salted S2K or an specifier MUST use a salt value, either a Salted S2K or an
Iterated-Salted S2K. The salt value will insure that the decryption Iterated-Salted S2K. The salt value will ensure that the decryption
key is not repeated even if the passphrase is reused. key is not repeated even if the passphrase is reused.
5.4. One-Pass Signature Packets (Tag 4) 5.4. One-Pass Signature Packets (Tag 4)
The One-Pass Signature packet precedes the signed data and contains The One-Pass Signature packet precedes the signed data and contains
enough information to allow the receiver to begin calculating any enough information to allow the receiver to begin calculating any
hashes needed to verify the signature. It allows the Signature hashes needed to verify the signature. It allows the Signature
Packet to be placed at the end of the message, so that the signer packet to be placed at the end of the message, so that the signer
can compute the entire signed message in one pass. can compute the entire signed message in one pass.
A One-Pass Signature does not interoperate with PGP 2.6.x or A One-Pass Signature does not interoperate with PGP 2.6.x or
earlier. earlier.
The body of this packet consists of: The body of this packet consists of:
- A one-octet version number. The current version is 3. - A one-octet version number. The current version is 3.
- A one-octet signature type. Signature types are described in - A one-octet signature type. Signature types are described in
section 5.2.1. Section 5.2.1.
- A one-octet number describing the hash algorithm used. - A one-octet number describing the hash algorithm used.
- A one-octet number describing the public key algorithm used. - A one-octet number describing the public-key algorithm used.
- An eight-octet number holding the key ID of the signing key. - An eight-octet number holding the Key ID of the signing key.
- A one-octet number holding a flag showing whether the signature - A one-octet number holding a flag showing whether the signature
is nested. A zero value indicates that the next packet is is nested. A zero value indicates that the next packet is
another One-Pass Signature packet that describes another another One-Pass Signature packet that describes another
signature to be applied to the same message data. signature to be applied to the same message data.
Note that if a message contains more than one one-pass signature, Note that if a message contains more than one one-pass signature,
then the signature packets bracket the message; that is, the first then the Signature packets bracket the message; that is, the first
signature packet after the message corresponds to the last one-pass Signature packet after the message corresponds to the last one-pass
packet and the final signature packet corresponds to the first packet and the final Signature packet corresponds to the first
one-pass packet. one-pass packet.
5.5. Key Material Packet 5.5. Key Material Packet
A key material packet contains all the information about a public or A key material packet contains all the information about a public or
private key. There are four variants of this packet type, and two private key. There are four variants of this packet type, and two
major versions. Consequently, this section is complex. major versions. Consequently, this section is complex.
5.5.1. Key Packet Variants 5.5.1. Key Packet Variants
5.5.1.1. Public Key Packet (Tag 6) 5.5.1.1. Public-Key Packet (Tag 6)
A Public Key packet starts a series of packets that forms an OpenPGP A Public-Key packet starts a series of packets that forms an OpenPGP
key (sometimes called an OpenPGP certificate). key (sometimes called an OpenPGP certificate).
5.5.1.2. Public Subkey Packet (Tag 14) 5.5.1.2. Public-Subkey Packet (Tag 14)
A Public Subkey packet (tag 14) has exactly the same format as a A Public-Subkey packet (tag 14) has exactly the same format as a
Public Key packet, but denotes a subkey. One or more subkeys may be Public-Key packet, but denotes a subkey. One or more subkeys may be
associated with a top-level key. By convention, the top-level key associated with a top-level key. By convention, the top-level key
provides signature services, and the subkeys provide encryption provides signature services, and the subkeys provide encryption
services. services.
Note: in PGP 2.6.x, tag 14 was intended to indicate a comment Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
packet. This tag was selected for reuse because no previous version packet. This tag was selected for reuse because no previous version
of PGP ever emitted comment packets but they did properly ignore of PGP ever emitted comment packets but they did properly ignore
them. Public Subkey packets are ignored by PGP 2.6.x and do not them. Public-Subkey packets are ignored by PGP 2.6.x and do not
cause it to fail, providing a limited degree of backward cause it to fail, providing a limited degree of backward
compatibility. compatibility.
5.5.1.3. Secret Key Packet (Tag 5) 5.5.1.3. Secret-Key Packet (Tag 5)
A Secret Key packet contains all the information that is found in a A Secret-Key packet contains all the information that is found in a
Public Key packet, including the public key material, but also Public-Key packet, including the public-key material, but also
includes the secret key material after all the public key fields. includes the secret-key material after all the public-key fields.
5.5.1.4. Secret Subkey Packet (Tag 7) 5.5.1.4. Secret-Subkey Packet (Tag 7)
A Secret Subkey packet (tag 7) is the subkey analog of the Secret A Secret-Subkey packet (tag 7) is the subkey analog of the Secret
Key packet, and has exactly the same format. Key packet and has exactly the same format.
5.5.2. Public Key Packet Formats 5.5.2. Public-Key Packet Formats
There are two versions of key-material packets. Version 3 packets There are two versions of key-material packets. Version 3 packets
were first generated by PGP 2.6. Version 4 keys first appeared in were first generated by PGP 2.6. Version 4 keys first appeared in
PGP 5.0, and are the preferred key version for OpenPGP. PGP 5.0 and are the preferred key version for OpenPGP.
OpenPGP implementations MUST create keys with version 4 format. V3 OpenPGP implementations MUST create keys with version 4 format. V3
keys are deprecated; an implementation MUST NOT generate a V3 key, keys are deprecated; an implementation MUST NOT generate a V3 key,
but MAY accept it. but MAY accept it.
A version 3 public key or public subkey packet contains: A version 3 public key or public-subkey packet contains:
- A one-octet version number (3). - A one-octet version number (3).
- A four-octet number denoting the time that the key was created. - A four-octet number denoting the time that the key was created.
- A two-octet number denoting the time in days that this key is - A two-octet number denoting the time in days that this key is
valid. If this number is zero, then it does not expire. valid. If this number is zero, then it does not expire.
- A one-octet number denoting the public key algorithm of this key - A one-octet number denoting the public-key algorithm of this key.
- A series of multiprecision integers comprising the key material: - A series of multiprecision integers comprising the key material:
- a multiprecision integer (MPI) of RSA public modulus n; - a multiprecision integer (MPI) of RSA public modulus n;
- an MPI of RSA public encryption exponent e. - an MPI of RSA public encryption exponent e.
V3 keys are deprecated. They contain three weaknesses in them. V3 keys are deprecated. They contain three weaknesses. First, it is
First, it is relatively easy to construct a V3 key that has the same relatively easy to construct a V3 key that has the same Key ID as any
key ID as any other key because the key ID is simply the low 64 bits other key because the Key ID is simply the low 64 bits of the public
of the public modulus. Secondly, because the fingerprint of a V3 key modulus. Secondly, because the fingerprint of a V3 key hashes the
hashes the key material, but not its length, there is an increased key material, but not its length, there is an increased opportunity
opportunity for fingerprint collisions. Third, there are weaknesses for fingerprint collisions. Third, there are weaknesses in the MD5
in the MD5 hash algorithm that make developers prefer other hash algorithm that make developers prefer other algorithms. See
algorithms. See below for a fuller discussion of key IDs and below for a fuller discussion of Key IDs and fingerprints.
fingerprints.
V2 keys are identical to the deprecated V3 keys except for the V2 keys are identical to the deprecated V3 keys except for the
version number. An implementation MUST NOT generate them and MAY version number. An implementation MUST NOT generate them and MAY
accept or reject them as it sees fit. accept or reject them as it sees fit.
The version 4 format is similar to the version 3 format except for The version 4 format is similar to the version 3 format except for
the absence of a validity period. This has been moved to the the absence of a validity period. This has been moved to the
signature packet. In addition, fingerprints of version 4 keys are Signature packet. In addition, fingerprints of version 4 keys are
calculated differently from version 3 keys, as described in section calculated differently from version 3 keys, as described in the
"Enhanced Key Formats." section "Enhanced Key Formats".
A version 4 packet contains:
- A one-octet version number (4). A version 4 packet contains:
- A four-octet number denoting the time that the key was created. - A one-octet version number (4).
- A one-octet number denoting the public key algorithm of this key - A four-octet number denoting the time that the key was created.
- A series of multiprecision integers comprising the key material. - A one-octet number denoting the public-key algorithm of this key.
This algorithm-specific portion is:
Algorithm Specific Fields for RSA public keys: - A series of multiprecision integers comprising the key material.
This algorithm-specific portion is:
- multiprecision integer (MPI) of RSA public modulus n; Algorithm-Specific Fields for RSA public keys:
- MPI of RSA public encryption exponent e. - multiprecision integer (MPI) of RSA public modulus n;
Algorithm Specific Fields for DSA public keys: - MPI of RSA public encryption exponent e.
- MPI of DSA prime p; Algorithm-Specific Fields for DSA public keys:
- MPI of DSA group order q (q is a prime divisor of p-1); - MPI of DSA prime p;
- MPI of DSA group generator g; - MPI of DSA group order q (q is a prime divisor of p-1);
- MPI of DSA public key value y (= g**x mod p where x is - MPI of DSA group generator g;
secret).
Algorithm Specific Fields for Elgamal public keys: - MPI of DSA public-key value y (= g**x mod p where x
is secret).
- MPI of Elgamal prime p; Algorithm-Specific Fields for Elgamal public keys:
- MPI of Elgamal group generator g; - MPI of Elgamal prime p;
- MPI of Elgamal public key value y (= g**x mod p where x is - MPI of Elgamal group generator g;
secret). - MPI of Elgamal public key value y (= g**x mod p where x
is secret).
5.5.3. Secret Key Packet Formats 5.5.3. Secret-Key Packet Formats
The Secret Key and Secret Subkey packets contain all the data of the The Secret-Key and Secret-Subkey packets contain all the data of the
Public Key and Public Subkey packets, with additional Public-Key and Public-Subkey packets, with additional algorithm-
algorithm-specific secret key data appended, usually in encrypted specific secret-key data appended, usually in encrypted form.
form.
The packet contains: The packet contains:
- A Public Key or Public Subkey packet, as described above - A Public-Key or Public-Subkey packet, as described above.
- One octet indicating string-to-key usage conventions. Zero - One octet indicating string-to-key usage conventions. Zero
indicates that the secret key data is not encrypted. 255 or 254 indicates that the secret-key data is not encrypted. 255 or 254
indicates that a string-to-key specifier is being given. Any indicates that a string-to-key specifier is being given. Any
other value is a symmetric-key encryption algorithm identifier. other value is a symmetric-key encryption algorithm identifier.
- [Optional] If string-to-key usage octet was 255 or 254, a - [Optional] If string-to-key usage octet was 255 or 254, a one-
one-octet symmetric encryption algorithm. octet symmetric encryption algorithm.
- [Optional] If string-to-key usage octet was 255 or 254, a - [Optional] If string-to-key usage octet was 255 or 254, a
string-to-key specifier. The length of the string-to-key string-to-key specifier. The length of the string-to-key
specifier is implied by its type, as described above. specifier is implied by its type, as described above.
- [Optional] If secret data is encrypted (string-to-key usage - [Optional] If secret data is encrypted (string-to-key usage octet
octet not zero), an Initial Vector (IV) of the same length as not zero), an Initial Vector (IV) of the same length as the
the cipher's block size. cipher's block size.
- Plain or encrypted multiprecision integers comprising the secret - Plain or encrypted multiprecision integers comprising the secret
key data. These algorithm-specific fields are as described key data. These algorithm-specific fields are as described
below. below.
- If the string-to-key usage octet is zero or 255, then a - If the string-to-key usage octet is zero or 255, then a two-octet
two-octet checksum of the plaintext of the algorithm-specific checksum of the plaintext of the algorithm-specific portion (sum
portion (sum of all octets, mod 65536). If the string-to-key of all octets, mod 65536). If the string-to-key usage octet was
usage octet was 254, then a 20-octet SHA-1 hash of the plaintext 254, then a 20-octet SHA-1 hash of the plaintext of the
of the algorithm-specific portion. This checksum or hash is algorithm-specific portion. This checksum or hash is encrypted
encrypted together with the algorithm-specific fields (if together with the algorithm-specific fields (if string-to-key
string-to-key usage octet is not zero). Note that for all other usage octet is not zero). Note that for all other values, a
values, a two-octet checksum is required. two-octet checksum is required.
Algorithm Specific Fields for RSA secret keys: Algorithm-Specific Fields for RSA secret keys:
- multiprecision integer (MPI) of RSA secret exponent d. - multiprecision integer (MPI) of RSA secret exponent d.
- MPI of RSA secret prime value p. - MPI of RSA secret prime value p.
- MPI of RSA secret prime value q (p < q). - MPI of RSA secret prime value q (p < q).
- MPI of u, the multiplicative inverse of p, mod q. - MPI of u, the multiplicative inverse of p, mod q.
Algorithm Specific Fields for DSA secret keys: Algorithm-Specific Fields for DSA secret keys:
- MPI of DSA secret exponent x. - MPI of DSA secret exponent x.
Algorithm Specific Fields for Elgamal secret keys: Algorithm-Specific Fields for Elgamal secret keys:
- MPI of Elgamal secret exponent x. - MPI of Elgamal secret exponent x.
Secret MPI values can be encrypted using a passphrase. If a Secret MPI values can be encrypted using a passphrase. If a string-
string-to-key specifier is given, that describes the algorithm for to-key specifier is given, that describes the algorithm for
converting the passphrase to a key, else a simple MD5 hash of the converting the passphrase to a key, else a simple MD5 hash of the
passphrase is used. Implementations MUST use a string-to-key passphrase is used. Implementations MUST use a string-to-key
specifier; the simple hash is for backward compatibility and is specifier; the simple hash is for backward compatibility and is
deprecated, though implementations MAY continue to use existing deprecated, though implementations MAY continue to use existing
private keys in the old format. The cipher for encrypting the MPIs private keys in the old format. The cipher for encrypting the MPIs
is specified in the secret key packet. is specified in the Secret-Key packet.
Encryption/decryption of the secret data is done in CFB mode using Encryption/decryption of the secret data is done in CFB mode using
the key created from the passphrase and the Initial Vector from the the key created from the passphrase and the Initial Vector from the
packet. A different mode is used with V3 keys (which are only RSA) packet. A different mode is used with V3 keys (which are only RSA)
than with other key formats. With V3 keys, the MPI bit count prefix than with other key formats. With V3 keys, the MPI bit count prefix
(i.e., the first two octets) is not encrypted. Only the MPI (i.e., the first two octets) is not encrypted. Only the MPI non-
non-prefix data is encrypted. Furthermore, the CFB state is prefix data is encrypted. Furthermore, the CFB state is
resynchronized at the beginning of each new MPI value, so that the resynchronized at the beginning of each new MPI value, so that the
CFB block boundary is aligned with the start of the MPI data. CFB block boundary is aligned with the start of the MPI data.
