draft-ietf-smime-3851bis-02.txt   draft-ietf-smime-3851bis-03.txt 
S/MIME WG Blake Ramsdell, SendMail S/MIME WG Blake Ramsdell, SendMail
Internet Draft Sean Turner, IECA Internet Draft Sean Turner, IECA
Intended Status: Standard Track May 12, 2008 Intended Status: Standard Track June 3, 2008
Obsoletes: 3851 (when approved) Obsoletes: 3851 (when approved)
Expires: November 12, 2008 Expires: December 3, 2008
Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2
Message Specification Message Specification
draft-ietf-smime-3851bis-02.txt draft-ietf-smime-3851bis-03.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2008). Copyright (C) The IETF Trust (2008).
Abstract Abstract
This document defines Secure/Multipurpose Internet Mail Extensions This document defines Secure/Multipurpose Internet Mail Extensions
(S/MIME) version 3.2. S/MIME provides a consistent way to send and (S/MIME) version 3.2. S/MIME provides a consistent way to send and
receive secure MIME data. Digital signatures provide authentication, receive secure MIME data. Digital signatures provide authentication,
message integrity, and non-repudiation with proof of origin. message integrity, and non-repudiation with proof of origin.
Encryption provides data confidentiality. Compression can be used to Encryption provides data confidentiality. Compression can be used to
reduce data size. This document obsoletes RFC 3851. reduce data size. This document obsoletes RFC 3851.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].
We define some additional terms here:
SHOULD+ This term means the same as SHOULD. However, it is likely
that an algorithm marked as SHOULD+ will be promoted at some
future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, an algorithm
marked as SHOULD- may be deprecated to a MAY in a future version
of this document.
MUST- This term means the same as MUST. However, we expect at some
point that this algorithm will no longer be a MUST in a future
document. Although its status will be determined at a later
time, it is reasonable to expect that if a future revision of a
document alters the status of a MUST- algorithm, it will remain
at least a SHOULD or a SHOULD-.
Discussion Discussion
This draft is being discussed on the 'ietf-smime' mailing list. To This draft is being discussed on the 'ietf-smime' mailing list. To
subscribe, send a message to ietf-smime-request@imc.org with the subscribe, send a message to ietf-smime-request@imc.org with the
single word subscribe in the body of the message. There is a Web site single word subscribe in the body of the message. There is a Web site
for the mailing list at <http://www.imc.org/ietf-smime/>. for the mailing list at <http://www.imc.org/ietf-smime/>.
Table of Contents Table of Contents
1. Introduction...................................................4 1. Introduction...................................................3
1.1. Specification Overview....................................4 1.1. Specification Overview....................................4
1.2. Definitions...............................................5 1.2. Definitions...............................................5
1.3. Compatibility with Prior Practice of S/MIME...............6 1.3. Conventions used in this document.........................6
1.4. Changes Since S/MIME v3...................................6 1.4. Compatibility with Prior Practice of S/MIME...............6
1.5. Changes Since S/MIME v3.1.................................7 1.5. Changes From S/MIME v3 to S/MIME v3.1.....................6
2. CMS Options....................................................7 1.6. Changes Since S/MIME v3.1.................................7
2. CMS Options....................................................8
2.1. DigestAlgorithmIdentifier.................................8 2.1. DigestAlgorithmIdentifier.................................8
2.2. SignatureAlgorithmIdentifier..............................8 2.2. SignatureAlgorithmIdentifier..............................8
2.3. KeyEncryptionAlgorithmIdentifier..........................8 2.3. KeyEncryptionAlgorithmIdentifier..........................9
2.4. General Syntax............................................9 2.4. General Syntax............................................9
2.4.1. Data Content Type....................................9 2.4.1. Data Content Type....................................9
2.4.2. SignedData Content Type..............................9 2.4.2. SignedData Content Type..............................9
2.4.3. EnvelopedData Content Type...........................9 2.4.3. EnvelopedData Content Type..........................10
2.4.4. CompressedData Content Type..........................9 2.4.4. CompressedData Content Type.........................10
2.5. Attributes and the SignerInfo Type.......................10 2.5. Attributes and the SignerInfo Type.......................10
2.5.1. Signing-Time Attribute..............................10 2.5.1. Signing-Time Attribute..............................11
2.5.2. SMIMECapabilities Attribute.........................11 2.5.2. SMIMECapabilities Attribute.........................11
2.5.3. Encryption Key Preference Attribute.................12 2.5.3. Encryption Key Preference Attribute.................12
2.5.3.1. Selection of Recipient Key Management 2.5.3.1. Selection of Recipient Key Management
Certificate....................................13 Certificate....................................13
2.6. SignerIdentifier SignerInfo Type.........................14 2.6. SignerIdentifier SignerInfo Type.........................14
2.7. ContentEncryptionAlgorithmIdentifier.....................14 2.7. ContentEncryptionAlgorithmIdentifier.....................14
2.7.1. Deciding Which Encryption Method To Use.............14 2.7.1. Deciding Which Encryption Method To Use.............14
2.7.1.1. Rule 1: Known Capabilities.....................15 2.7.1.1. Rule 1: Known Capabilities.....................15
2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of 2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version
S/MIME..................................................15 of S/MIME..............................16
2.7.2. Choosing Weak Encryption............................16 2.7.2. Choosing Weak Encryption............................16
2.7.3. Multiple Recipients.................................16 2.7.3. Multiple Recipients.................................16
3. Creating S/MIME Messages......................................16 3. Creating S/MIME Messages......................................16
3.1. Preparing the MIME Entity for Signing, Enveloping or 3.1. Preparing the MIME Entity for Signing, Enveloping or
Compressing..............................................17 Compressing..............................................17
3.1.1. Canonicalization....................................18 3.1.1. Canonicalization....................................18
3.1.2. Transfer Encoding...................................19 3.1.2. Transfer Encoding...................................19
3.1.3. Transfer Encoding for Signing Using multipart/signed19 3.1.3. Transfer Encoding for Signing Using
multipart/signed....................................20
3.1.4. Sample Canonical MIME Entity........................20 3.1.4. Sample Canonical MIME Entity........................20
3.2. The application/pkcs7-mime Type..........................21 3.2. The application/pkcs7-mime Media Type....................21
3.2.1. The name and filename Parameters....................22 3.2.1. The name and filename Parameters....................22
3.2.2. The smime-type parameter............................23 3.2.2. The smime-type parameter............................23
3.3. Creating an Enveloped-only Message.......................23 3.3. Creating an Enveloped-only Message.......................24
3.4. Creating a Signed-only Message...........................24 3.4. Creating a Signed-only Message...........................24
3.4.1. Choosing a Format for Signed-only Messages..........24 3.4.1. Choosing a Format for Signed-only Messages..........25