With V4 keys, a simpler method is used. All secret MPI values are With V4 keys, a simpler method is used. All secret MPI values are
encrypted in CFB mode, including the MPI bitcount prefix. encrypted in CFB mode, including the MPI bitcount prefix.
The two-octet checksum that follows the algorithm-specific portion The two-octet checksum that follows the algorithm-specific portion is
is the algebraic sum, mod 65536, of the plaintext of all the the algebraic sum, mod 65536, of the plaintext of all the algorithm-
algorithm-specific octets (including MPI prefix and data). With V3 specific octets (including MPI prefix and data). With V3 keys, the
keys, the checksum is stored in the clear. With V4 keys, the checksum is stored in the clear. With V4 keys, the checksum is
checksum is encrypted like the algorithm-specific data. This value encrypted like the algorithm-specific data. This value is used to
is used to check that the passphrase was correct. However, this check that the passphrase was correct. However, this checksum is
checksum is deprecated; an implementation SHOULD NOT use it, but deprecated; an implementation SHOULD NOT use it, but should rather
should rather use the SHA-1 hash denoted with a usage octet of 254. use the SHA-1 hash denoted with a usage octet of 254. The reason for
The reason for this is that there are some attacks that involve this is that there are some attacks that involve undetectably
undetectably modifying the secret key. modifying the secret key.
5.6. Compressed Data Packet (Tag 8) 5.6. Compressed Data Packet (Tag 8)
The Compressed Data packet contains compressed data. Typically, this The Compressed Data packet contains compressed data. Typically, this
packet is found as the contents of an encrypted packet, or following packet is found as the contents of an encrypted packet, or following
a Signature or One-Pass Signature packet, and contains a literal a Signature or One-Pass Signature packet, and contains a literal data
data packet. packet.
The body of this packet consists of: The body of this packet consists of:
- One octet that gives the algorithm used to compress the packet. - One octet that gives the algorithm used to compress the packet.
- The remainder of the packet is compressed data. - Compressed data, which makes up the remainder of the packet.
A Compressed Data Packet's body contains an block that compresses A Compressed Data Packet's body contains an block that compresses
some set of packets. See section "Packet Composition" for details on some set of packets. See section "Packet Composition" for details on
how messages are formed. how messages are formed.
ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]
blocks. Note that PGP V2.6 uses 13 bits of compression. If an DEFLATE blocks. Note that PGP V2.6 uses 13 bits of compression. If
implementation uses more bits of compression, PGP V2.6 cannot an implementation uses more bits of compression, PGP V2.6 cannot
decompress it. decompress it.
ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-
blocks. style blocks.
BZip2-compressed packets are compressed using the BZip2 [BZ2] BZip2-compressed packets are compressed using the BZip2 [BZ2]
algorithm. algorithm.
5.7. Symmetrically Encrypted Data Packet (Tag 9) 5.7. Symmetrically Encrypted Data Packet (Tag 9)
The Symmetrically Encrypted Data packet contains data encrypted with The Symmetrically Encrypted Data packet contains data encrypted with
a symmetric-key algorithm. When it has been decrypted, it contains a symmetric-key algorithm. When it has been decrypted, it contains
other packets (usually a literal data packet or compressed data other packets (usually a literal data packet or compressed data
packet, but in theory other Symmetrically Encrypted Data Packets or packet, but in theory other Symmetrically Encrypted Data packets or
sequences of packets that form whole OpenPGP messages). sequences of packets that form whole OpenPGP messages).
The body of this packet consists of: The body of this packet consists of:
- Encrypted data, the output of the selected symmetric-key cipher - Encrypted data, the output of the selected symmetric-key cipher
operating in OpenPGP's variant of Cipher Feedback (CFB) mode. operating in OpenPGP's variant of Cipher Feedback (CFB) mode.
The symmetric cipher used may be specified in an Public-Key or The symmetric cipher used may be specified in a Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data Packet. In that case, the cipher Symmetrically Encrypted Data packet. In that case, the cipher
algorithm octet is prefixed to the session key before it is algorithm octet is prefixed to the session key before it is
encrypted. If no packets of these types precede the encrypted data, encrypted. If no packets of these types precede the encrypted data,
the IDEA algorithm is used with the session key calculated as the the IDEA algorithm is used with the session key calculated as the MD5
MD5 hash of the passphrase, though this use is deprecated. hash of the passphrase, though this use is deprecated.
The data is encrypted in CFB mode, with a CFB shift size equal to The data is encrypted in CFB mode, with a CFB shift size equal to the
the cipher's block size. The Initial Vector (IV) is specified as all cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes a string of length zeros. Instead of using an IV, OpenPGP prefixes a string of length
equal to the block size of the cipher plus two to the data before it equal to the block size of the cipher plus two to the data before it
is encrypted. The first block-size octets (for example, 8 octets for is encrypted. The first block-size octets (for example, 8 octets for
a 64-bit block length) are random, and the following two octets are a 64-bit block length) are random, and the following two octets are
copies of the last two octets of the IV. For example, in an 8 octet copies of the last two octets of the IV. For example, in an 8-octet
block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15 octet 8. In a cipher of length 16, octet 17 is a repeat of octet 15
and octet 18 is a repeat of octet 16. As a pedantic clarification, and octet 18 is a repeat of octet 16. As a pedantic clarification,
in both these examples, we consider the first octet to be numbered in both these examples, we consider the first octet to be numbered 1.
1.
After encrypting the first block-size-plus-two octets, the CFB state After encrypting the first block-size-plus-two octets, the CFB state
is resynchronized. The last block-size octets of ciphertext are is resynchronized. The last block-size octets of ciphertext are
passed through the cipher and the block boundary is reset. passed through the cipher and the block boundary is reset.
The repetition of 16 bits in the random data prefixed to the message The repetition of 16 bits in the random data prefixed to the message
allows the receiver to immediately check whether the session key is allows the receiver to immediately check whether the session key is
incorrect. See the Security Considerations section for hints on the incorrect. See the "Security Considerations" section for hints on
proper use of this "quick check." the proper use of this "quick check".
5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
An experimental version of PGP used this packet as the Literal An experimental version of PGP used this packet as the Literal
packet, but no released version of PGP generated Literal packets packet, but no released version of PGP generated Literal packets with
with this tag. With PGP 5.x, this packet has been re-assigned and is this tag. With PGP 5.x, this packet has been reassigned and is
reserved for use as the Marker packet. reserved for use as the Marker packet.
The body of this packet consists of: The body of this packet consists of:
- The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8). - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
Such a packet MUST be ignored when received. It may be placed at the Such a packet MUST be ignored when received. It may be placed at the
beginning of a message that uses features not available in PGP 2.6.x beginning of a message that uses features not available in PGP 2.6.x
in order to cause that version to report that newer software is in order to cause that version to report that newer software is
necessary to process the message. necessary to process the message.
5.9. Literal Data Packet (Tag 11) 5.9. Literal Data Packet (Tag 11)
A Literal Data packet contains the body of a message; data that is A Literal Data packet contains the body of a message; data that is
not to be further interpreted. not to be further interpreted.
The body of this packet consists of: The body of this packet consists of:
- A one-octet field that describes how the data is formatted. - A one-octet field that describes how the data is formatted.
If it is a 'b' (0x62), then the literal packet contains binary data. If it is a 'b' (0x62), then the Literal packet contains binary data.
If it is a 't' (0x74), then it contains text data, and thus may need If it is a 't' (0x74), then it contains text data, and thus may need
line ends converted to local form, or other text-mode changes. The line ends converted to local form, or other text-mode changes. The
tag 'u' (0x75) means the same as 't', but also indicates that tag 'u' (0x75) means the same as 't', but also indicates that
implementation believes that the literal data contains UTF-8 text. implementation believes that the literal data contains UTF-8 text.
Early versions of PGP also defined a value of 'l' as a 'local' mode Early versions of PGP also defined a value of 'l' as a 'local' mode
for machine-local conversions. RFC 1991 incorrectly stated this for machine-local conversions. RFC 1991 [RFC1991] incorrectly stated
local mode flag as '1' (ASCII numeral one). Both of these local this local mode flag as '1' (ASCII numeral one). Both of these local
modes are deprecated. modes are deprecated.
- File name as a string (one-octet length, followed by a file - File name as a string (one-octet length, followed by a file
name). This may be a zero-length string. Commonly, if the source name). This may be a zero-length string. Commonly, if the
of the encrypted data is a file, this will be the name of the source of the encrypted data is a file, this will be the name of
encrypted file. An implementation MAY consider the file name in the encrypted file. An implementation MAY consider the file name
the literal packet to be a more authoritative name than the in the Literal packet to be a more authoritative name than the
actual file name. actual file name.
If the special name "_CONSOLE" is used, the message is considered to If the special name "_CONSOLE" is used, the message is considered to
be "for your eyes only". This advises that the message data is be "for your eyes only". This advises that the message data is
unusually sensitive, and the receiving program should process it unusually sensitive, and the receiving program should process it more
more carefully, perhaps avoiding storing the received data to disk, carefully, perhaps avoiding storing the received data to disk, for
for example. example.
- A four-octet number that indicates a date associated with the - A four-octet number that indicates a date associated with the
literal data. Commonly, the date might be the modification date literal data. Commonly, the date might be the modification date
of a file, or the time the packet was created, or a zero that of a file, or the time the packet was created, or a zero that
indicates no specific time. indicates no specific time.
- The remainder of the packet is literal data. - The remainder of the packet is literal data.
Text data is stored with <CR><LF> text endings (i.e. network-normal Text data is stored with <CR><LF> text endings (i.e., network-
line endings). These should be converted to native line endings by normal line endings). These should be converted to native line
the receiving software. endings by the receiving software.
5.10. Trust Packet (Tag 12) 5.10. Trust Packet (Tag 12)
The Trust packet is used only within keyrings and is not normally The Trust packet is used only within keyrings and is not normally
exported. Trust packets contain data that record the user's exported. Trust packets contain data that record the user's
specifications of which key holders are trustworthy introducers, specifications of which key holders are trustworthy introducers,
along with other information that implementing software uses for along with other information that implementing software uses for
trust information. The format of trust packets is defined by a given trust information. The format of Trust packets is defined by a given
implementation. implementation.
Trust packets SHOULD NOT be emitted to output streams that are Trust packets SHOULD NOT be emitted to output streams that are
transferred to other users, and they SHOULD be ignored on any input transferred to other users, and they SHOULD be ignored on any input
other than local keyring files. other than local keyring files.
5.11. User ID Packet (Tag 13) 5.11. User ID Packet (Tag 13)
A User ID packet consists of UTF-8 text that is intended to A User ID packet consists of UTF-8 text that is intended to represent
represent the name and email address of the key holder. By the name and email address of the key holder. By convention, it
convention, it includes an RFC 2822 mail name-addr, but there are no includes an RFC 2822 [RFC2822] mail name-addr, but there are no
restrictions on its content. The packet length in the header restrictions on its content. The packet length in the header
specifies the length of the User ID. specifies the length of the User ID.
5.12. User Attribute Packet (Tag 17) 5.12. User Attribute Packet (Tag 17)
The User Attribute packet is a variation of the User ID packet. It The User Attribute packet is a variation of the User ID packet. It
is capable of storing more types of data than the User ID packet is capable of storing more types of data than the User ID packet,
which is limited to text. Like the User ID packet, a User Attribute which is limited to text. Like the User ID packet, a User Attribute
packet may be certified by the key owner ("self-signed") or any packet may be certified by the key owner ("self-signed") or any other
other key owner who cares to certify it. Except as noted, a User key owner who cares to certify it. Except as noted, a User Attribute
Attribute packet may be used anywhere that a User ID packet may be packet may be used anywhere that a User ID packet may be used.
used.
While User Attribute packets are not a required part of the OpenPGP While User Attribute packets are not a required part of the OpenPGP
standard, implementations SHOULD provide at least enough standard, implementations SHOULD provide at least enough
compatibility to properly handle a certification signature on the compatibility to properly handle a certification signature on the
User Attribute packet. A simple way to do this is by treating the User Attribute packet. A simple way to do this is by treating the
User Attribute packet as a User ID packet with opaque contents, but User Attribute packet as a User ID packet with opaque contents, but
an implementation may use any method desired. an implementation may use any method desired.
The User Attribute packet is made up of one or more attribute The User Attribute packet is made up of one or more attribute
subpackets. Each subpacket consists of a subpacket header and a subpackets. Each subpacket consists of a subpacket header and a
body. The header consists of: body. The header consists of:
- the subpacket length (1, 2, or 5 octets) - the subpacket length (1, 2, or 5 octets)
- the subpacket type (1 octet) - the subpacket type (1 octet)
and is followed by the subpacket specific data. and is followed by the subpacket specific data.
The only currently defined subpacket type is 1, signifying an image. The only currently defined subpacket type is 1, signifying an image.
An implementation SHOULD ignore any subpacket of a type that it does An implementation SHOULD ignore any subpacket of a type that it does
not recognize. Subpacket types 100 through 110 are reserved for not recognize. Subpacket types 100 through 110 are reserved for
private or experimental use. private or experimental use.
5.12.1. The Image Attribute Subpacket 5.12.1. The Image Attribute Subpacket
The image attribute subpacket is used to encode an image, presumably The Image Attribute subpacket is used to encode an image, presumably
(but not required to be) that of the key owner. (but not required to be) that of the key owner.
The image attribute subpacket begins with an image header. The first The Image Attribute subpacket begins with an image header. The first
two octets of the image header contain the length of the image two octets of the image header contain the length of the image
header. Note that unlike other multi-octet numerical values in this header. Note that unlike other multi-octet numerical values in this
document, due to an historical accident this value is encoded as a document, due to a historical accident this value is encoded as a
little-endian number. The image header length is followed by a little-endian number. The image header length is followed by a
single octet for the image header version. The only currently single octet for the image header version. The only currently
defined version of the image header is 1, which is a 16 octet image defined version of the image header is 1, which is a 16-octet image
header. The first three octets of a version 1 image header are thus header. The first three octets of a version 1 image header are thus
0x10 0x00 0x01. 0x10, 0x00, 0x01.
The fourth octet of a version 1 image header designates the encoding The fourth octet of a version 1 image header designates the encoding
format of the image. The only currently defined encoding format is format of the image. The only currently defined encoding format is
the value 1 to indicate JPEG. Image format types 100 through 110 are the value 1 to indicate JPEG. Image format types 100 through 110 are
reserved for private or experimental use. The rest of the version 1 reserved for private or experimental use. The rest of the version 1
image header is made up of 12 reserved octets, all of which MUST be image header is made up of 12 reserved octets, all of which MUST be
set to 0. set to 0.
The rest of the image subpacket contains the image itself. As the The rest of the image subpacket contains the image itself. As the
only currently defined image type is JPEG, the image is encoded in only currently defined image type is JPEG, the image is encoded in
the JPEG File Interchange Format (JFIF), a standard file format for the JPEG File Interchange Format (JFIF), a standard file format for
JPEG images. [JFIF] JPEG images [JFIF].
An implementation MAY try and determine the type of an image by An implementation MAY try to determine the type of an image by
examination of the image data if it is unable to handle a particular examination of the image data if it is unable to handle a particular
version of the image header or if a specified encoding format value version of the image header or if a specified encoding format value
is not recognized. is not recognized.
5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) 5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)
The Symmetrically Encrypted Integrity Protected Data Packet is a The Symmetrically Encrypted Integrity Protected Data packet is a
variant of the Symmetrically Encrypted Data Packet. It is a new variant of the Symmetrically Encrypted Data packet. It is a new
feature created for OpenPGP that addresses the problem of detecting feature created for OpenPGP that addresses the problem of detecting a
a modification to encrypted data. It is used in combination with a modification to encrypted data. It is used in combination with a
Modification Detection Code Packet. Modification Detection Code packet.
There is a corresponding feature in the features signature subpacket There is a corresponding feature in the features Signature subpacket
that denotes that an implementation can properly use this packet that denotes that an implementation can properly use this packet
type. An implementation MUST support decrypting these packets and type. An implementation MUST support decrypting these packets and
SHOULD prefer generating them to the older Symmetrically Encrypted SHOULD prefer generating them to the older Symmetrically Encrypted
Data Packet when possible. Since this data packet protects against Data packet when possible. Since this data packet protects against
modification attacks, this standard encourages its proliferation. modification attacks, this standard encourages its proliferation.
While blanket adoption of this data packet would create While blanket adoption of this data packet would create
interoperability problems, rapid adoption is nevertheless important. interoperability problems, rapid adoption is nevertheless important.
An implementation SHOULD specifically denote support for this An implementation SHOULD specifically denote support for this packet,
packet, but it MAY infer it from other mechanisms. but it MAY infer it from other mechanisms.
For example, an implementation might infer from the use of a cipher For example, an implementation might infer from the use of a cipher
such as AES or Twofish that a user supports this feature. It might such as Advanced Encryption Standard (AES) or Twofish that a user
place in the unhashed portion of another user's key signature a supports this feature. It might place in the unhashed portion of
features subpacket. It might also present a user with an opportunity another user's key signature a Features subpacket. It might also
to regenerate their own self-signature with a features subpacket. present a user with an opportunity to regenerate their own self-
signature with a Features subpacket.
This packet contains data encrypted with a symmetric-key algorithm This packet contains data encrypted with a symmetric-key algorithm
and protected against modification by the SHA-1 hash algorithm. When and protected against modification by the SHA-1 hash algorithm. When
it has been decrypted, it will typically contain other packets it has been decrypted, it will typically contain other packets (often
(often a literal data packet or compressed data packet). The last a Literal Data packet or Compressed Data packet). The last decrypted
decrypted packet in this packet's payload MUST be a Modification packet in this packet's payload MUST be a Modification Detection Code
Detection Code packet. packet.
The body of this packet consists of: The body of this packet consists of:
- A one-octet version number. The only currently defined value is - A one-octet version number. The only currently defined value is
1. 1.
- Encrypted data, the output of the selected symmetric-key cipher - Encrypted data, the output of the selected symmetric-key cipher
operating in Cipher Feedback mode with shift amount equal to the operating in Cipher Feedback mode with shift amount equal to the
block size of the cipher (CFB-n where n is the block size). block size of the cipher (CFB-n where n is the block size).
The symmetric cipher used MUST be specified in a Public-Key or The symmetric cipher used MUST be specified in a Public-Key or
Symmetric-Key Encrypted Session Key packet that precedes the Symmetric-Key Encrypted Session Key packet that precedes the
Symmetrically Encrypted Data Packet. In either case, the cipher Symmetrically Encrypted Data packet. In either case, the cipher
algorithm octet is prefixed to the session key before it is algorithm octet is prefixed to the session key before it is
encrypted. encrypted.
The data is encrypted in CFB mode, with a CFB shift size equal to The data is encrypted in CFB mode, with a CFB shift size equal to the
the cipher's block size. The Initial Vector (IV) is specified as all cipher's block size. The Initial Vector (IV) is specified as all
zeros. Instead of using an IV, OpenPGP prefixes an octet string to zeros. Instead of using an IV, OpenPGP prefixes an octet string to
the data before it is encrypted. The length of the octet string the data before it is encrypted. The length of the octet string
equals the block size of the cipher in octets, plus two. The first equals the block size of the cipher in octets, plus two. The first
octets in the group, of length equal to the block size of the octets in the group, of length equal to the block size of the cipher,
cipher, are random; the last two octets are each copies of their 2nd are random; the last two octets are each copies of their 2nd
preceding octet. For example, with a cipher whose block size is 128 preceding octet. For example, with a cipher whose block size is 128
bits or 16 octets, the prefix data will contain 16 random octets, bits or 16 octets, the prefix data will contain 16 random octets,
then two more octets, which are copies of the 15th and 16th octets, then two more octets, which are copies of the 15th and 16th octets,
respectively. Unlike the Symmetrically Encrypted Data Packet, no respectively. Unlike the Symmetrically Encrypted Data Packet, no
special CFB resynchronization is done after encrypting this prefix special CFB resynchronization is done after encrypting this prefix
data. See OpenPGP CFB Mode below for more details. data. See "OpenPGP CFB Mode" below for more details.
The repetition of 16 bits in the random data prefixed to the message The repetition of 16 bits in the random data prefixed to the message
allows the receiver to immediately check whether the session key is allows the receiver to immediately check whether the session key is
incorrect. incorrect.
The plaintext of the data to be encrypted is passed through the The plaintext of the data to be encrypted is passed through the SHA-1
SHA-1 hash function, and the result of the hash is appended to the hash function, and the result of the hash is appended to the
plaintext in a Modification Detection Code packet. The input to the plaintext in a Modification Detection Code packet. The input to the
hash function includes the prefix data described above; it includes hash function includes the prefix data described above; it includes
all of the plaintext, and then also includes two octets of values all of the plaintext, and then also includes two octets of values
0xD3, 0x14. These represent the encoding of a Modification Detection 0xD3, 0x14. These represent the encoding of a Modification Detection
Code packet tag and length field of 20 octets. Code packet tag and length field of 20 octets.
The resulting hash value is stored in a Modification Detection Code The resulting hash value is stored in a Modification Detection Code
packet which MUST use the two octet encoding just given to represent (MDC) packet, which MUST use the two octet encoding just given to
its tag and length field. The body of the MDC packet is the 20 octet represent its tag and length field. The body of the MDC packet is
output of the SHA-1 hash. the 20-octet output of the SHA-1 hash.
The Modification Detection Code packet is appended to the plaintext The Modification Detection Code packet is appended to the plaintext
and encrypted along with the plaintext using the same CFB context. and encrypted along with the plaintext using the same CFB context.
During decryption, the plaintext data should be hashed with SHA-1, During decryption, the plaintext data should be hashed with SHA-1,
including the prefix data as well as the packet tag and length field including the prefix data as well as the packet tag and length field
of the Modification Detection Code packet. The body of the MDC of the Modification Detection Code packet. The body of the MDC
packet, upon decryption, is compared with the result of the SHA-1 packet, upon decryption, is compared with the result of the SHA-1
hash. hash.
Any failure of the MDC indicates that the message has been modified Any failure of the MDC indicates that the message has been modified
and MUST be treated as a security problem. Failures include a and MUST be treated as a security problem. Failures include a
difference in the hash values, but also the absence of an MDC difference in the hash values, but also the absence of an MDC packet,
packet, or an MDC packet in any position other than the end of the or an MDC packet in any position other than the end of the plaintext.
plaintext. Any failure SHOULD be reported to the user. Any failure SHOULD be reported to the user.
Note: future designs of new versions of this packet should consider Note: future designs of new versions of this packet should consider
rollback attacks since it will be possible for an attacker to change rollback attacks since it will be possible for an attacker to change
the version back to 1. the version back to 1.
NON-NORMATIVE EXPLANATION NON-NORMATIVE EXPLANATION
The MDC system, as packets 18 and 19 are called, were created to The MDC system, as packets 18 and 19 are called, were created to
provide an integrity mechanism that is less strong than a provide an integrity mechanism that is less strong than a
signature, yet stronger than bare CFB encryption. signature, yet stronger than bare CFB encryption.
It is a limitation of CFB encryption that damage to the It is a limitation of CFB encryption that damage to the ciphertext
ciphertext will corrupt the affected cipher blocks and the block will corrupt the affected cipher blocks and the block following.
following. Additionally, if data is removed from the end of a Additionally, if data is removed from the end of a CFB-encrypted
CFB-encrypted block, that removal is undetectable. (Note also block, that removal is undetectable. (Note also that CBC mode has
that CBC mode has a similar limitation, but data removed from a similar limitation, but data removed from the front of the block
the front of the block is undetectable.) is undetectable.)
The obvious way to protect or authenticate an encrypted block is The obvious way to protect or authenticate an encrypted block is
to digitally sign it. However, many people do not wish to to digitally sign it. However, many people do not wish to
habitually sign data, for a large number of reasons beyond the habitually sign data, for a large number of reasons beyond the
scope of this document. Suffice it to say that many people scope of this document. Suffice it to say that many people
consider properties such as deniability to be as valuable as consider properties such as deniability to be as valuable as
integrity. integrity.
OpenPGP addresses this desire to have more security than raw OpenPGP addresses this desire to have more security than raw
encryption and yet preserve deniability with the MDC system. An encryption and yet preserve deniability with the MDC system. An
MDC is intentionally not a MAC. Its name was not selected by MDC is intentionally not a MAC. Its name was not selected by
accident. It is analogous to a checksum. accident. It is analogous to a checksum.
Despite the fact that it is a relatively modest system, it has Despite the fact that it is a relatively modest system, it has
proved itself in the real world. It is an effective defense to proved itself in the real world. It is an effective defense to
several attacks that have surfaced since it has been created. It several attacks that have surfaced since it has been created. It
has met its modest goals admirably. has met its modest goals admirably.
Consequently, because it is a modest security system, it has Consequently, because it is a modest security system, it has
modest requirements on the hash function(s) it employs. It does modest requirements on the hash function(s) it employs. It does
not rely on a hash function being collision-free, it relies on a not rely on a hash function being collision-free, it relies on a
hash function being one-way. If a forger, Frank, wishes to send hash function being one-way. If a forger, Frank, wishes to send
Alice a (digitally) unsigned message that says, "I've always Alice a (digitally) unsigned message that says, "I've always
secretly loved you, signed Bob" it is far easier for him to secretly loved you, signed Bob", it is far easier for him to
construct a new message than it is to modify anything construct a new message than it is to modify anything intercepted
intercepted from Bob. (Note also that if Bob wishes to from Bob. (Note also that if Bob wishes to communicate secretly
communicate secretly with Alice, but without authentication nor with Alice, but without authentication or identification and with
identification and with a threat model that includes forgers, he a threat model that includes forgers, he has a problem that
has a problem that transcends mere cryptography.) transcends mere cryptography.)
Note also that unlike nearly every other OpenPGP subsystem, Note also that unlike nearly every other OpenPGP subsystem, there
there are no parameters in the MDC system. It hard-defines SHA-1 are no parameters in the MDC system. It hard-defines SHA-1 as its
as its hash function. This is not an accident. It is an hash function. This is not an accident. It is an intentional
intentional choice to avoid downgrade and cross-grade attacks choice to avoid downgrade and cross-grade attacks while making a
while making a simple, fast system. (A downgrade attack would be simple, fast system. (A downgrade attack would be an attack that
an attack that replaced SHA-256 with SHA-1, for example. A replaced SHA-256 with SHA-1, for example. A cross-grade attack
cross-grade attack would replace SHA-1 with another 160-bit would replace SHA-1 with another 160-bit hash, such as RIPE-
hash, such as RIPE-MD/160, for example.) MD/160, for example.)
However, given the present state of hash function cryptanalysis However, given the present state of hash function cryptanalysis
and cryptography, it may be desirable to upgrade the MDC system and cryptography, it may be desirable to upgrade the MDC system to
to a new hash function. See section 10.5 in the IANA a new hash function. See Section 13.11 in the "IANA
considerations for guidance. Considerations" for guidance.
5.14. Modification Detection Code Packet (Tag 19) 5.14. Modification Detection Code Packet (Tag 19)
The Modification Detection Code packet contains a SHA-1 hash of The Modification Detection Code packet contains a SHA-1 hash of
plaintext data which is used to detect message modification. It is plaintext data, which is used to detect message modification. It is
only used with a Symmetrically Encrypted Integrity Protected Data only used with a Symmetrically Encrypted Integrity Protected Data
packet. The Modification Detection Code packet MUST be the last packet. The Modification Detection Code packet MUST be the last
packet in the plaintext data which is encrypted in the Symmetrically packet in the plaintext data that is encrypted in the Symmetrically
Encrypted Integrity Protected Data packet, and MUST appear in no Encrypted Integrity Protected Data packet, and MUST appear in no
other place. other place.
A Modification Detection Code packet MUST have a length of 20 A Modification Detection Code packet MUST have a length of 20 octets.
octets.
The body of this packet consists of: The body of this packet consists of:
- A 20-octet SHA-1 hash of the preceding plaintext data of the - A 20-octet SHA-1 hash of the preceding plaintext data of the
Symmetrically Encrypted Integrity Protected Data packet, Symmetrically Encrypted Integrity Protected Data packet,
including prefix data, the tag octet, and length octet of the including prefix data, the tag octet, and length octet of the
Modification Detection Code packet. Modification Detection Code packet.
Note that the Modification Detection Code packet MUST always use a Note that the Modification Detection Code packet MUST always use a
new-format encoding of the packet tag, and a one-octet encoding of new format encoding of the packet tag, and a one-octet encoding of
the packet length. The reason for this is that the hashing rules for the packet length. The reason for this is that the hashing rules for
modification detection include a one-octet tag and one-octet length modification detection include a one-octet tag and one-octet length
in the data hash. While this is a bit restrictive, it reduces in the data hash. While this is a bit restrictive, it reduces
complexity. complexity.
6. Radix-64 Conversions 6. Radix-64 Conversions
As stated in the introduction, OpenPGP's underlying native As stated in the introduction, OpenPGP's underlying native
representation for objects is a stream of arbitrary octets, and some representation for objects is a stream of arbitrary octets, and some
systems desire these objects to be immune to damage caused by systems desire these objects to be immune to damage caused by
character set translation, data conversions, etc. character set translation, data conversions, etc.
In principle, any printable encoding scheme that met the In principle, any printable encoding scheme that met the requirements
requirements of the unsafe channel would suffice, since it would not of the unsafe channel would suffice, since it would not change the
change the underlying binary bit streams of the native OpenPGP data underlying binary bit streams of the native OpenPGP data structures.
structures. The OpenPGP standard specifies one such printable The OpenPGP standard specifies one such printable encoding scheme to
encoding scheme to ensure interoperability. ensure interoperability.