3.4.2. Signing Using application/pkcs7-mime with SignedData25 3.4.2. Signing Using application/pkcs7-mime with
SignedData..........................................25
3.4.3. Signing Using the multipart/signed Format...........26 3.4.3. Signing Using the multipart/signed Format...........26
3.4.3.1. The application/pkcs7-signature MIME Type......26 3.4.3.1. The application/pkcs7-signature Media Type.....26
3.4.3.2. Creating a multipart/signed Message............26 3.4.3.2. Creating a multipart/signed Message............26
3.4.3.3. Sample multipart/signed Message................27 3.4.3.3. Sample multipart/signed Message................28
3.5. Creating an Compressed-only Message......................28 3.5. Creating an Compressed-only Message......................28
3.6. Multiple Operations......................................29 3.6. Multiple Operations......................................29
3.7. Creating a Certificate Management Message................29 3.7. Creating a Certificate Management Message................30
3.8. Registration Requests....................................30 3.8. Registration Requests....................................30
3.9. Identifying an S/MIME Message............................30 3.9. Identifying an S/MIME Message............................31
4. Certificate Processing........................................31 4. Certificate Processing........................................31
4.1. Key Pair Generation......................................31 4.1. Key Pair Generation......................................31
4.2. Signature Generation.....................................32
4.3. Signature Verification...................................32
4.4. Encryption...............................................32
4.5. Decryption...............................................32
5. IANA Considerations...........................................32 5. IANA Considerations...........................................32
6. Security Considerations.......................................32 6. Security Considerations.......................................33
Appendix A. ASN.1 Module.........................................34 Appendix A. ASN.1 Module.........................................35
Appendix B. References...........................................36 Appendix B. References...........................................37
B.1. Normative References.....................................36 B.1. Normative References.....................................37
B.2. Informative References...................................37 B.2. Informative References...................................39
Appendix C. Acknowledgements.....................................40
1. Introduction 1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
consistent way to send and receive secure MIME data. Based on the consistent way to send and receive secure MIME data. Based on the
popular Internet MIME standard, S/MIME provides the following popular Internet MIME standard, S/MIME provides the following
cryptographic security services for electronic messaging cryptographic security services for electronic messaging
applications: authentication, message integrity and non-repudiation applications: authentication, message integrity and non-repudiation
of origin (using digital signatures), and data confidentiality (using of origin (using digital signatures), and data confidentiality (using
encryption). encryption). As a supplementary service, S/MIME provides for message
compression.
S/MIME can be used by traditional mail user agents (MUAs) to add S/MIME can be used by traditional mail user agents (MUAs) to add
cryptographic security services to mail that is sent, and to cryptographic security services to mail that is sent, and to
interpret cryptographic security services in mail that is received. interpret cryptographic security services in mail that is received.
However, S/MIME is not restricted to mail; it can be used with any However, S/MIME is not restricted to mail; it can be used with any
transport mechanism that transports MIME data, such as HTTP. As transport mechanism that transports MIME data, such as HTTP or SIP.
such, S/MIME takes advantage of the object-based features of MIME and As such, S/MIME takes advantage of the object-based features of MIME
allows secure messages to be exchanged in mixed-transport systems. and allows secure messages to be exchanged in mixed-transport
systems.
Further, S/MIME can be used in automated message transfer agents that Further, S/MIME can be used in automated message transfer agents that
use cryptographic security services that do not require any human use cryptographic security services that do not require any human
intervention, such as the signing of software-generated documents and intervention, such as the signing of software-generated documents and
the encryption of FAX messages sent over the Internet. the encryption of FAX messages sent over the Internet.
1.1. Specification Overview 1.1. Specification Overview
This document describes a protocol for adding cryptographic signature This document describes a protocol for adding cryptographic signature
and encryption services to MIME data. The MIME standard [MIME-SPEC] and encryption services to MIME data. The MIME standard [MIME-SPEC]
provides a general structure for the content type of Internet provides a general structure for the content of Internet messages and
messages and allows extensions for new content type applications. allows extensions for new content type based applications.
This specification defines how to create a MIME body part that has This specification defines how to create a MIME body part that has
been cryptographically enhanced according to CMS [CMS], which is been cryptographically enhanced according to CMS [CMS], which is
derived from PKCS #7 [PKCS-7]. This specification also defines the derived from PKCS #7 [PKCS-7]. This specification also defines the
application/pkcs7-mime MIME type that can be used to transport those application/pkcs7-mime media type that can be used to transport those
body parts. body parts.
This document also discusses how to use the multipart/signed MIME This document also discusses how to use the multipart/signed media
type defined in [MIME-SECURE] to transport S/MIME signed messages. type defined in [MIME-SECURE] to transport S/MIME signed messages.
multipart/signed is used in conjunction with the application/pkcs7- multipart/signed is used in conjunction with the application/pkcs7-
signature MIME type, which is used to transport a detached S/MIME signature media type, which is used to transport a detached S/MIME
signature. signature.
In order to create S/MIME messages, an S/MIME agent MUST follow the In order to create S/MIME messages, an S/MIME agent MUST follow the
specifications in this document, as well as the specifications listed specifications in this document, as well as the specifications listed
in the Cryptographic Message Syntax document [CMS], [CMSALG], in the Cryptographic Message Syntax document [CMS], [CMSALG],
[RSAPSS], [RSAOAEP], and [CMS-SHA2]. [RSAPSS], [RSAOAEP], and [CMS-SHA2].
Throughout this specification, there are requirements and Throughout this specification, there are requirements and
recommendations made for how receiving agents handle incoming recommendations made for how receiving agents handle incoming
messages. There are separate requirements and recommendations for messages. There are separate requirements and recommendations for
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Receiving agent: Software that interprets and processes S/MIME CMS Receiving agent: Software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS content types, or both. objects, MIME body parts that contain CMS content types, or both.
Sending agent: Software that creates S/MIME CMS content types, MIME Sending agent: Software that creates S/MIME CMS content types, MIME
body parts that contain CMS content types, or both. body parts that contain CMS content types, or both.
S/MIME agent: User software that is a receiving agent, a sending S/MIME agent: User software that is a receiving agent, a sending
agent, or both. agent, or both.