OpenPGP's Radix-64 encoding is composed of two parts: a base64 OpenPGP's Radix-64 encoding is composed of two parts: a base64
encoding of the binary data, and a checksum. The base64 encoding is encoding of the binary data and a checksum. The base64 encoding is
identical to the MIME base64 content-transfer-encoding [RFC2045]. identical to the MIME base64 content-transfer-encoding [RFC2045].
The checksum is a 24-bit CRC converted to four characters of The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to
radix-64 encoding by the same MIME base64 transformation, preceded four characters of radix-64 encoding by the same MIME base64
by an equals sign (=). The CRC is computed by using the generator transformation, preceded by an equal sign (=). The CRC is computed
0x864CFB and an initialization of 0xB704CE. The accumulation is done by using the generator 0x864CFB and an initialization of 0xB704CE.
on the data before it is converted to radix-64, rather than on the The accumulation is done on the data before it is converted to
converted data. A sample implementation of this algorithm is in the radix-64, rather than on the converted data. A sample implementation
next section. of this algorithm is in the next section.
The checksum with its leading equal sign MAY appear on the first The checksum with its leading equal sign MAY appear on the first line
line after the Base64 encoded data. after the base64 encoded data.
Rationale for CRC-24: The size of 24 bits fits evenly into printable Rationale for CRC-24: The size of 24 bits fits evenly into printable
base64. The nonzero initialization can detect more errors than a base64. The nonzero initialization can detect more errors than a
zero initialization. zero initialization.
6.1. An Implementation of the CRC-24 in "C" 6.1. An Implementation of the CRC-24 in "C"
#define CRC24_INIT 0xb704ceL #define CRC24_INIT 0xB704CEL
#define CRC24_POLY 0x1864cfbL #define CRC24_POLY 0x1864CFBL
typedef long crc24; typedef long crc24;
crc24 crc_octets(unsigned char *octets, size_t len) crc24 crc_octets(unsigned char *octets, size_t len)
{ {
crc24 crc = CRC24_INIT; crc24 crc = CRC24_INIT;
int i; int i;
while (len--) { while (len--) {
crc ^= (*octets++) << 16; crc ^= (*octets++) << 16;
for (i = 0; i < 8; i++) { for (i = 0; i < 8; i++) {
crc <<= 1; crc <<= 1;
if (crc & 0x1000000) if (crc & 0x1000000)
crc ^= CRC24_POLY; crc ^= CRC24_POLY;
} }
} }
return crc & 0xffffffL; return crc & 0xFFFFFFL;
} }
6.2. Forming ASCII Armor 6.2. Forming ASCII Armor
When OpenPGP encodes data into ASCII Armor, it puts specific headers When OpenPGP encodes data into ASCII Armor, it puts specific headers
around the Radix-64 encoded data, so OpenPGP can reconstruct the around the Radix-64 encoded data, so OpenPGP can reconstruct the data
data later. An OpenPGP implementation MAY use ASCII armor to protect later. An OpenPGP implementation MAY use ASCII armor to protect raw
raw binary data. OpenPGP informs the user what kind of data is binary data. OpenPGP informs the user what kind of data is encoded
encoded in the ASCII armor through the use of the headers. in the ASCII armor through the use of the headers.
Concatenating the following data creates ASCII Armor: Concatenating the following data creates ASCII Armor:
- An Armor Header Line, appropriate for the type of data - An Armor Header Line, appropriate for the type of data
- Armor Headers - Armor Headers
- A blank (zero-length, or containing only whitespace) line - A blank (zero-length, or containing only whitespace) line
- The ASCII-Armored data - The ASCII-Armored data
- An Armor Checksum - An Armor Checksum
- The Armor Tail, which depends on the Armor Header Line. - The Armor Tail, which depends on the Armor Header Line
An Armor Header Line consists of the appropriate header line text An Armor Header Line consists of the appropriate header line text
surrounded by five (5) dashes ('-', 0x2D) on either side of the surrounded by five (5) dashes ('-', 0x2D) on either side of the
header line text. The header line text is chosen based upon the type header line text. The header line text is chosen based upon the type
of data that is being encoded in Armor, and how it is being encoded. of data that is being encoded in Armor, and how it is being encoded.
Header line texts include the following strings: Header line texts include the following strings:
BEGIN PGP MESSAGE BEGIN PGP MESSAGE
Used for signed, encrypted, or compressed files. Used for signed, encrypted, or compressed files.
BEGIN PGP PUBLIC KEY BLOCK BEGIN PGP PUBLIC KEY BLOCK
Used for armoring public keys Used for armoring public keys.
BEGIN PGP PRIVATE KEY BLOCK BEGIN PGP PRIVATE KEY BLOCK
Used for armoring private keys Used for armoring private keys.
BEGIN PGP MESSAGE, PART X/Y BEGIN PGP MESSAGE, PART X/Y
Used for multi-part messages, where the armor is split amongst Y Used for multi-part messages, where the armor is split amongst Y
parts, and this is the Xth part out of Y. parts, and this is the Xth part out of Y.
BEGIN PGP MESSAGE, PART X BEGIN PGP MESSAGE, PART X
Used for multi-part messages, where this is the Xth part of an Used for multi-part messages, where this is the Xth part of an
unspecified number of parts. Requires the MESSAGE-ID Armor unspecified number of parts. Requires the MESSAGE-ID Armor
Header to be used. Header to be used.
BEGIN PGP SIGNATURE BEGIN PGP SIGNATURE
Used for detached signatures, OpenPGP/MIME signatures, and Used for detached signatures, OpenPGP/MIME signatures, and
cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE cleartext signatures. Note that PGP 2.x uses BEGIN PGP MESSAGE
for detached signatures. for detached signatures.
Note that all these Armor Header Lines are to consist of a complete Note that all these Armor Header Lines are to consist of a complete
line. That is to say, there is always a line ending preceding the line. That is to say, there is always a line ending preceding the
starting five dashes, and following the ending five dashes. The starting five dashes, and following the ending five dashes. The
header lines, therefore, MUST start at the beginning of a line, and header lines, therefore, MUST start at the beginning of a line, and
MUST NOT have text other than whitespace following them on the same MUST NOT have text other than whitespace following them on the same
line. These line endings are considered a part of the Armor Header line. These line endings are considered a part of the Armor Header
Line for the purposes of determining the content they delimit. This Line for the purposes of determining the content they delimit. This
is particularly important when computing a cleartext signature (see is particularly important when computing a cleartext signature (see
below). below).
The Armor Headers are pairs of strings that can give the user or the The Armor Headers are pairs of strings that can give the user or the
receiving OpenPGP implementation some information about how to receiving OpenPGP implementation some information about how to decode
decode or use the message. The Armor Headers are a part of the or use the message. The Armor Headers are a part of the armor, not a
armor, not a part of the message, and hence are not protected by any part of the message, and hence are not protected by any signatures
signatures applied to the message. applied to the message.
The format of an Armor Header is that of a key-value pair. A colon The format of an Armor Header is that of a key-value pair. A colon
(':' 0x38) and a single space (0x20) separate the key and value. (':' 0x38) and a single space (0x20) separate the key and value.
OpenPGP should consider improperly formatted Armor Headers to be OpenPGP should consider improperly formatted Armor Headers to be
corruption of the ASCII Armor. Unknown keys should be reported to corruption of the ASCII Armor. Unknown keys should be reported to
the user, but OpenPGP should continue to process the message. the user, but OpenPGP should continue to process the message.
Note that some transport methods are sensitive to line length. While Note that some transport methods are sensitive to line length. While
there is a limit of 76 characters for the Radix-64 data (section there is a limit of 76 characters for the Radix-64 data (Section
6.3), there is no limit to the length of Armor Headers. Care should 6.3), there is no limit to the length of Armor Headers. Care should
be taken that the Armor Headers are short enough to survive be taken that the Armor Headers are short enough to survive
transport. One way to do this is to repeat an Armor Header key transport. One way to do this is to repeat an Armor Header key
multiple times with different values for each so that no one line is multiple times with different values for each so that no one line is
overly long. overly long.
Currently defined Armor Header Keys are: Currently defined Armor Header Keys are as follows:
- "Version", that states the OpenPGP implementation and version - "Version", which states the OpenPGP implementation and version
used to encode the message. used to encode the message.
- "Comment", a user-defined comment. OpenPGP defines all text to - "Comment", a user-defined comment. OpenPGP defines all text to
be in UTF-8. A comment may be any UTF-8 string. However, the be in UTF-8. A comment may be any UTF-8 string. However, the
whole point of armoring is to provide seven-bit-clean data. whole point of armoring is to provide seven-bit-clean data.
Consequently, if a comment has characters that are outside the Consequently, if a comment has characters that are outside the
US-ASCII range of UTF, they may very well not survive transport. US-ASCII range of UTF, they may very well not survive transport.
- "MessageID", a 32-character string of printable characters. The - "MessageID", a 32-character string of printable characters. The
string must be the same for all parts of a multi-part message string must be the same for all parts of a multi-part message
that uses the "PART X" Armor Header. MessageID strings should be that uses the "PART X" Armor Header. MessageID strings should be
unique enough that the recipient of the mail can associate all unique enough that the recipient of the mail can associate all
the parts of a message with each other. A good checksum or the parts of a message with each other. A good checksum or
cryptographic hash function is sufficient. cryptographic hash function is sufficient.
The MessageID SHOULD NOT appear unless it is in a multi-part The MessageID SHOULD NOT appear unless it is in a multi-part
message. If it appears at all, it MUST be computed from the message. If it appears at all, it MUST be computed from the
finished (encrypted, signed, etc.) message in a deterministic finished (encrypted, signed, etc.) message in a deterministic
fashion, rather than contain a purely random value. This is to fashion, rather than contain a purely random value. This is to
allow the legitimate recipient to determine that the MessageID allow the legitimate recipient to determine that the MessageID
cannot serve as a covert means of leaking cryptographic key cannot serve as a covert means of leaking cryptographic key
information. information.
- "Hash", a comma-separated list of hash algorithms used in this - "Hash", a comma-separated list of hash algorithms used in this
message. This is used only in cleartext signed messages. message. This is used only in cleartext signed messages.
- "Charset", a description of the character set that the plaintext - "Charset", a description of the character set that the plaintext
is in. Please note that OpenPGP defines text to be in UTF-8. An is in. Please note that OpenPGP defines text to be in UTF-8. An
implementation will get best results by translating into and out implementation will get best results by translating into and out
of UTF-8. However, there are many instances where this is easier of UTF-8. However, there are many instances where this is easier
said than done. Also, there are communities of users who have no said than done. Also, there are communities of users who have no
need for UTF-8 because they are all happy with a character set need for UTF-8 because they are all happy with a character set
like ISO Latin-5 or a Japanese character set. In such instances, like ISO Latin-5 or a Japanese character set. In such instances,
an implementation MAY override the UTF-8 default by using this an implementation MAY override the UTF-8 default by using this
header key. An implementation MAY implement this key and any header key. An implementation MAY implement this key and any
translations it cares to; an implementation MAY ignore it and translations it cares to; an implementation MAY ignore it and
assume all text is UTF-8. assume all text is UTF-8.
The Armor Tail Line is composed in the same manner as the Armor The Armor Tail Line is composed in the same manner as the Armor
Header Line, except the string "BEGIN" is replaced by the string Header Line, except the string "BEGIN" is replaced by the string
"END". "END".
6.3. Encoding Binary in Radix-64 6.3. Encoding Binary in Radix-64
The encoding process represents 24-bit groups of input bits as The encoding process represents 24-bit groups of input bits as output
output strings of 4 encoded characters. Proceeding from left to strings of 4 encoded characters. Proceeding from left to right, a
right, a 24-bit input group is formed by concatenating three 8-bit 24-bit input group is formed by concatenating three 8-bit input
input groups. These 24 bits are then treated as four concatenated groups. These 24 bits are then treated as four concatenated 6-bit
6-bit groups, each of which is translated into a single digit in the groups, each of which is translated into a single digit in the
Radix-64 alphabet. When encoding a bit stream with the Radix-64 Radix-64 alphabet. When encoding a bit stream with the Radix-64
encoding, the bit stream must be presumed to be ordered with the encoding, the bit stream must be presumed to be ordered with the most
most-significant-bit first. That is, the first bit in the stream significant bit first. That is, the first bit in the stream will be
will be the high-order bit in the first 8-bit octet, and the eighth the high-order bit in the first 8-bit octet, and the eighth bit will
bit will be the low-order bit in the first 8-bit octet, and so on. be the low-order bit in the first 8-bit octet, and so on.
+--first octet--+-second octet--+--third octet--+ +--first octet--+-second octet--+--third octet--+
|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0| |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
+-----------+---+-------+-------+---+-----------+ +-----------+---+-------+-------+---+-----------+
|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0| |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
+--1.index--+--2.index--+--3.index--+--4.index--+ +--1.index--+--2.index--+--3.index--+--4.index--+
Each 6-bit group is used as an index into an array of 64 printable Each 6-bit group is used as an index into an array of 64 printable
characters from the table below. The character referenced by the characters from the table below. The character referenced by the
index is placed in the output string. index is placed in the output string.
Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z 0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0 1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1 2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2 3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3 4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4 5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5 6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6 7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7 8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8 9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9 10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 + 11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 / 12 M 29 d 46 u 63 /
13 N 30 e 47 v 13 N 30 e 47 v
14 O 31 f 48 w (pad) = 14 O 31 f 48 w (pad) =
15 P 32 g 49 x 15 P 32 g 49 x
16 Q 33 h 50 y 16 Q 33 h 50 y
The encoded output stream must be represented in lines of no more The encoded output stream must be represented in lines of no more
than 76 characters each. than 76 characters each.
Special processing is performed if fewer than 24 bits are available Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. There are three possibilities: at the end of the data being encoded. There are three possibilities:
1. The last data group has 24 bits (3 octets). No special 1. The last data group has 24 bits (3 octets). No special processing
processing is needed. is needed.
2. The last data group has 16 bits (2 octets). The first two 6-bit 2. The last data group has 16 bits (2 octets). The first two 6-bit
groups are processed as above. The third (incomplete) data group groups are processed as above. The third (incomplete) data group
has two zero-value bits added to it, and is processed as above. has two zero-value bits added to it, and is processed as above. A
A pad character (=) is added to the output. pad character (=) is added to the output.
3. The last data group has 8 bits (1 octet). The first 6-bit group 3. The last data group has 8 bits (1 octet). The first 6-bit group
is processed as above. The second (incomplete) data group has is processed as above. The second (incomplete) data group has
four zero-value bits added to it, and is processed as above. Two four zero-value bits added to it, and is processed as above. Two
pad characters (=) are added to the output. pad characters (=) are added to the output.
6.4. Decoding Radix-64 6.4. Decoding Radix-64
In Radix-64 data, characters other than those in the table, line In Radix-64 data, characters other than those in the table, line
breaks, and other white space probably indicate a transmission breaks, and other white space probably indicate a transmission error,
error, about which a warning message or even a message rejection about which a warning message or even a message rejection might be
might be appropriate under some circumstances. Decoding software appropriate under some circumstances. Decoding software must ignore
must ignore all white space. all white space.
Because it is used only for padding at the end of the data, the Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the occurrence of any "=" characters may be taken as evidence that the
end of the data has been reached (without truncation in transit). No end of the data has been reached (without truncation in transit). No
such assurance is possible, however, when the number of octets such assurance is possible, however, when the number of octets
transmitted was a multiple of three and no "=" characters are transmitted was a multiple of three and no "=" characters are
present. present.
6.5. Examples of Radix-64 6.5. Examples of Radix-64
Input data: 0x14fb9c03d97e Input data: 0x14FB9C03D97E
Hex: 1 4 f b 9 c | 0 3 d 9 7 e Hex: 1 4 F B 9 C | 0 3 D 9 7 E
8-bit: 00010100 11111011 10011100 | 00000011 11011001 11111110 8-bit: 00010100 11111011 10011100 | 00000011 11011001 11111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100111 111110 6-bit: 000101 001111 101110 011100 | 000000 111101 100111 111110
Decimal: 5 15 46 28 0 61 37 62 Decimal: 5 15 46 28 0 61 37 62
Output: F P u c A 9 l + Output: F P u c A 9 l +
Input data: 0x14fb9c03d9 Input data: 0x14FB9C03D9
Hex: 1 4 f b 9 c | 0 3 d 9 Hex: 1 4 F B 9 C | 0 3 D 9
8-bit: 00010100 11111011 10011100 | 00000011 11011001 8-bit: 00010100 11111011 10011100 | 00000011 11011001
pad with 00 pad with 00
6-bit: 000101 001111 101110 011100 | 000000 111101 100100 6-bit: 000101 001111 101110 011100 | 000000 111101 100100
Decimal: 5 15 46 28 0 61 36 Decimal: 5 15 46 28 0 61 36
pad with = pad with =
Output: F P u c A 9 k = Output: F P u c A 9 k =
Input data: 0x14fb9c03 Input data: 0x14FB9C03
Hex: 1 4 f b 9 c | 0 3 Hex: 1 4 F B 9 C | 0 3
8-bit: 00010100 11111011 10011100 | 00000011 8-bit: 00010100 11111011 10011100 | 00000011
pad with 0000 pad with 0000
6-bit: 000101 001111 101110 011100 | 000000 110000 6-bit: 000101 001111 101110 011100 | 000000 110000
Decimal: 5 15 46 28 0 48 Decimal: 5 15 46 28 0 48
pad with = = pad with = =
Output: F P u c A w = = Output: F P u c A w = =
6.6. Example of an ASCII Armored Message 6.6. Example of an ASCII Armored Message
-----BEGIN PGP MESSAGE----- -----BEGIN PGP MESSAGE-----
Version: OpenPrivacy 0.99 Version: OpenPrivacy 0.99
yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
vBSFjNSiVHsuAA== vBSFjNSiVHsuAA==
=njUN =njUN
-----END PGP MESSAGE----- -----END PGP MESSAGE-----
Note that this example has extra indenting; an actual armored Note that this example has extra indenting; an actual armored message
message would have no leading whitespace. would have no leading whitespace.
7. Cleartext signature framework
It is desirable to be able to sign a textual octet stream without
ASCII armoring the stream itself, so the signed text is still
readable without special software. In order to bind a signature to
such a cleartext, this framework is used. (Note that this framework
is not intended to be reversible. RFC 3156 defines another way to
sign cleartext messages for environments that support MIME.)
The cleartext signed message consists of: 7. Cleartext Signature Framework
- The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a It is desirable to be able to sign a textual octet stream without
single line, ASCII armoring the stream itself, so the signed text is still
readable without special software. In order to bind a signature to
such a cleartext, this framework is used. (Note that this framework
is not intended to be reversible. RFC 3156 [RFC3156] defines another
way to sign cleartext messages for environments that support MIME.)
The cleartext signed message consists of:
- One or more "Hash" Armor Headers, - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
single line,
- Exactly one empty line not included into the message digest, - One or more "Hash" Armor Headers,
- The dash-escaped cleartext that is included into the message - Exactly one empty line not included into the message digest,
digest,
- The ASCII armored signature(s) including the '-----BEGIN PGP - The dash-escaped cleartext that is included into the message
SIGNATURE-----' Armor Header and Armor Tail Lines. digest,
If the "Hash" armor header is given, the specified message digest - The ASCII armored signature(s) including the '-----BEGIN PGP
algorithm(s) are used for the signature. If there are no such SIGNATURE-----' Armor Header and Armor Tail Lines.
headers, MD5 is used. If MD5 is the only hash used, then an
implementation MAY omit this header for improved V2.x compatibility.
If more than one message digest is used in the signature, the "Hash"
armor header contains a comma-delimited list of used message
digests.
Current message digest names are described below with the algorithm If the "Hash" Armor Header is given, the specified message digest
IDs. algorithm(s) are used for the signature. If there are no such
headers, MD5 is used. If MD5 is the only hash used, then an
implementation MAY omit this header for improved V2.x compatibility.
If more than one message digest is used in the signature, the "Hash"
armor header contains a comma-delimited list of used message digests.
An implementation SHOULD add a line break after the cleartext, but Current message digest names are described below with the algorithm
MAY omit it if the cleartext ends with a line break. This is for IDs.
visual clarity.
7.1. Dash-Escaped Text An implementation SHOULD add a line break after the cleartext, but
MAY omit it if the cleartext ends with a line break. This is for
visual clarity.
The cleartext content of the message must also be dash-escaped. 7.1. Dash-Escaped Text
Dash escaped cleartext is the ordinary cleartext where every line The cleartext content of the message must also be dash-escaped.
starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
(0x2D) and space ' ' (0x20). This prevents the parser from
recognizing armor headers of the cleartext itself. An implementation
MAY dash escape any line, SHOULD dash escape lines commencing "From"
followed by a space, and MUST dash escape any line commencing in a
dash. The message digest is computed using the cleartext itself, not
the dash escaped form.
As with binary signatures on text documents, a cleartext signature Dash-escaped cleartext is the ordinary cleartext where every line
is calculated on the text using canonical <CR><LF> line endings. The starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP (0x2D) and space ' ' (0x20). This prevents the parser from
SIGNATURE-----' line that terminates the signed text is not recognizing armor headers of the cleartext itself. An implementation
considered part of the signed text. MAY dash-escape any line, SHOULD dash-escape lines commencing "From"
followed by a space, and MUST dash-escape any line commencing in a
dash. The message digest is computed using the cleartext itself, not
the dash-escaped form.
When reversing dash-escaping, an implementation MUST strip the As with binary signatures on text documents, a cleartext signature is
string "- " if it occurs at the beginning of a line, and SHOULD warn calculated on the text using canonical <CR><LF> line endings. The
on "-" and any character other than a space at the beginning of a line ending (i.e., the <CR><LF>) before the '-----BEGIN PGP
line. SIGNATURE-----' line that terminates the signed text is not
considered part of the signed text.
Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at When reversing dash-escaping, an implementation MUST strip the string
the end of any line is removed when the cleartext signature is "- " if it occurs at the beginning of a line, and SHOULD warn on "-"
generated. and any character other than a space at the beginning of a line.
8. Regular Expressions Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
the end of any line is removed when the cleartext signature is
generated.
A regular expression is zero or more branches, separated by '|'. It 8. Regular Expressions
matches anything that matches one of the branches.
A branch is zero or more pieces, concatenated. It matches a match A regular expression is zero or more branches, separated by '|'. It
for the first, followed by a match for the second, etc. matches anything that matches one of the branches.
A piece is an atom possibly followed by '*', '+', or '?'. An atom A branch is zero or more pieces, concatenated. It matches a match
followed by '*' matches a sequence of 0 or more matches of the atom. for the first, followed by a match for the second, etc.
An atom followed by '+' matches a sequence of 1 or more matches of
the atom. An atom followed by '?' matches a match of the atom, or
the null string.
An atom is a regular expression in parentheses (matching a match for A piece is an atom possibly followed by '*', '+', or '?'. An atom
the regular expression), a range (see below), '.' (matching any followed by '*' matches a sequence of 0 or more matches of the atom.
single character), '^' (matching the null string at the beginning of An atom followed by '+' matches a sequence of 1 or more matches of
the input string), '$' (matching the null string at the end of the the atom. An atom followed by '?' matches a match of the atom, or
input string), a '\' followed by a single character (matching that the null string.
character), or a single character with no other significance
(matching that character).
A range is a sequence of characters enclosed in '[]'. It normally An atom is a regular expression in parentheses (matching a match for
matches any single character from the sequence. If the sequence the regular expression), a range (see below), '.' (matching any
begins with '^', it matches any single character not from the rest single character), '^' (matching the null string at the beginning of
of the sequence. If two characters in the sequence are separated by the input string), '$' (matching the null string at the end of the
'-', this is shorthand for the full list of ASCII characters between input string), a '\' followed by a single character (matching that
them (e.g. '[0-9]' matches any decimal digit). To include a literal character), or a single character with no other significance
']' in the sequence, make it the first character (following a (matching that character).
possible '^'). To include a literal '-', make it the first or last
character.
9. Constants A range is a sequence of characters enclosed in '[]'. It normally
matches any single character from the sequence. If the sequence
begins with '^', it matches any single character not from the rest of
the sequence. If two characters in the sequence are separated
by '-', this is shorthand for the full list of ASCII characters
between them (e.g., '[0-9]' matches any decimal digit). To include a
literal ']' in the sequence, make it the first character (following a
possible '^'). To include a literal '-', make it the first or last
character.
This section describes the constants used in OpenPGP. 9. Constants
Note that these tables are not exhaustive lists; an implementation This section describes the constants used in OpenPGP.
MAY implement an algorithm not on these lists, so long as the
algorithm number(s) are chosen from the private or experimental
algorithm range.
See the section "Notes on Algorithms" below for more discussion of Note that these tables are not exhaustive lists; an implementation
the algorithms. MAY implement an algorithm not on these lists, so long as the
algorithm numbers are chosen from the private or experimental
algorithm range.
9.1. Public Key Algorithms See the section "Notes on Algorithms" below for more discussion of
the algorithms.
ID Algorithm 9.1. Public-Key Algorithms
-- ---------
1 - RSA (Encrypt or Sign) [HAC]
2 - RSA Encrypt-Only [HAC]
3 - RSA Sign-Only [HAC]
16 - Elgamal (Encrypt-Only), see [ELGAMAL] [HAC]
17 - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
18 - Reserved for Elliptic Curve
19 - Reserved for ECDSA
20 - Reserved (formerly Elgamal Encrypt or Sign)
21 - Reserved for Diffie-Hellman (X9.42,
as defined for IETF-S/MIME)
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement DSA for signatures, and Elgamal for ID Algorithm
encryption. Implementations SHOULD implement RSA keys (1). RSA -- ---------
Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be 1 - RSA (Encrypt or Sign) [HAC]
generated, but may be interpreted. See Section 13.5. See Section 2 - RSA Encrypt-Only [HAC]
13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt 3 - RSA Sign-Only [HAC]
or Sign (20), and X9.42 (21). Implementations MAY implement any 16 - Elgamal (Encrypt-Only) [ELGAMAL] [HAC]
other algorithm. 17 - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
18 - Reserved for Elliptic Curve
19 - Reserved for ECDSA
20 - Reserved (formerly Elgamal Encrypt or Sign)
21 - Reserved for Diffie-Hellman (X9.42,
as defined for IETF-S/MIME)
100 to 110 - Private/Experimental algorithm
9.2. Symmetric Key Algorithms Implementations MUST implement DSA for signatures, and Elgamal for
encryption. Implementations SHOULD implement RSA keys (1). RSA
Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be
generated, but may be interpreted. See Section 13.5. See Section
13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt or
Sign (20), and X9.42 (21). Implementations MAY implement any other
algorithm.
ID Algorithm 9.2. Symmetric-Key Algorithms
-- ---------
0 - Plaintext or unencrypted data
1 - IDEA [IDEA]
2 - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
168 bit key derived from 192)
3 - CAST5 (128 bit key, as per RFC 2144)
4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
5 - Reserved
6 - Reserved
7 - AES with 128-bit key [AES]
8 - AES with 192-bit key
9 - AES with 256-bit key
10 - Twofish with 256-bit key [TWOFISH]
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement TripleDES. Implementations SHOULD ID Algorithm
implement AES-128 and CAST5. Implementations that interoperate with -- ---------
PGP 2.6 or earlier need to support IDEA, as that is the only 0 - Plaintext or unencrypted data
symmetric cipher those versions use. Implementations MAY implement 1 - IDEA [IDEA]
any other algorithm. 2 - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
168 bit key derived from 192)
3 - CAST5 (128 bit key, as per [RFC2144])
4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
5 - Reserved
6 - Reserved
7 - AES with 128-bit key [AES]
8 - AES with 192-bit key
9 - AES with 256-bit key
10 - Twofish with 256-bit key [TWOFISH]
100 to 110 - Private/Experimental algorithm
9.3. Compression Algorithms Implementations MUST implement TripleDES. Implementations SHOULD
implement AES-128 and CAST5. Implementations that interoperate with
PGP 2.6 or earlier need to support IDEA, as that is the only
symmetric cipher those versions use. Implementations MAY implement
any other algorithm.
ID Algorithm 9.3. Compression Algorithms
-- ---------
0 - Uncompressed
1 - ZIP [RFC 1951]
2 - ZLIB [RFC 1950]
3 - BZip2 [BZ2]
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement uncompressed data. Implementations ID Algorithm
SHOULD implement ZIP. Implementations MAY implement any other -- ---------
algorithm. 0 - Uncompressed
1 - ZIP [RFC1951]
2 - ZLIB [RFC1950]
3 - BZip2 [BZ2]
100 to 110 - Private/Experimental algorithm
9.4. Hash Algorithms Implementations MUST implement uncompressed data. Implementations
SHOULD implement ZIP. Implementations MAY implement any other
algorithm.
ID Algorithm Text Name 9.4. Hash Algorithms
-- --------- ---- ----
1 - MD5 [HAC] "MD5"
2 - SHA-1 [FIPS180] "SHA1"
3 - RIPE-MD/160 [HAC] "RIPEMD160"
4 - Reserved
5 - Reserved
6 - Reserved
7 - Reserved
8 - SHA256 [FIPS180] "SHA256"
9 - SHA384 [FIPS180] "SHA384"
10 - SHA512 [FIPS180] "SHA512"
11 - SHA224 [FIPS180] "SHA224"
100 to 110 - Private/Experimental algorithm.
Implementations MUST implement SHA-1. Implementations MAY implement ID Algorithm Text Name
other algorithms. MD5 is deprecated. -- --------- ---------
1 - MD5 [HAC] "MD5"
2 - SHA-1 [FIPS180] "SHA1"
3 - RIPE-MD/160 [HAC] "RIPEMD160"
4 - Reserved
5 - Reserved
6 - Reserved
7 - Reserved
8 - SHA256 [FIPS180] "SHA256"
9 - SHA384 [FIPS180] "SHA384"
10 - SHA512 [FIPS180] "SHA512"
11 - SHA224 [FIPS180] "SHA224"
100 to 110 - Private/Experimental algorithm
10. IANA Considerations Implementations MUST implement SHA-1. Implementations MAY implement
other algorithms. MD5 is deprecated.