1.3. Compatibility with Prior Practice of S/MIME 1.3. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].
We define some additional terms here:
SHOULD+ This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD+ will be promoted at
some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, the authors
expect a requirement marked as SHOULD- will be demoted to a MAY
in a future version of this document.
MUST- This term means the same as MUST. However, the authors
expect that this requirement will no longer be a MUST in a future
document. Although its status will be determined at a later
time, it is reasonable to expect that if a future revision of a
document alters the status of a MUST- requirement, it will remain
at least a SHOULD or a SHOULD-.
1.4. Compatibility with Prior Practice of S/MIME
S/MIME version 3.2 agents SHOULD attempt to have the greatest S/MIME version 3.2 agents SHOULD attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME. interoperability possible with agents for prior versions of S/MIME.
S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive S/MIME version 2 is described in RFC 2311 through RFC 2315 , S/MIME
S/MIME version 3 is described in RFC 2630 through RFC 2634 inclusive, version 3 is described in RFC 2630 through RFC 2634, and S/MIME
and S/MIME version 3.1 is described in RFC 3850 through 3851 version 3.1 is described in RFC 3850, RFC 3851, and RFC 2634. RFC
inclusive and RFC 2634. RFC 2311 also has historical information 2311 also has historical information about the development of S/MIME.
about the development of S/MIME.
1.4. Changes Since S/MIME v3 1.5. Changes From S/MIME v3 to S/MIME v3.1
The RSA public key algorithm was changed to a MUST implement key The RSA public key algorithm was changed to a MUST implement key
wrapping algorithm, and the Diffie-Hellman algorithm changed to a wrapping algorithm, and the Diffie-Hellman algorithm changed to a
SHOULD implement. SHOULD implement.
The AES symmetric encryption algorithm has been included as a SHOULD The AES symmetric encryption algorithm has been included as a SHOULD
implement. implement.
The RSA public key algorithm was changed to a MUST implement The RSA public key algorithm was changed to a MUST implement
signature algorithm. signature algorithm.
Ambiguous language about the use of "empty" SignedData messages to Ambiguous language about the use of "empty" SignedData messages to
transmit certificates was clarified to reflect that transmission of transmit certificates was clarified to reflect that transmission of
certificate revocation lists is also allowed. certificate revocation lists is also allowed.
The use of binary encoding for some MIME entities is now explicitly The use of binary encoding for some MIME entities is now explicitly
discussed. discussed.
Header protection through the use of the message/rfc822 MIME type has Header protection through the use of the message/rfc822 media type
been added. has been added.
Use of the CompressedData CMS type is allowed, along with required Use of the CompressedData CMS type is allowed, along with required
MIME type and file extension additions. media type and file extension additions.
1.5. Changes Since S/MIME v3.1 1.6. Changes Since S/MIME v3.1
Added definitions for SHOULD+, SHOULD-, and MUST-. Editorial changes, e.g., replaced "MIME type" with "media type",
content-type with Content-Type.
Moved "Conventions Used in This Document to Section 1.2." Added
definitions for SHOULD+, SHOULD-, and MUST-.
Sec 1.1 and Appendix A: Added references to RFCs for RSA-PSS, RSA- Sec 1.1 and Appendix A: Added references to RFCs for RSA-PSS, RSA-
OAEP, and SHA2 CMS Algorithms. Added CMS Multiple Signers OAEP, and SHA2 CMS Algorithms. Added CMS Multiple Signers
Clarification to CMS reference. Clarification to CMS reference.
Sec 1.3: Added references to S/MIME MSG 3.1 RFCs. Sec 1.3: Added references to S/MIME MSG 3.1 RFCs.
Sec 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5 made Sec 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5 made
SHOULD-. SHOULD-.
Sec 2.2 (signature algorithms): RSA with SHA256 added as the MUST, Sec 2.2 (signature algorithms): RSA with SHA256 added as the MUST,
RSA with SHA-1 changed to MUST-, DSA with SHA-1, and RSA with MD5 RSA with SHA-1 changed to MUST-, DSA with SHA-1, and RSA with MD5
changed to SHOULD-, and RSA-PSS with SHA-256. Also added note about changed to SHOULD-, and RSA-PSS with SHA-256. Also added note about
what S/MIME v3.1 clients support. what S/MIME v3.1 clients support.
Sec 2.3 (key encryption): DH changed to SHOULD- and RSA-OAEP added as Sec 2.3 (key encryption): DH changed to SHOULD- and RSA-OAEP added as
SHOULD+. SHOULD+.
Sec 2.5.1: Added requirement that receiving agents MUST support both
GeneralizedTime and UTCTime.
Sec 2.5.2: Replaced reference "sha1WithRSAEncrption" with Sec 2.5.2: Replaced reference "sha1WithRSAEncrption" with
"sha256WithRSAEncryption" and "AES-CBC" in parantheticals. "sha256WithRSAEncryption" and "AES-CBC".
Sec 2.5.2.1, 2.7, 2.7.1, Appendix A: reference to RC2/40 removed. Sec 2.5.2.1, 2.7, 2.7.1, Appendix A: reference to RC2/40 removed.
Sec 2.7 (content encryption): AES-128 CBC added as MUST, AES-192 and Sec 2.7 (content encryption): AES-128 CBC added as MUST, AES-192 and
AES-256 CBC SHOULD+, tripleDES now SHOULD-. AES-256 CBC SHOULD+, tripleDES now SHOULD-.
Sec 4: Updated reference to CERT v3.2. Sec 4: Updated reference to CERT v3.2.
Sec 4.1: Updated RSA key size discussion. Moved last 4 sentences to Sec 4.1: Updated RSA key size discussion. Moved last 4 sentences to
security considerations. Updated reference to randomness requirements security considerations. Updated reference to randomness requirements
for security. for security.
2. CMS Options 2. CMS Options
CMS allows for a wide variety of options in content and algorithm CMS allows for a wide variety of options in content, attributes, and
support. This section puts forth a number of support requirements algorithm support. This section puts forth a number of support
and recommendations in order to achieve a base level of requirements and recommendations in order to achieve a base level of
interoperability among all S/MIME implementations. [CMSALG] and [CMS- interoperability among all S/MIME implementations. [CMSALG] and [CMS-
SHA2] provides additional details regarding the use of the SHA2] provides additional details regarding the use of the
cryptographic algorithms. cryptographic algorithms. [ESS] provides additional details
regarding the use of additional attributes.