OpenPGP is highly parameterized and consequently there are a number 10. IANA Considerations
of considerations for allocating parameters for extensions. This
section describes how IANA should look at extensions to the protocol
as described in this document.
10.1. New String-to-Key specifier types OpenPGP is highly parameterized, and consequently there are a number
of considerations for allocating parameters for extensions. This
section describes how IANA should look at extensions to the protocol
as described in this document.
OpenPGP S2K specifiers contain a mechanism for new algorithms to 10.1. New String-to-Key Specifier Types
turn a string into a key. This specification creates a registry of
S2K specifier types. The registry includes the S2K type, the name of
the S2K and a reference to the defining specification. The initial
values for this registry can be found in 3.7.1. Adding a new S2K
specifier MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
10.2. New Packets OpenPGP S2K specifiers contain a mechanism for new algorithms to turn
a string into a key. This specification creates a registry of S2K
specifier types. The registry includes the S2K type, the name of the
S2K, and a reference to the defining specification. The initial
values for this registry can be found in Section 3.7.1. Adding a new
S2K specifier MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
Major new features of OpenPGP are defined though new packet types. 10.2. New Packets
This specification creates a registry of packet types. The registry
includes the packet type, the name of the packet and a reference to
the defining specification. The initial values for this registry can
be found in 4.3. Adding a new packet type MUST be done through the
IETF CONSENSUS method, as described in [RFC2434].
10.2.1. User Attribute Types Major new features of OpenPGP are defined through new packet types.
This specification creates a registry of packet types. The registry
includes the packet type, the name of the packet, and a reference to
the defining specification. The initial values for this registry can
be found in Section 4.3. Adding a new packet type MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
The User Attribute packet permits an extensible mechanism for other 10.2.1. User Attribute Types
types of certificate identification. This specification creates a
registry of User Attribute types. The registry includes the User
Attribute type, the name of the User Attribute and a reference to
the defining specification. The initial values for this registry can
be found in 5.12. Adding a new User Attribute type MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
10.2.1.1. Image Format Subpacket Types The User Attribute packet permits an extensible mechanism for other
types of certificate identification. This specification creates a
registry of User Attribute types. The registry includes the User
Attribute type, the name of the User Attribute, and a reference to
the defining specification. The initial values for this registry can
be found in Section 5.12. Adding a new User Attribute type MUST be
done through the IETF CONSENSUS method, as described in [RFC2434].
Within User Attribute packets, there is an extensible mechanism for 10.2.1.1. Image Format Subpacket Types
other types of image-based user attributes. This specification
creates a registry of Image Attribute subpacket types. The registry
includes the Image Attribute subpacket type, the name of the Image
Attribute subpacket and a reference to the defining specification.
The initial values for this registry can be found in 5.12.1. Adding
a new Image Attribute subpacket type MUST be done through the IETF
CONSENSUS method, as described in [RFC2434].
10.2.2. New Signature Subpackets Within User Attribute packets, there is an extensible mechanism for
other types of image-based user attributes. This specification
creates a registry of Image Attribute subpacket types. The registry
includes the Image Attribute subpacket type, the name of the Image
Attribute subpacket, and a reference to the defining specification.
The initial values for this registry can be found in Section 5.12.1.
Adding a new Image Attribute subpacket type MUST be done through the
IETF CONSENSUS method, as described in [RFC2434].
OpenPGP signatures contain a mechanism for signed (or unsigned) data 10.2.2. New Signature Subpackets
to be added to them for a variety of purposes in the signature
subpackets as discussed in section 5.2.3.1. This specification
creates a registry of signature subpacket types. The registry
includes the signature subpacket type, the name of the subpacket and
a reference to the defining specification. The initial values for
this registry can be found in 5.2.3.1. Adding a new signature
subpacket MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
10.2.2.1. Signature Notation Data Subpackets OpenPGP signatures contain a mechanism for signed (or unsigned) data
to be added to them for a variety of purposes in the Signature
subpackets as discussed in Section 5.2.3.1. This specification
creates a registry of Signature subpacket types. The registry
includes the Signature subpacket type, the name of the subpacket, and
a reference to the defining specification. The initial values for
this registry can be found in Section 5.2.3.1. Adding a new
Signature subpacket MUST be done through the IETF CONSENSUS method,
as described in [RFC2434].
OpenPGP signatures further contain a mechanism for extensions in 10.2.2.1. Signature Notation Data Subpackets
signatures. These are the Notation Data subpackets, which contain a
key/value pair. Notations contain a user space which is completely
unmanaged and an IETF space.
This specification creates a registry of Signature Notation Data OpenPGP signatures further contain a mechanism for extensions in
types. The registry includes the Signature Notation Data type, the signatures. These are the Notation Data subpackets, which contain a
name of the Signature Notation Data, its allowed values, and a key/value pair. Notations contain a user space that is completely
reference to the defining specification. The initial values for this unmanaged and an IETF space.
registry can be found in 5.2.3.16. Adding a new Signature Notation
Data subpacket MUST be done through the EXPERT REVIEW method, as
described in [RFC2434].
10.2.2.2. Key Server Preference Extensions This specification creates a registry of Signature Notation Data
types. The registry includes the Signature Notation Data type, the
name of the Signature Notation Data, its allowed values, and a
reference to the defining specification. The initial values for this
registry can be found in Section 5.2.3.16. Adding a new Signature
Notation Data subpacket MUST be done through the EXPERT REVIEW
method, as described in [RFC2434].
OpenPGP signatures contain a mechanism for preferences to be 10.2.2.2. Key Server Preference Extensions
specified about key servers. This specification creates a registry
of key server preferences. The registry includes the key server
preference, the name of the preference and a reference to the
defining specification. The initial values for this registry can be
found in 5.2.3.17. Adding a new key server preference MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
10.2.2.3. Key Flags Extensions OpenPGP signatures contain a mechanism for preferences to be
specified about key servers. This specification creates a registry
of key server preferences. The registry includes the key server
preference, the name of the preference, and a reference to the
defining specification. The initial values for this registry can be
found in Section 5.2.3.17. Adding a new key server preference MUST
be done through the IETF CONSENSUS method, as described in [RFC2434].
OpenPGP signatures contain a mechanism for flags to be specified 10.2.2.3. Key Flags Extensions
about key usage. This specification creates a registry of key usage
flags. The registry includes the key flags value, the name of the
flag and a reference to the defining specification. The initial
values for this registry can be found in 5.2.3.21. Adding a new key
usage flag MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
10.2.2.4. Reason For Revocation Extensions OpenPGP signatures contain a mechanism for flags to be specified
about key usage. This specification creates a registry of key usage
flags. The registry includes the key flags value, the name of the
flag, and a reference to the defining specification. The initial
values for this registry can be found in Section 5.2.3.21. Adding a
new key usage flag MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
OpenPGP signatures contain a mechanism for flags to be specified 10.2.2.4. Reason for Revocation Extensions
about why a key was revoked. This specification creates a registry
of reason-for-revocation flags. The registry includes the
reason-for-revocation flags value, the name of the flag and a
reference to the defining specification. The initial values for this
registry can be found in 5.2.3.23. Adding a new feature flag MUST be
done through the IETF CONSENSUS method, as described in [RFC2434].
10.2.2.5. Implementation Features OpenPGP signatures contain a mechanism for flags to be specified
about why a key was revoked. This specification creates a registry
of "Reason for Revocation" flags. The registry includes the "Reason
for Revocation" flags value, the name of the flag, and a reference to
the defining specification. The initial values for this registry can
be found in Section 5.2.3.23. Adding a new feature flag MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
OpenPGP signatures contain a mechanism for flags to be specified 10.2.2.5. Implementation Features
stating which optional features an implementation supports. This
specification creates a registry of feature-implementation flags.
The registry includes the feature-implementation flags value, the
name of the flag and a reference to the defining specification. The
initial values for this registry can be found in 5.2.3.24. Adding a
new feature-implementation flag MUST be done through the IETF
CONSENSUS method, as described in [RFC2434].
Also see section 10.6 for more information about when feature flags OpenPGP signatures contain a mechanism for flags to be specified
are needed. stating which optional features an implementation supports. This
specification creates a registry of feature-implementation flags.
The registry includes the feature-implementation flags value, the
name of the flag, and a reference to the defining specification. The
initial values for this registry can be found in Section 5.2.3.24.
Adding a new feature-implementation flag MUST be done through the
IETF CONSENSUS method, as described in [RFC2434].
10.2.3. New Packet Versions Also see Section 13.12 for more information about when feature flags
are needed.
The core OpenPGP packets all have version numbers, and can be 10.2.3. New Packet Versions
revised by introducing a new version of an existing packet. This
specification creates a registry of packet types. The registry
includes the packet type, the number of the version and a reference
to the defining specification. The initial values for this registry
can be found in 5. Adding a new packet version MUST be done through
the IETF CONSENSUS method, as described in [RFC2434].
10.3. New Algorithms The core OpenPGP packets all have version numbers, and can be revised
by introducing a new version of an existing packet. This
specification creates a registry of packet types. The registry
includes the packet type, the number of the version, and a reference
to the defining specification. The initial values for this registry
can be found in Section 5. Adding a new packet version MUST be done
through the IETF CONSENSUS method, as described in [RFC2434].
Chapter 9 lists the core algorithms that OpenPGP uses. Adding in a 10.3. New Algorithms
new algorithm is usually simple. For example, adding in a new
symmetric cipher usually would not need anything more than
allocating a constant for that cipher. If that cipher had other than
a 64-bit or 128-bit block size, there might need to be additional
documentation describing how OpenPGP-CFB mode would be adjusted.
Similarly, when DSA was expanded from a maximum of 1024-bit public
keys to 3072-bit public keys, the revision of FIPS 186 contained
enough information itself to allow implementation. Changes to this
document were emphasis more than required.
10.3.1. Public Key Algorithms Section 9 lists the core algorithms that OpenPGP uses. Adding in a
new algorithm is usually simple. For example, adding in a new
symmetric cipher usually would not need anything more than allocating
a constant for that cipher. If that cipher had other than a 64-bit
or 128-bit block size, there might need to be additional
documentation describing how OpenPGP-CFB mode would be adjusted.
Similarly, when DSA was expanded from a maximum of 1024-bit public
keys to 3072-bit public keys, the revision of FIPS 186 contained
enough information itself to allow implementation. Changes to this
document were made mainly for emphasis.
OpenPGP specifies a number of public key algorithms. This 10.3.1. Public-Key Algorithms
specification creates a registry of public key algorithm
identifiers. The registry includes the algorithm name, its key sizes
and parameters, and a reference to the defining specification. The
initial values for this registry can be found in section 9. Adding a
new public key algorithm MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
10.3.2. Symmetric Key Algorithms OpenPGP specifies a number of public-key algorithms. This
specification creates a registry of public-key algorithm identifiers.
The registry includes the algorithm name, its key sizes and
parameters, and a reference to the defining specification. The
initial values for this registry can be found in Section 9. Adding a
new public-key algorithm MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
OpenPGP specifies a number of symmetric key algorithms. This 10.3.2. Symmetric-Key Algorithms
specification creates a registry of symmetric key algorithm
identifiers. The registry includes the algorithm name, its key sizes
and block size, and a reference to the defining specification. The
initial values for this registry can be found in section 9. Adding a
new symmetric key algorithm MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
10.3.3. Hash Algorithms OpenPGP specifies a number of symmetric-key algorithms. This
specification creates a registry of symmetric-key algorithm
identifiers. The registry includes the algorithm name, its key sizes
and block size, and a reference to the defining specification. The
initial values for this registry can be found in Section 9. Adding a
new symmetric-key algorithm MUST be done through the IETF CONSENSUS
method, as described in [RFC2434].
OpenPGP specifies a number of hash algorithms. This specification 10.3.3. Hash Algorithms
creates a registry of hash algorithm identifiers. The registry
includes the algorithm name, a text representation of that name, its
block size, an OID hash prefix, and a reference to the defining
specification. The initial values for this registry can be found in
section 9 for the algorithm identifiers and text names, and section
5.2.2 for the OIDs and expanded signature prefixes. Adding a new
hash algorithm MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
10.3.4. Compression Algorithms OpenPGP specifies a number of hash algorithms. This specification
creates a registry of hash algorithm identifiers. The registry
includes the algorithm name, a text representation of that name, its
block size, an OID hash prefix, and a reference to the defining
specification. The initial values for this registry can be found in
Section 9 for the algorithm identifiers and text names, and Section
5.2.2 for the OIDs and expanded signature prefixes. Adding a new
hash algorithm MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
OpenPGP specifies a number of compression algorithms. This 10.3.4. Compression Algorithms
specification creates a registry of compression algorithm
identifiers. The registry includes the algorithm name, and a
reference to the defining specification. The initial values for this
registry can be found in section 9.3. Adding a new compression key
algorithm MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
11. Packet Composition OpenPGP specifies a number of compression algorithms. This
specification creates a registry of compression algorithm
identifiers. The registry includes the algorithm name and a
reference to the defining specification. The initial values for this
registry can be found in Section 9.3. Adding a new compression key
algorithm MUST be done through the IETF CONSENSUS method, as
described in [RFC2434].
OpenPGP packets are assembled into sequences in order to create 11. Packet Composition
messages and to transfer keys. Not all possible packet sequences are
meaningful and correct. This section describes the rules for how
packets should be placed into sequences.
11.1. Transferable Public Keys OpenPGP packets are assembled into sequences in order to create
messages and to transfer keys. Not all possible packet sequences are
meaningful and correct. This section describes the rules for how
packets should be placed into sequences.
OpenPGP users may transfer public keys. The essential elements of a 11.1. Transferable Public Keys
transferable public key are:
- One Public Key packet OpenPGP users may transfer public keys. The essential elements of a
transferable public key are as follows:
- Zero or more revocation signatures - One Public-Key packet
- One or more User ID packets - Zero or more revocation signatures
- One or more User ID packets
- After each User ID packet, zero or more signature packets - After each User ID packet, zero or more Signature packets
(certifications) (certifications)
- Zero or more User Attribute packets - Zero or more User Attribute packets
- After each User Attribute packet, zero or more signature packets - After each User Attribute packet, zero or more Signature packets
(certifications) (certifications)
- Zero or more Subkey packets - Zero or more Subkey packets
- After each Subkey packet, one signature packet, plus optionally - After each Subkey packet, one Signature packet, plus optionally a
a revocation. revocation
The Public Key packet occurs first. Each of the following User ID The Public-Key packet occurs first. Each of the following User ID
packets provides the identity of the owner of this public key. If packets provides the identity of the owner of this public key. If
there are multiple User ID packets, this corresponds to multiple there are multiple User ID packets, this corresponds to multiple
means of identifying the same unique individual user; for example, a means of identifying the same unique individual user; for example, a
user may have more than one email address, and construct a User ID user may have more than one email address, and construct a User ID
for each one. for each one.