2.1. DigestAlgorithmIdentifier 2.1. DigestAlgorithmIdentifier
Sending and receiving agents MUST support SHA-256 [CMS-SHA2] and Sending and receiving agents MUST support SHA-256 [CMS-SHA2] and
SHOULD- support SHA-1 [CMSALG]. Receiving agents SHOULD- support MD5 SHOULD- support SHA-1 [CMSALG]. Receiving agents SHOULD- support MD5
[CMSALG] for the purpose of providing backward compatibility with [CMSALG] for the purpose of providing backward compatibility with
MD5-digested S/MIME v2 SignedData objects. MD5-digested S/MIME v2 SignedData objects.
2.2. SignatureAlgorithmIdentifier 2.2. SignatureAlgorithmIdentifier
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Receiving and sending agents: Receiving and sending agents:
- MUST support RSA Encryption, as specified in [CMSALG] - MUST support RSA Encryption, as specified in [CMSALG]
- SHOULD+ support RSA-OAEP, as specified in [RSAOAEP] - SHOULD+ support RSA-OAEP, as specified in [RSAOAEP]
- SHOULD- support DH ephemeral-static mode, as specified - SHOULD- support DH ephemeral-static mode, as specified
in [CMSALG]. in [CMSALG].
Note that S/MIME v3.1 clients might only implement key encryption and Note that S/MIME v3.1 clients might only implement key encryption and
decryption using rsaEncryption algorithm. Note that S/MIME v3 clients decryption using the rsaEncryption algorithm. Note that S/MIME v3
might only implement key encryption and decryption using the Diffie- clients might only implement key encryption and decryption using the
Hellman algorithm. Also note that S/MIME v2 clients are only capable Diffie-Hellman algorithm. Also note that S/MIME v2 clients are only
of decrypting content-encryption keys using the rsaEncryption capable of decrypting content-encryption keys using the rsaEncryption
algorithm. algorithm.
2.4. General Syntax 2.4. General Syntax
There are several CMS content types. Of these, only the Data, There are several CMS content types. Of these, only the Data,
SignedData, EnvelopedData, and CompressedData content types are SignedData, EnvelopedData, and CompressedData content types are
currently used for S/MIME. currently used for S/MIME.
2.4.1. Data Content Type 2.4.1. Data Content Type
Sending agents MUST use the id-data content type identifier to Sending agents MUST use the id-data content type identifier to
identify the "inner" MIME message content. For example, when identify the "inner" MIME message content. For example, when
applying a digital signature to MIME data, the CMS SignedData applying a digital signature to MIME data, the CMS SignedData
encapContentInfo eContentType MUST include the id-data object encapContentInfo eContentType MUST include the id-data object
identifier and the MIME content MUST be stored in the SignedData identifier and the media type MUST be stored in the SignedData
encapContentInfo eContent OCTET STRING (unless the sending agent is encapContentInfo eContent OCTET STRING (unless the sending agent is
using multipart/signed, in which case the eContent is absent, per using multipart/signed, in which case the eContent is absent, per
section 3.4.3 of this document). As another example, when applying section 3.4.3 of this document). As another example, when applying
encryption to MIME data, the CMS EnvelopedData encryptedContentInfo encryption to MIME data, the CMS EnvelopedData encryptedContentInfo
contentType MUST include the id-data object identifier and the contentType MUST include the id-data object identifier and the
encrypted MIME content MUST be stored in the EnvelopedData encrypted MIME content MUST be stored in the EnvelopedData
encryptedContentInfo encryptedContent OCTET STRING. encryptedContentInfo encryptedContent OCTET STRING.
2.4.2. SignedData Content Type 2.4.2. SignedData Content Type
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This content type is used to apply data confidentiality to a message. This content type is used to apply data confidentiality to a message.
A sender needs to have access to a public key for each intended A sender needs to have access to a public key for each intended
message recipient to use this service. message recipient to use this service.
2.4.4. CompressedData Content Type 2.4.4. CompressedData Content Type
This content type is used to apply data compression to a message. This content type is used to apply data compression to a message.
This content type does not provide authentication, message integrity, This content type does not provide authentication, message integrity,
non-repudiation, or data confidentiality, and is only used to reduce non-repudiation, or data confidentiality, and is only used to reduce
message size. the message's size.
See section 3.6 for further guidance on the use of this type in See section 3.6 for further guidance on the use of this type in
conjunction with other CMS types. conjunction with other CMS types.
2.5. Attributes and the SignerInfo Type 2.5. Attributes and the SignerInfo Type
The SignerInfo type allows the inclusion of unsigned and signed The SignerInfo type allows the inclusion of unsigned and signed
attributes to be included along with a signature. attributes along with a signature.
Receiving agents MUST be able to handle zero or one instance of each Receiving agents MUST be able to handle zero or one instance of each
of the signed attributes listed here. Sending agents SHOULD generate of the signed attributes listed here. Sending agents SHOULD generate
one instance of each of the following signed attributes in each one instance of each of the following signed attributes in each
S/MIME message: S/MIME message:
- signingTime (section 2.5.1 in this document) - signingTime (section 2.5.1 in this document)
- sMIMECapabilities (section 2.5.2 in this document) - sMIMECapabilities (section 2.5.2 in this document)
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Further, receiving agents SHOULD be able to handle zero or one Further, receiving agents SHOULD be able to handle zero or one
instance in the signingCertificate signed attribute, as defined in instance in the signingCertificate signed attribute, as defined in
section 5 of [ESS]. section 5 of [ESS].
Sending agents SHOULD generate one instance of the signingCertificate Sending agents SHOULD generate one instance of the signingCertificate
signed attribute in each SignerInfo structure. signed attribute in each SignerInfo structure.
Additional attributes and values for these attributes might be Additional attributes and values for these attributes might be
defined in the future. Receiving agents SHOULD handle attributes or defined in the future. Receiving agents SHOULD handle attributes or
values that it does not recognize in a graceful manner. values that they do not recognize in a graceful manner.
Interactive sending agents that include signed attributes that are Interactive sending agents that include signed attributes that are
not listed here SHOULD display those attributes to the user, so that not listed here SHOULD display those attributes to the user, so that
the user is aware of all of the data being signed. the user is aware of all of the data being signed.
2.5.1. Signing-Time Attribute 2.5.1. Signing-Time Attribute
The signing-time attribute is used to convey the time that a message The signing-time attribute is used to convey the time that a message
was signed. The time of signing will most likely be created by a was signed. The time of signing will most likely be created by a
message originator and therefore is only as trustworthy as the message originator and therefore is only as trustworthy as the
originator. originator.
Sending agents MUST encode signing time through the year 2049 as Sending agents MUST encode signing time through the year 2049 as
UTCTime; signing times in 2050 or later MUST be encoded as UTCTime; signing times in 2050 or later MUST be encoded as
GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST
interpret the year field (YY) as follows: interpret the year field (YY) as follows:
If YY is greater than or equal to 50, the year is interpreted as If YY is greater than or equal to 50, the year is interpreted as
19YY; if YY is less than 50, the year is interpreted as 20YY. 19YY; if YY is less than 50, the year is interpreted as 20YY.
Receiving agents MUST be able to process signing-time attributes that
are encoded in either UTCTime or GeneralizedTime.
2.5.2. SMIMECapabilities Attribute 2.5.2. SMIMECapabilities Attribute
The SMIMECapabilities attribute includes signature algorithms (such The SMIMECapabilities attribute includes signature algorithms (such
as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES- as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-
CBC"), and key encipherment algorithms (such as "rsaEncryption"). CBC"), and key encipherment algorithms (such as "rsaEncryption").
There are also several identifiers which indicate support for other There are also several identifiers which indicate support for other
optional features such as binary encoding and compression. The optional features such as binary encoding and compression. The
SMIMECapabilities were designed to be flexible and extensible so SMIMECapabilities were designed to be flexible and extensible so
that, in the future, a means of identifying other capabilities and that, in the future, a means of identifying other capabilities and
preferences such as certificates can be added in a way that will not preferences such as certificates can be added in a way that will not
skipping to change at page 15, line 46 skipping to change at page 16, line 10
intended recipient) that the sending agent knows how to encrypt. The intended recipient) that the sending agent knows how to encrypt. The
sending agent SHOULD use one of the capabilities in the list if the sending agent SHOULD use one of the capabilities in the list if the
agent reasonably expects the recipient to be able to decrypt the agent reasonably expects the recipient to be able to decrypt the
message. message.
2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME 2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME
If the following two conditions are met: If the following two conditions are met:
- The sending agent has no knowledge of the encryption capabilities - The sending agent has no knowledge of the encryption capabilities
of the recipient; and, of the recipient, and
- The sending agent has no knowledge of the version of S/MIME of the - The sending agent has no knowledge of the version of S/MIME of the
recipient, then the sending agent SHOULD use AES 128 because it recipient,
is a stronger algorithm and is required by S/MIME v3.2. If the
sending agent chooses not to use AES 128 in this step, it SHOULD then the sending agent SHOULD use AES 128 because it is a stronger
use tripleDES. algorithm and is required by S/MIME v3.2. If the sending agent
chooses not to use AES 128 in this step, it SHOULD use tripleDES.
2.7.2. Choosing Weak Encryption 2.7.2. Choosing Weak Encryption
All algorithms that use 40 bit keys are considered by many to be weak All algorithms that use 40 bit keys are considered by many to be weak
encryption. A sending agent that is controlled by a human SHOULD encryption. A sending agent that is controlled by a human SHOULD
allow a human sender to determine the risks of sending data using allow a human sender to determine the risks of sending data using
weak encryption algorithm before sending the data, and possibly allow weak encryption algorithm before sending the data, and possibly allow
the human to use a stronger encryption method such as tripleDES or the human to use a stronger encryption method such as tripleDES or
AES. AES.
skipping to change at page 16, line 31 skipping to change at page 16, line 43
than one message. Please note that if the sending agent chooses to than one message. Please note that if the sending agent chooses to
send a message encrypted with a strong algorithm, and then send the send a message encrypted with a strong algorithm, and then send the
same message encrypted with a weak algorithm, someone watching the same message encrypted with a weak algorithm, someone watching the
communications channel could learn the contents of the strongly- communications channel could learn the contents of the strongly-
encrypted message simply by decrypting the weakly-encrypted message. encrypted message simply by decrypting the weakly-encrypted message.
3. Creating S/MIME Messages 3. Creating S/MIME Messages
This section describes the S/MIME message formats and how they are This section describes the S/MIME message formats and how they are
created. S/MIME messages are a combination of MIME bodies and CMS created. S/MIME messages are a combination of MIME bodies and CMS
content types. Several MIME types as well as several CMS content content types. Several media types as well as several CMS content
types are used. The data to be secured is always a canonical MIME types are used. The data to be secured is always a canonical MIME
entity. The MIME entity and other data, such as certificates and entity. The MIME entity and other data, such as certificates and
algorithm identifiers, are given to CMS processing facilities which algorithm identifiers, are given to CMS processing facilities which
produce a CMS object. Finally, the CMS object is wrapped in MIME. produce a CMS object. Finally, the CMS object is wrapped in MIME.
The Enhanced Security Services for S/MIME [ESS] document provides The Enhanced Security Services for S/MIME [ESS] document provides
descriptions of how nested, secured S/MIME messages are formatted. descriptions of how nested, secured S/MIME messages are formatted.
ESS provides a description of how a triple-wrapped S/MIME message is ESS provides a description of how a triple-wrapped S/MIME message is
formatted using multipart/signed and application/pkcs7-mime for the formatted using multipart/signed and application/pkcs7-mime for the
signatures. signatures.
skipping to change at page 17, line 10 skipping to change at page 17, line 21
choosing among these formats are also described. choosing among these formats are also described.
The reader of this section is expected to understand MIME as The reader of this section is expected to understand MIME as
described in [MIME-SPEC] and [MIME-SECURE]. described in [MIME-SPEC] and [MIME-SECURE].
3.1. Preparing the MIME Entity for Signing, Enveloping or Compressing 3.1. Preparing the MIME Entity for Signing, Enveloping or Compressing
S/MIME is used to secure MIME entities. A MIME entity can be a sub- S/MIME is used to secure MIME entities. A MIME entity can be a sub-
part, sub-parts of a message, or the whole message with all its sub- part, sub-parts of a message, or the whole message with all its sub-
parts. A MIME entity that is the whole message includes only the parts. A MIME entity that is the whole message includes only the
MIME headers and MIME body, and does not include the RFC-822 headers. MIME message headers and MIME body, and does not include the RFC-822
Note that S/MIME can also be used to secure MIME entities used in header. Note that S/MIME can also be used to secure MIME entities
applications other than Internet mail. If protection of the RFC-822 used in applications other than Internet mail. If protection of the
headers is required, the use of the message/rfc822 MIME type is RFC-822 header is required, the use of the message/rfc822 media type
explained later in this section. is explained later in this section.