Immediately following each User ID packet, there are zero or more Immediately following each User ID packet, there are zero or more
signature packets. Each signature packet is calculated on the Signature packets. Each Signature packet is calculated on the
immediately preceding User ID packet and the initial Public Key immediately preceding User ID packet and the initial Public-Key
packet. The signature serves to certify the corresponding public key packet. The signature serves to certify the corresponding public key
and User ID. In effect, the signer is testifying to his or her and User ID. In effect, the signer is testifying to his or her
belief that this public key belongs to the user identified by this belief that this public key belongs to the user identified by this
User ID. User ID.
Within the same section as the User ID packets, there are zero or Within the same section as the User ID packets, there are zero or
more User Attribute packets. Like the User ID packets, a User more User Attribute packets. Like the User ID packets, a User
Attribute packet is followed by zero or more signature packets Attribute packet is followed by zero or more Signature packets
calculated on the immediately preceding User Attribute packet and calculated on the immediately preceding User Attribute packet and the
the initial Public Key packet. initial Public-Key packet.
User Attribute packets and User ID packets may be freely intermixed User Attribute packets and User ID packets may be freely intermixed
in this section, so long as the signatures that follow them are in this section, so long as the signatures that follow them are
maintained on the proper User Attribute or User ID packet. maintained on the proper User Attribute or User ID packet.
After the User ID or Attribute packets there may be zero or more After the User ID packet or Attribute packet, there may be zero or
Subkey packets. In general, subkeys are provided in cases where the more Subkey packets. In general, subkeys are provided in cases where
top-level public key is a signature-only key. However, any V4 key the top-level public key is a signature-only key. However, any V4
may have subkeys, and the subkeys may be encryption-only keys, key may have subkeys, and the subkeys may be encryption-only keys,
signature-only keys, or general-purpose keys. V3 keys MUST NOT have signature-only keys, or general-purpose keys. V3 keys MUST NOT have
subkeys. subkeys.
Each Subkey packet MUST be followed by one Signature packet, which Each Subkey packet MUST be followed by one Signature packet, which
should be a subkey binding signature issued by the top level key. should be a subkey binding signature issued by the top-level key.
For subkeys that can issue signatures, the subkey binding signature For subkeys that can issue signatures, the subkey binding signature
MUST contain an embedded signature subpacket with a primary key MUST contain an Embedded Signature subpacket with a primary key
binding signature (0x19) issued by the subkey on the top level key. binding signature (0x19) issued by the subkey on the top-level key.
Subkey and Key packets may each be followed by a revocation Subkey and Key packets may each be followed by a revocation Signature
Signature packet to indicate that the key is revoked. Revocation packet to indicate that the key is revoked. Revocation signatures
signatures are only accepted if they are issued by the key itself, are only accepted if they are issued by the key itself, or by a key
or by a key that is authorized to issue revocations via a revocation that is authorized to issue revocations via a Revocation Key
key subpacket in a self-signature by the top level key. subpacket in a self-signature by the top-level key.
Transferable public key packet sequences may be concatenated to Transferable public-key packet sequences may be concatenated to allow
allow transferring multiple public keys in one operation. transferring multiple public keys in one operation.
11.2. Transferable Secret Keys 11.2. Transferable Secret Keys
OpenPGP users may transfer secret keys. The format of a transferable OpenPGP users may transfer secret keys. The format of a transferable
secret key is the same as a transferable public key except that secret key is the same as a transferable public key except that
secret key and secret subkey packets are used instead of the public secret-key and secret-subkey packets are used instead of the public
key and public subkey packets. Implementations SHOULD include key and public-subkey packets. Implementations SHOULD include self-
self-signatures on any user IDs and subkeys, as this allows for a signatures on any user IDs and subkeys, as this allows for a complete
complete public key to be automatically extracted from the public key to be automatically extracted from the transferable secret
transferable secret key. Implementations MAY choose to omit the key. Implementations MAY choose to omit the self-signatures,
self-signatures, especially if a transferable public key accompanies especially if a transferable public key accompanies the transferable
the transferable secret key. secret key.
11.3. OpenPGP Messages 11.3. OpenPGP Messages
An OpenPGP message is a packet or sequence of packets that An OpenPGP message is a packet or sequence of packets that
corresponds to the following grammatical rules (comma represents corresponds to the following grammatical rules (comma represents
sequential composition, and vertical bar separates alternatives): sequential composition, and vertical bar separates alternatives):
OpenPGP Message :- Encrypted Message | Signed Message | OpenPGP Message :- Encrypted Message | Signed Message |
Compressed Message | Literal Message. Compressed Message | Literal Message.
Compressed Message :- Compressed Data Packet. Compressed Message :- Compressed Data Packet.
Literal Message :- Literal Data Packet. Literal Message :- Literal Data Packet.
ESK :- Public Key Encrypted Session Key Packet | ESK :- Public-Key Encrypted Session Key Packet |
Symmetric-Key Encrypted Session Key Packet. Symmetric-Key Encrypted Session Key Packet.
ESK Sequence :- ESK | ESK Sequence, ESK. ESK Sequence :- ESK | ESK Sequence, ESK.
Encrypted Data :- Symmetrically Encrypted Data Packet | Encrypted Data :- Symmetrically Encrypted Data Packet |
Symmetrically Encrypted Integrity Protected Data Packet Symmetrically Encrypted Integrity Protected Data Packet
Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data. Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
One-Pass Signed Message :- One-Pass Signature Packet, One-Pass Signed Message :- One-Pass Signature Packet,
OpenPGP Message, Corresponding Signature Packet. OpenPGP Message, Corresponding Signature Packet.
Signed Message :- Signature Packet, OpenPGP Message | Signed Message :- Signature Packet, OpenPGP Message |
One-Pass Signed Message. One-Pass Signed Message.
In addition, decrypting a Symmetrically Encrypted Data Packet or a In addition, decrypting a Symmetrically Encrypted Data packet or a
Symmetrically Encrypted Integrity Protected Data Packet as well as Symmetrically Encrypted Integrity Protected Data packet as well as
decompressing a Compressed Data packet must yield a valid OpenPGP decompressing a Compressed Data packet must yield a valid OpenPGP
Message. Message.
11.4. Detached Signatures 11.4. Detached Signatures
Some OpenPGP applications use so-called "detached signatures." For Some OpenPGP applications use so-called "detached signatures". For
example, a program bundle may contain a file, and with it a second example, a program bundle may contain a file, and with it a second
file that is a detached signature of the first file. These detached file that is a detached signature of the first file. These detached
signatures are simply a signature packet stored separately from the signatures are simply a Signature packet stored separately from the
data that they are a signature of. data for which they are a signature.
12. Enhanced Key Formats 12. Enhanced Key Formats
12.1. Key Structures 12.1. Key Structures
The format of an OpenPGP V3 key is as follows. Entries in square The format of an OpenPGP V3 key is as follows. Entries in square
brackets are optional and ellipses indicate repetition. brackets are optional and ellipses indicate repetition.
RSA Public Key RSA Public Key
[Revocation Self Signature] [Revocation Self Signature]
User ID [Signature ...] User ID [Signature ...]
[User ID [Signature ...] ...] [User ID [Signature ...] ...]
Each signature certifies the RSA public key and the preceding User Each signature certifies the RSA public key and the preceding User
ID. The RSA public key can have many User IDs and each User ID can ID. The RSA public key can have many User IDs and each User ID can
have many signatures. V3 keys are deprecated. Implementations MUST have many signatures. V3 keys are deprecated. Implementations MUST
NOT generate new V3 keys, but MAY continue to use existing ones. NOT generate new V3 keys, but MAY continue to use existing ones.
The format of an OpenPGP V4 key that uses multiple public keys is The format of an OpenPGP V4 key that uses multiple public keys is
similar except that the other keys are added to the end as "subkeys" similar except that the other keys are added to the end as "subkeys"
of the primary key. of the primary key.
Primary-Key Primary-Key
[Revocation Self Signature] [Revocation Self Signature]
[Direct Key Signature...] [Direct Key Signature...]
User ID [Signature ...] User ID [Signature ...]
[User ID [Signature ...] ...] [User ID [Signature ...] ...]
[User Attribute [Signature ...] ...] [User Attribute [Signature ...] ...]
[[Subkey [Binding-Signature-Revocation] [[Subkey [Binding-Signature-Revocation]
Primary-Key-Binding-Signature] ...] Primary-Key-Binding-Signature] ...]
A subkey always has a single signature after it that is issued using A subkey always has a single signature after it that is issued using
the primary key to tie the two keys together. This binding signature the primary key to tie the two keys together. This binding signature
may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can may be in either V3 or V4 format, but SHOULD be V4. Subkeys that can
issue signatures MUST have a V4 binding signature due to the issue signatures MUST have a V4 binding signature due to the REQUIRED
REQUIRED embedded primary key binding signature. embedded primary key binding signature.
In the above diagram, if the binding signature of a subkey has been In the above diagram, if the binding signature of a subkey has been
revoked, the revoked key may be removed, leaving only one key. revoked, the revoked key may be removed, leaving only one key.
In a V4 key, the primary key MUST be a key capable of certification. In a V4 key, the primary key MUST be a key capable of certification.
The subkeys may be keys of any other type. There may be other The subkeys may be keys of any other type. There may be other
constructions of V4 keys, too. For example, there may be a constructions of V4 keys, too. For example, there may be a single-
single-key RSA key in V4 format, a DSA primary key with an RSA key RSA key in V4 format, a DSA primary key with an RSA encryption
encryption key, or RSA primary key with an Elgamal subkey, etc. key, or RSA primary key with an Elgamal subkey, etc.
It is also possible to have a signature-only subkey. This permits a It is also possible to have a signature-only subkey. This permits a
primary key that collects certifications (key signatures) but is primary key that collects certifications (key signatures), but is
used only used for certifying subkeys that are used for encryption used only for certifying subkeys that are used for encryption and
and signatures. signatures.
12.2. Key IDs and Fingerprints 12.2. Key IDs and Fingerprints
For a V3 key, the eight-octet key ID consists of the low 64 bits of For a V3 key, the eight-octet Key ID consists of the low 64 bits of
the public modulus of the RSA key. the public modulus of the RSA key.
The fingerprint of a V3 key is formed by hashing the body (but not The fingerprint of a V3 key is formed by hashing the body (but not
the two-octet length) of the MPIs that form the key material (public the two-octet length) of the MPIs that form the key material (public
modulus n, followed by exponent e) with MD5. Note that both V3 keys modulus n, followed by exponent e) with MD5. Note that both V3 keys
and MD5 are deprecated. and MD5 are deprecated.
A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99, A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
followed by the two-octet packet length, followed by the entire followed by the two-octet packet length, followed by the entire
Public Key packet starting with the version field. The key ID is the Public-Key packet starting with the version field. The Key ID is the
low order 64 bits of the fingerprint. Here are the fields of the low-order 64 bits of the fingerprint. Here are the fields of the
hash material, with the example of a DSA key: hash material, with the example of a DSA key:
a.1) 0x99 (1 octet) a.1) 0x99 (1 octet)
a.2) high order length octet of (b)-(f) (1 octet) a.2) high-order length octet of (b)-(e) (1 octet)
a.3) low-order length octet of (b)-(e) (1 octet)
a.3) low order length octet of (b)-(f) (1 octet)
b) version number = 4 (1 octet); b) version number = 4 (1 octet);
c) time stamp of key creation (4 octets); c) timestamp of key creation (4 octets);
d) algorithm (1 octet): 17 = DSA (example); d) algorithm (1 octet): 17 = DSA (example);
e) Algorithm specific fields. e) Algorithm-specific fields.
Algorithm Specific Fields for DSA keys (example): Algorithm-Specific Fields for DSA keys (example):
e.1) MPI of DSA prime p; e.1) MPI of DSA prime p;
e.2) MPI of DSA group order q (q is a prime divisor of p-1); e.2) MPI of DSA group order q (q is a prime divisor of p-1);
e.3) MPI of DSA group generator g; e.3) MPI of DSA group generator g;
e.4) MPI of DSA public key value y (= g**x mod p where x is secret). e.4) MPI of DSA public-key value y (= g**x mod p where x is secret).
Note that it is possible for there to be collisions of key IDs -- Note that it is possible for there to be collisions of Key IDs -- two
two different keys with the same key ID. Note that there is a much different keys with the same Key ID. Note that there is a much
smaller, but still non-zero probability that two different keys have smaller, but still non-zero, probability that two different keys have
the same fingerprint. the same fingerprint.
Also note that if V3 and V4 format keys share the same RSA key Also note that if V3 and V4 format keys share the same RSA key
material, they will have different key IDs as well as different material, they will have different Key IDs as well as different
fingerprints. fingerprints.
Finally, the key ID and fingerprint of a subkey are calculated in Finally, the Key ID and fingerprint of a subkey are calculated in the
the same way as for a primary key, including the 0x99 as the first same way as for a primary key, including the 0x99 as the first octet
octet (even though this is not a valid packet ID for a public (even though this is not a valid packet ID for a public subkey).
subkey).
13. Notes on Algorithms 13. Notes on Algorithms
13.1. PKCS#1 Encoding In OpenPGP 13.1. PKCS#1 Encoding in OpenPGP
This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
EMSA-PKCS1-v1_5. However, the calling conventions of these functions EMSA-PKCS1-v1_5. However, the calling conventions of these functions
has changed in the past. To avoid potential confusion and has changed in the past. To avoid potential confusion and
interoperability problems, we are including local copies in this interoperability problems, we are including local copies in this
document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC-3447 document, adapted from those in PKCS#1 v2.1 [RFC3447]. RFC 3447
should be treated as the ultimate authority on PKCS#1 for OpenPGP. should be treated as the ultimate authority on PKCS#1 for OpenPGP.
Nonetheless, we believe that there is value in having a Nonetheless, we believe that there is value in having a self-
self-contained document that avoids problems in the future with contained document that avoids problems in the future with needed
needed changes in the conventions. changes in the conventions.
13.1.1. EME-PKCS1-v1_5-ENCODE 13.1.1. EME-PKCS1-v1_5-ENCODE
Input: Input:
k = the length in octets of the key modulus k = the length in octets of the key modulus
M = message to be encoded, an octet string of length mLen, where M = message to be encoded, an octet string of length mLen, where
mLen <= k - 11 mLen <= k - 11
Output: Output:
EM = encoded message, an octet string of length k EM = encoded message, an octet string of length k
Error: "message too long" Error: "message too long"
1. Length checking: If mLen > k - 11, output "message too long" and 1. Length checking: If mLen > k - 11, output "message too long" and
stop. stop.
2. Generate an octet string PS of length k - mLen - 3 consisting of 2. Generate an octet string PS of length k - mLen - 3 consisting of
pseudo-randomly generated nonzero octets. The length of PS will pseudo-randomly generated nonzero octets. The length of PS will
be at least eight octets. be at least eight octets.
3. Concatenate PS, the message M, and other padding to form an 3. Concatenate PS, the message M, and other padding to form an
encoded message EM of length k octets as encoded message EM of length k octets as
EM = 0x00 || 0x02 || PS || 0x00 || M. EM = 0x00 || 0x02 || PS || 0x00 || M.
4. Output EM. 4. Output EM.
13.1.2. EME-PKCS1-v1_5-DECODE 13.1.2. EME-PKCS1-v1_5-DECODE
Input: Input:
EM = encoded message, an octet string EM = encoded message, an octet string
Output: Output:
M = message, an octet string M = message, an octet string
Error: "decryption error" Error: "decryption error"
To decode an EME-PKCS1_v1_5 message, separate the encoded message EM To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
into an octet string PS consisting of nonzero octets and a message M into an octet string PS consisting of nonzero octets and a message M
as as follows
EM = 0x00 || 0x02 || PS || 0x00 || M. EM = 0x00 || 0x02 || PS || 0x00 || M.
If the first octet of EM does not have hexadecimal value 0x00, if If the first octet of EM does not have hexadecimal value 0x00, if the
the second octet of EM does not have hexadecimal value 0x02, if second octet of EM does not have hexadecimal value 0x02, if there is
there is no octet with hexadecimal value 0x00 to separate PS from M, no octet with hexadecimal value 0x00 to separate PS from M, or if the
or if the length of PS is less than 8 octets, output "decryption length of PS is less than 8 octets, output "decryption error" and
error" and stop. See also the security note in section 13 regarding stop. See also the security note in Section 14 regarding differences
differences in reporting between a decryption error and a padding in reporting between a decryption error and a padding error.
error.
13.1.3. EMSA-PKCS1-v1_5 13.1.3. EMSA-PKCS1-v1_5
This encoding method is deterministic and only has an encoding This encoding method is deterministic and only has an encoding
operation. operation.
Option: Option:
Hash hash function (hLen denotes the length in octets of the hash Hash - a hash function in which hLen denotes the length in octets of
function output) the hash function output
Input: Input:
M = message to be encoded M = message to be encoded
mL = intended length in octets of the encoded message, at least tLen mL = intended length in octets of the encoded message, at least tLen
+ 11, where tLen is the octet length of the DER encoding T of a + 11, where tLen is the octet length of the DER encoding T of a
certain value computed during the encoding operation certain value computed during the encoding operation
Output: Output:
EM = encoded message, an octet string of length emLen EM = encoded message, an octet string of length emLen
Errors: "message too long"; "intended encoded message length too Errors: "message too long"; "intended encoded message length too
short" short"
Steps: Steps:
1. Apply the hash function to the message M to produce a hash value 1. Apply the hash function to the message M to produce a hash value
H: H:
H = Hash(M). H = Hash(M).
If the hash function outputs "message too long," output "message If the hash function outputs "message too long," output "message
too long" and stop. too long" and stop.
2. Using the list in section 5.2.2, produce an ASN.1 DER value for 2. Using the list in Section 5.2.2, produce an ASN.1 DER value for
the hash function used. Let T be the full hash prefix from the hash function used. Let T be the full hash prefix from
section 5.2.2, and let tLen be the length in octets of T. Section 5.2.2, and let tLen be the length in octets of T.
3. If emLen < tLen + 11, output "intended encoded message length 3. If emLen < tLen + 11, output "intended encoded message length
too short" and stop. too short" and stop.
4. Generate an octet string PS consisting of emLen - tLen - 3 4. Generate an octet string PS consisting of emLen - tLen - 3
octets with hexadecimal value 0xff. The length of PS will be at octets with hexadecimal value 0xFF. The length of PS will be at
least 8 octets. least 8 octets.
5. Concatenate PS, the hash prefix T, and other padding to form the 5. Concatenate PS, the hash prefix T, and other padding to form the
encoded message EM as encoded message EM as
EM = 0x00 || 0x01 || PS || 0x00 || T. EM = 0x00 || 0x01 || PS || 0x00 || T.
6. Output EM. 6. Output EM.
13.2. Symmetric Algorithm Preferences 13.2. Symmetric Algorithm Preferences
The symmetric algorithm preference is an ordered list of algorithms The symmetric algorithm preference is an ordered list of algorithms
that the keyholder accepts. Since it is found on a self-signature, that the keyholder accepts. Since it is found on a self-signature,
it is possible that a keyholder may have multiple, different it is possible that a keyholder may have multiple, different
preferences. For example, Alice may have TripleDES only specified preferences. For example, Alice may have TripleDES only specified
for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
for "alice@home.org". Note that it is also possible for preferences "alice@home.org". Note that it is also possible for preferences to
to be in a subkey's binding signature. be in a subkey's binding signature.
Since TripleDES is the MUST-implement algorithm, if it is not Since TripleDES is the MUST-implement algorithm, if it is not
explicitly in the list, it is tacitly at the end. However, it is explicitly in the list, it is tacitly at the end. However, it is
good form to place it there explicitly. Note also that if an good form to place it there explicitly. Note also that if an
implementation does not implement the preference, then it is implementation does not implement the preference, then it is
implicitly a TripleDES-only implementation. implicitly a TripleDES-only implementation.
An implementation MUST NOT use a symmetric algorithm that is not in An implementation MUST NOT use a symmetric algorithm that is not in
the recipient's preference list. When encrypting to more than one the recipient's preference list. When encrypting to more than one
recipient, the implementation finds a suitable algorithm by taking recipient, the implementation finds a suitable algorithm by taking
the intersection of the preferences of the recipients. Note that the the intersection of the preferences of the recipients. Note that the
MUST-implement algorithm, TripleDES, ensures that the intersection MUST-implement algorithm, TripleDES, ensures that the intersection is
is not null. The implementation may use any mechanism to pick an not null. The implementation may use any mechanism to pick an
algorithm in the intersection. algorithm in the intersection.
If an implementation can decrypt a message that a keyholder doesn't If an implementation can decrypt a message that a keyholder doesn't
have in their preferences, the implementation SHOULD decrypt the have in their preferences, the implementation SHOULD decrypt the
message anyway, but MUST warn the keyholder that the protocol has message anyway, but MUST warn the keyholder that the protocol has
been violated. For example, suppose that Alice, above, has software been violated. For example, suppose that Alice, above, has software
that implements all algorithms in this specification. Nonetheless, that implements all algorithms in this specification. Nonetheless,
she prefers subsets for work or home. If she is sent a message she prefers subsets for work or home. If she is sent a message
encrypted with IDEA, which is not in her preferences, the software encrypted with IDEA, which is not in her preferences, the software
warns her that someone sent her an IDEA-encrypted message, but it warns her that someone sent her an IDEA-encrypted message, but it
would ideally decrypt it anyway. would ideally decrypt it anyway.
13.3. Other Algorithm Preferences 13.3. Other Algorithm Preferences
Other algorithm preferences work similarly to the symmetric Other algorithm preferences work similarly to the symmetric algorithm
algorithm preference, in that they specify which algorithms the preference, in that they specify which algorithms the keyholder
keyholder accepts. There are two interesting cases that other accepts. There are two interesting cases that other comments need to
comments need to be made about, though, the compression preferences be made about, though, the compression preferences and the hash
and the hash preferences. preferences.
13.3.1. Compression Preferences 13.3.1. Compression Preferences
Compression has been an integral part of PGP since its first days. Compression has been an integral part of PGP since its first days.
OpenPGP and all previous versions of PGP have offered compression.
In this specification, the default is for messages to be compressed,
although an implementation is not required to do so. Consequently,
the compression preference gives a way for a keyholder to request
that messages not be compressed, presumably because they are using a
minimal implementation that does not include compression.
Additionally, this gives a keyholder a way to state that it can
support alternate algorithms.
OpenPGP and all previous versions of PGP have offered compression. Like the algorithm preferences, an implementation MUST NOT use an
In this specification, the default is for messages to be compressed, algorithm that is not in the preference vector. If the preferences
although an implementation is not required to do so. Consequently, are not present, then they are assumed to be [ZIP(1),
the compression preference gives a way for a keyholder to request Uncompressed(0)].
that messages not be compressed, presumably because they are using a
minimal implementation that does not include compression.
Additionally, this gives a keyholder a way to state that it can
support alternate algorithms.
Like the algorithm preferences, an implementation MUST NOT use an Additionally, an implementation MUST implement this preference to the
algorithm that is not in the preference vector. If the preferences degree of recognizing when to send an uncompressed message. A robust
are not present, then they are assumed to be [ZIP(1), implementation would satisfy this requirement by looking at the
UNCOMPRESSED(0)]. recipient's preference and acting accordingly. A minimal
implementation can satisfy this requirement by never generating a
compressed message, since all implementations can handle messages
that have not been compressed.
Additionally, an implementation MUST implement this preference to 13.3.2. Hash Algorithm Preferences
the degree of recognizing when to send an uncompressed message. A
robust implementation would satisfy this requirement by looking at
the recipient's preference and acting accordingly. A minimal
implementation can satisfy this requirement by never generating a
compressed message, since all implementations can handle messages
that have not been compressed.
13.3.2. Hash Algorithm Preferences Typically, the choice of a hash algorithm is something the signer
does, rather than the verifier, because a signer rarely knows who is
going to be verifying the signature. This preference, though, allows
a protocol based upon digital signatures ease in negotiation.
Typically, the choice of a hash algorithm is something the signer Thus, if Alice is authenticating herself to Bob with a signature, it
does, rather than the verifier, because a signer rarely knows who is makes sense for her to use a hash algorithm that Bob's software uses.
going to be verifying the signature. This preference, though, allows This preference allows Bob to state in his key which algorithms Alice
a protocol based upon digital signatures ease in negotiation. may use.
Thus, if Alice is authenticating herself to Bob with a signature, it Since SHA1 is the MUST-implement hash algorithm, if it is not
makes sense for her to use a hash algorithm that Bob's software explicitly in the list, it is tacitly at the end. However, it is
uses. This preference allows Bob to state in his key which good form to place it there explicitly.
algorithms Alice may use.
Since SHA1 is the MUST-implement hash algorithm, if it is not 13.4. Plaintext
explicitly in the list, it is tacitly at the end. However, it is
good form to place it there explicitly.
13.4. Plaintext Algorithm 0, "plaintext", may only be used to denote secret keys that
are stored in the clear. Implementations MUST NOT use plaintext in
Symmetrically Encrypted Data packets; they must use Literal Data
packets to encode unencrypted or literal data.
Algorithm 0, "plaintext," may only be used to denote secret keys 13.5. RSA
that are stored in the clear. Implementations MUST NOT use plaintext
in Symmetrically Encrypted Data Packets; they must use Literal Data
Packets to encode unencrypted or literal data.
13.5. RSA There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only
keys. These types are deprecated. The "key flags" subpacket in a
signature is a much better way to express the same idea, and
generalizes it to all algorithms. An implementation SHOULD NOT
create such a key, but MAY interpret it.
There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only An implementation SHOULD NOT implement RSA keys of size less than
keys. These types are deprecated. The "key flags" subpacket in a 1024 bits.
signature is a much better way to express the same idea, and
generalizes it to all algorithms. An implementation SHOULD NOT
create such a key, but MAY interpret it.
An implementation SHOULD NOT implement RSA keys of size less than 13.6. DSA
1024 bits.
13.6. DSA An implementation SHOULD NOT implement DSA keys of size less than
1024 bits. It MUST NOT implement a DSA key with a q size of less
than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the
q size MUST be a multiple of 8 bits. The Digital Signature Standard
(DSS) [FIPS186] specifies that DSA be used in one of the following
ways:
An implementation SHOULD NOT implement DSA keys of size less than * 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384, or
1024 bits. It MUST NOT implement a DSA key with a q size of less SHA-512 hash
than 160 bits. DSA keys MUST also be a multiple of 64 bits, and the
q size MUST be a multiple of 8 bits. The Digital Signature Standard
(DSS) [FIPS186] specifies that DSA be used in one of the following
ways:
* 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384 or * 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384, or SHA-512
SHA-512 hash hash
* 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384 or SHA-512 * 2048-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash
hash
* 2048-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash * 3072-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash
* 3072-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash The above key and q size pairs were chosen to best balance the
strength of the key with the strength of the hash. Implementations
SHOULD use one of the above key and q size pairs when generating DSA
keys. If DSS compliance is desired, one of the specified SHA hashes
must be used as well. [FIPS186] is the ultimate authority on DSS,
and should be consulted for all questions of DSS compliance.
The above key and q size pairs were chosen to best balance the Note that earlier versions of this standard only allowed a 160-bit q
strength of the key with the strength of the hash. Implementations with no truncation allowed, so earlier implementations may not be
SHOULD use one of the above key and q size pairs when generating DSA able to handle signatures with a different q size or a truncated
keys. If DSS compliance is desired, one of the specified SHA hashes hash.
must be used as well. [FIPS186] is the ultimate authority on DSS,
and should be consulted for all questions of DSS compliance.
Note that earlier versions of this standard only allowed a 160-bit q 13.7. Elgamal
with no truncation allowed, so earlier implementations may not be
able to handle signatures with a different q size or a truncated
hash.
13.7. Elgamal An implementation SHOULD NOT implement Elgamal keys of size less than
1024 bits.
An implementation SHOULD NOT implement Elgamal keys of size less 13.8. Reserved Algorithm Numbers
than 1024 bits.
13.8. Reserved Algorithm Numbers A number of algorithm IDs have been reserved for algorithms that
would be useful to use in an OpenPGP implementation, yet there are
issues that prevent an implementer from actually implementing the
algorithm. These are marked in Section 9.1, "Public-Key Algorithms",
as "reserved for".
A number of algorithm IDs have been reserved for algorithms that The reserved public-key algorithms, Elliptic Curve (18), ECDSA (19),
would be useful to use in an OpenPGP implementation, yet there are and X9.42 (21), do not have the necessary parameters, parameter
issues that prevent an implementer from actually implementing the order, or semantics defined.
algorithm. These are marked in the Public Algorithms section as
"(reserved for)".
The reserved public key algorithms, Elliptic Curve (18), ECDSA (19), Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
and X9.42 (21) do not have the necessary parameters, parameter with a public-key identifier of 20. These are no longer permitted.
order, or semantics defined. An implementation MUST NOT generate such keys. An implementation
MUST NOT generate Elgamal signatures. See [BLEICHENBACHER].
Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures 13.9. OpenPGP CFB Mode
with a public key identifier of 20. These are no longer permitted.
An implementation MUST NOT generate such keys. An implementation
MUST NOT generate Elgamal signatures. See [BLEICHENBACHER].
13.9. OpenPGP CFB mode OpenPGP does symmetric encryption using a variant of Cipher Feedback
mode (CFB mode). This section describes the procedure it uses in
detail. This mode is what is used for Symmetrically Encrypted Data
Packets; the mechanism used for encrypting secret-key material is