The MIME entity that is secured and described in this section can be The MIME entity that is secured and described in this section can be
thought of as the "inside" MIME entity. That is, it is the thought of as the "inside" MIME entity. That is, it is the
"innermost" object in what is possibly a larger MIME message. "innermost" object in what is possibly a larger MIME message.
Processing "outside" MIME entities into CMS content types is Processing "outside" MIME entities into CMS content types is
described in Section 3.2, 3.4, and elsewhere. described in Section 3.2, 3.4, and elsewhere.
The procedure for preparing a MIME entity is given in [MIME-SPEC]. The procedure for preparing a MIME entity is given in [MIME-SPEC].
The same procedure is used here with some additional restrictions The same procedure is used here with some additional restrictions
when signing. Description of the procedures from [MIME-SPEC] are when signing. Description of the procedures from [MIME-SPEC] are
skipping to change at page 18, line 7 skipping to change at page 18, line 20
conventions. conventions.
Step 2. The leaf parts of the MIME entity are converted to Step 2. The leaf parts of the MIME entity are converted to
canonical form. canonical form.
Step 3. Appropriate transfer encoding is applied to the leaves Step 3. Appropriate transfer encoding is applied to the leaves
of the MIME entity. of the MIME entity.
When an S/MIME message is received, the security services on the When an S/MIME message is received, the security services on the
message are processed, and the result is the MIME entity. That MIME message are processed, and the result is the MIME entity. That MIME
entity is typically passed to a MIME-capable user agent where, it is entity is typically passed to a MIME-capable user agent where it is
further decoded and presented to the user or receiving application. further decoded and presented to the user or receiving application.
In order to protect outer, non-content related message headers (for In order to protect outer, non-content related message headers (for
nstance, the "Subject", "To", "From" and "CC" fields), the sending instance, the "Subject", "To", "From" and "Cc" fields), the sending
client MAY wrap a full MIME message in a message/rfc822 wrapper in client MAY wrap a full MIME message in a message/rfc822 wrapper in
order to apply S/MIME security services to these headers. It is up order to apply S/MIME security services to these header fields. It
to the receiving client to decide how to present these "inner" is up to the receiving client to decide how to present this "inner"
headers along with the unprotected "outer" headers. header along with the unprotected "outer" header.
When an S/MIME message is received, if the top-level protected MIME When an S/MIME message is received, if the top-level protected MIME
entity has a Content-Type of message/rfc822, it can be assumed that entity has a Content-Type of message/rfc822, it can be assumed that
the intent was to provide header protection. This entity SHOULD be the intent was to provide header protection. This entity SHOULD be
presented as the top-level message, taking into account header presented as the top-level message, taking into account header
merging issues as previously discussed. merging issues as previously discussed.
3.1.1. Canonicalization 3.1.1. Canonicalization
Each MIME entity MUST be converted to a canonical form that is Each MIME entity MUST be converted to a canonical form that is
uniquely and unambiguously representable in the environment where the uniquely and unambiguously representable in the environment where the
signature is created and the environment where the signature will be signature is created and the environment where the signature will be
verified. MIME entities MUST be canonicalized for enveloping and verified. MIME entities MUST be canonicalized for enveloping and
compressing as well as signing. compressing as well as signing.
The exact details of canonicalization depend on the actual MIME type The exact details of canonicalization depend on the actual media type
and subtype of an entity, and are not described here. Instead, the and subtype of an entity, and are not described here. Instead, the
standard for the particular MIME type SHOULD be consulted. For standard for the particular media type SHOULD be consulted. For
example, canonicalization of type text/plain is different from example, canonicalization of type text/plain is different from
canonicalization of audio/basic. Other than text types, most types canonicalization of audio/basic. Other than text types, most types
have only one representation regardless of computing platform or have only one representation regardless of computing platform or
environment which can be considered their canonical representation. environment which can be considered their canonical representation.
In general, canonicalization will be performed by the non-security In general, canonicalization will be performed by the non-security
part of the sending agent rather than the S/MIME implementation. part of the sending agent rather than the S/MIME implementation.
The most common and important canonicalization is for text, which is The most common and important canonicalization is for text, which is
often represented differently in different environments. MIME often represented differently in different environments. MIME
entities of major type "text" MUST have both their line endings and entities of major type "text" MUST have both their line endings and
character set canonicalized. The line ending MUST be the pair of character set canonicalized. The line ending MUST be the pair of
characters <CR><LF>, and the charset SHOULD be a registered charset characters <CR><LF>, and the charset SHOULD be a registered charset
[CHARSETS]. The details of the canonicalization are specified in [CHARSETS]. The details of the canonicalization are specified in
[MIME-SPEC]. The chosen charset SHOULD be named in the charset [MIME-SPEC]. The chosen charset SHOULD be named in the charset
skipping to change at page 19, line 15 skipping to change at page 19, line 28
Note that some charsets such as ISO-2022 have multiple Note that some charsets such as ISO-2022 have multiple
representations for the same characters. When preparing such text representations for the same characters. When preparing such text
for signing, the canonical representation specified for the charset for signing, the canonical representation specified for the charset
MUST be used. MUST be used.
3.1.2. Transfer Encoding 3.1.2. Transfer Encoding
When generating any of the secured MIME entities below, except the When generating any of the secured MIME entities below, except the
signing using the multipart/signed format, no transfer encoding is signing using the multipart/signed format, no transfer encoding is
required at all. S/MIME implementations MUST be able to deal with required at all. S/MIME implementations MUST be able to deal with
binary MIME objects. If no Content-Transfer-Encoding header is binary MIME objects. If no Content-Transfer-Encoding header field is
present, the transfer encoding is presumed to be 7BIT. present, the transfer encoding is presumed to be 7BIT.
S/MIME implementations SHOULD however use transfer encoding described S/MIME implementations SHOULD however use transfer encoding described
in section 3.1.3 for all MIME entities they secure. The reason for in section 3.1.3 for all MIME entities they secure. The reason for
securing only 7-bit MIME entities, even for enveloped data that are securing only 7-bit MIME entities, even for enveloped data that are
not exposed to the transport, is that it allows the MIME entity to be not exposed to the transport, is that it allows the MIME entity to be
handled in any environment without changing it. For example, a handled in any environment without changing it. For example, a
trusted gateway might remove the envelope, but not the signature, of trusted gateway might remove the envelope, but not the signature, of
a message, and then forward the signed message on to the end a message, and then forward the signed message on to the end
recipient so that they can verify the signatures directly. If the recipient so that they can verify the signatures directly. If the
skipping to change at page 21, line 26 skipping to change at page 21, line 39
Content-Type: image/jpeg Content-Type: image/jpeg
Content-Transfer-Encoding: base64 Content-Transfer-Encoding: base64
iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC// iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
HOxEa44b+EI= HOxEa44b+EI=
--bar-- --bar--
3.2. The application/pkcs7-mime Type 3.2. The application/pkcs7-mime Media Type
The application/pkcs7-mime type is used to carry CMS content types The application/pkcs7-mime media type is used to carry CMS content
including EnvelopedData, SignedData, and CompressedData. The details types including EnvelopedData, SignedData, and CompressedData. The
of constructing these entities are described in subsequent sections. details of constructing these entities are described in subsequent
This section describes the general characteristics of the sections. This section describes the general characteristics of the
application/pkcs7-mime type. application/pkcs7-mime media type.
The carried CMS object always contains a MIME entity that is prepared The carried CMS object always contains a MIME entity that is prepared
as described in section 3.1 if the eContentType is id-data. Other as described in section 3.1 if the eContentType is id-data. Other
contents MAY be carried when the eContentType contains different contents MAY be carried when the eContentType contains different
values. See [ESS] for an example of this with signed receipts. values. See [ESS] for an example of this with signed receipts.
Since CMS content types are binary data, in most cases base-64 Since CMS content types are binary data, in most cases base-64
transfer encoding is appropriate, in particular, when used with SMTP transfer encoding is appropriate, in particular, when used with SMTP
transport. The transfer encoding used depends on the transport transport. The transfer encoding used depends on the transport
through which the object is to be sent, and is not a characteristic through which the object is to be sent, and is not a characteristic
of the MIME type. of the media type.
Note that this discussion refers to the transfer encoding of the CMS Note that this discussion refers to the transfer encoding of the CMS
object or "outside" MIME entity. It is completely distinct from, and object or "outside" MIME entity. It is completely distinct from, and
unrelated to, the transfer encoding of the MIME entity secured by the unrelated to, the transfer encoding of the MIME entity secured by the
CMS object, the "inside" object, which is described in section 3.1. CMS object, the "inside" object, which is described in section 3.1.
Because there are several types of application/pkcs7-mime objects, a Because there are several types of application/pkcs7-mime objects, a
sending agent SHOULD do as much as possible to help a receiving agent sending agent SHOULD do as much as possible to help a receiving agent
know about the contents of the object without forcing the receiving know about the contents of the object without forcing the receiving
agent to decode the ASN.1 for the object. The MIME headers of all agent to decode the ASN.1 for the object. The Content-Type header
application/pkcs7-mime objects SHOULD include the optional "smime- field of all application/pkcs7-mime objects SHOULD include the
type" parameter, as described in the following sections. optional "smime-type" parameter, as described in the following
sections.
3.2.1. The name and filename Parameters 3.2.1. The name and filename Parameters
For the application/pkcs7-mime, sending agents SHOULD emit the For the application/pkcs7-mime, sending agents SHOULD emit the
optional "name" parameter to the Content-Type field for compatibility optional "name" parameter to the Content-Type field for compatibility
with older systems. Sending agents SHOULD also emit the optional with older systems. Sending agents SHOULD also emit the optional
Content-Disposition field [CONTDISP] with the "filename" parameter. Content-Disposition field [CONTDISP] with the "filename" parameter.
If a sending agent emits the above parameters, the value of the If a sending agent emits the above parameters, the value of the
parameters SHOULD be a file name with the appropriate extension: parameters SHOULD be a file name with the appropriate extension:
MIME Type File Extension Media Type File Extension
application/pkcs7-mime (SignedData, EnvelopedData) .p7m application/pkcs7-mime (SignedData, EnvelopedData) .p7m
application/pkcs7-mime (degenerate SignedData .p7c application/pkcs7-mime (degenerate SignedData .p7c
certificate management message) certificate management message)
application/pkcs7-mime (CompressedData) .p7z application/pkcs7-mime (CompressedData) .p7z
application/pkcs7-signature (SignedData) .p7s application/pkcs7-signature (SignedData) .p7s
In addition, the file name SHOULD be limited to eight characters In addition, the file name SHOULD be limited to eight characters
followed by a three letter extension. The eight character filename followed by a three letter extension. The eight character filename
base can be any distinct name; the use of the filename base "smime" base can be any distinct name; the use of the filename base "smime"
SHOULD be used to indicate that the MIME entity is associated with SHOULD be used to indicate that the MIME entity is associated with
S/MIME. S/MIME.
Including a file name serves two purposes. It facilitates easier use Including a file name serves two purposes. It facilitates easier use
of S/MIME objects as files on disk. It also can convey type of S/MIME objects as files on disk. It also can convey type
information across gateways. When a MIME entity of type information across gateways. When a MIME entity of type
application/pkcs7-mime (for example) arrives at a gateway that has no application/pkcs7-mime (for example) arrives at a gateway that has no
special knowledge of S/MIME, it will default the entity's MIME type special knowledge of S/MIME, it will default the entity's media type
to application/octet-stream and treat it as a generic attachment, to application/octet-stream and treat it as a generic attachment,
thus losing the type information. However, the suggested filename thus losing the type information. However, the suggested filename
for an attachment is often carried across a gateway. This often for an attachment is often carried across a gateway. This often
allows the receiving systems to determine the appropriate application allows the receiving systems to determine the appropriate application
to hand the attachment off to, in this case, a stand-alone S/MIME to hand the attachment off to, in this case, a stand-alone S/MIME
processing application. Note that this mechanism is provided as a processing application. Note that this mechanism is provided as a
convenience for implementations in certain environments. A proper convenience for implementations in certain environments. A proper
S/MIME implementation MUST use the MIME types and MUST NOT rely on S/MIME implementation MUST use the media types and MUST NOT rely on
the file extensions. the file extensions.
3.2.2. The smime-type parameter 3.2.2. The smime-type parameter
The application/pkcs7-mime content type defines the optional "smime- The application/pkcs7-mime content type defines the optional "smime-
type" parameter. The intent of this parameter is to convey details type" parameter. The intent of this parameter is to convey details
about the security applied (signed or enveloped) along with about the security applied (signed or enveloped) along with
information about the contained content. This specification defines information about the contained content. This specification defines
the following smime-types. the following smime-types.
skipping to change at page 23, line 33 skipping to change at page 23, line 41
1. If both signing and encryption can be applied to the content, 1. If both signing and encryption can be applied to the content,
then two values for smime-type SHOULD be assigned "signed-*" and then two values for smime-type SHOULD be assigned "signed-*" and
"encrypted-*". If one operation can be assigned then this can be "encrypted-*". If one operation can be assigned then this can be
omitted. Thus since "certs-only" can only be signed, "signed-" omitted. Thus since "certs-only" can only be signed, "signed-"
is omitted. is omitted.
2. A common string for a content OID SHOULD be assigned. We use 2. A common string for a content OID SHOULD be assigned. We use
"data" for the id-data content OID when MIME is the inner "data" for the id-data content OID when MIME is the inner
content. content.
3. If no common string is assigned. Then the common string of 3. If no common string is assigned, then the common string of
"OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1" "OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1"
would be DES40). would be DES40).
It is explicitly intended that this field be a suitable hint for mail It is explicitly intended that this field be a suitable hint for mail
client applications to indicate whether a message is "signed" or client applications to indicate whether a message is "signed" or
"encrypted" without having to tunnel into the CMS payload. "encrypted" without having to tunnel into the CMS payload.
3.3. Creating an Enveloped-only Message 3.3. Creating an Enveloped-only Message
This section describes the format for enveloping a MIME entity This section describes the format for enveloping a MIME entity
skipping to change at page 25, line 21 skipping to change at page 25, line 32
message might have. message might have.
Messages signed using the SignedData format cannot be viewed by a Messages signed using the SignedData format cannot be viewed by a
recipient unless they have S/MIME facilities. However, the recipient unless they have S/MIME facilities. However, the
SignedData format protects the message content from being changed by SignedData format protects the message content from being changed by
benign intermediate agents. Such agents might do line wrapping or benign intermediate agents. Such agents might do line wrapping or
content-transfer encoding changes which would break the signature. content-transfer encoding changes which would break the signature.
3.4.2. Signing Using application/pkcs7-mime with SignedData 3.4.2. Signing Using application/pkcs7-mime with SignedData
This signing format uses the application/pkcs7-mime MIME type. The This signing format uses the application/pkcs7-mime media type. The
steps to create this format are: steps to create this format are:
Step 1. The MIME entity is prepared according to section 3.1. Step 1. The MIME entity is prepared according to section 3.1.
Step 2. The MIME entity and other required data is processed Step 2. The MIME entity and other required data is processed
into a CMS object of type SignedData. into a CMS object of type SignedData.
Step 3. The SignedData object is wrapped in a CMS ContentInfo Step 3. The SignedData object is wrapped in a CMS ContentInfo
object. object.
skipping to change at page 26, line 9 skipping to change at page 26, line 21
567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7 567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH 77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
6YT64V0GhIGfHfQbnj75 6YT64V0GhIGfHfQbnj75
3.4.3. Signing Using the multipart/signed Format 3.4.3. Signing Using the multipart/signed Format
This format is a clear-signing format. Recipients without any S/MIME This format is a clear-signing format. Recipients without any S/MIME
or CMS processing facilities are able to view the message. It makes or CMS processing facilities are able to view the message. It makes
use of the multipart/signed MIME type described in [MIME-SECURE]. The use of the multipart/signed media type described in [MIME-SECURE].
multipart/signed MIME type has two parts. The first part contains The multipart/signed media type has two parts. The first part
the MIME entity that is signed; the second part contains the contains the MIME entity that is signed; the second part contains the
"detached signature" CMS SignedData object in which the "detached signature" CMS SignedData object in which the
encapContentInfo eContent field is absent. encapContentInfo eContent field is absent.
3.4.3.1. The application/pkcs7-signature MIME Type 3.4.3.1. The application/pkcs7-signature Media Type
This MIME type always contains a CMS ContentInfo containing a single This media type always contains a CMS ContentInfo containing a single
CMS object of type SignedData. The SignedData encapContentInfo CMS object of type SignedData. The SignedData encapContentInfo
eContent field MUST be absent. The signerInfos field contains the eContent field MUST be absent. The signerInfos field contains the
signatures for the MIME entity. signatures for the MIME entity.
The file extension for signed-only messages using application/pkcs7- The file extension for signed-only messages using application/pkcs7-
signature is ".p7s". signature is ".p7s".
3.4.3.2. Creating a multipart/signed Message 3.4.3.2. Creating a multipart/signed Message
Step 1. The MIME entity to be signed is prepared according to Step 1. The MIME entity to be signed is prepared according to
skipping to change at page 26, line 45 skipping to change at page 27, line 12
multipart/signed message with no processing other than that multipart/signed message with no processing other than that
described in section 3.1. described in section 3.1.
Step 4. Transfer encoding is applied to the "detached signature" Step 4. Transfer encoding is applied to the "detached signature"
CMS SignedData object and it is inserted into a MIME entity of CMS SignedData object and it is inserted into a MIME entity of
type application/pkcs7-signature. type application/pkcs7-signature.
Step 5. The MIME entity of the application/pkcs7-signature is Step 5. The MIME entity of the application/pkcs7-signature is
inserted into the second part of the multipart/signed entity. inserted into the second part of the multipart/signed entity.
The multipart/signed Content type has two required parameters: the The multipart/signed Content-Type has two required parameters: the
protocol parameter and the micalg parameter. protocol parameter and the micalg parameter.
The protocol parameter MUST be "application/pkcs7-signature". Note The protocol parameter MUST be "application/pkcs7-signature". Note
that quotation marks are required around the protocol parameter that quotation marks are required around the protocol parameter
because MIME requires that the "/" character in the parameter value because MIME requires that the "/" character in the parameter value
MUST be quoted. MUST be quoted.
The micalg parameter allows for one-pass processing when the The micalg parameter allows for one-pass processing when the
signature is being verified. The value of the micalg parameter is signature is being verified. The value of the micalg parameter is
dependent on the message digest algorithm(s) used in the calculation dependent on the message digest algorithm(s) used in the calculation
skipping to change at page 27, line 29 skipping to change at page 27, line 44
SHA-384 sha384 SHA-384 sha384
SHA-512 sha512 SHA-512 sha512
Any other (defined separately in algorithm profile or "unknown" Any other (defined separately in algorithm profile or "unknown"
if not defined) if not defined)
(Historical note: some early implementations of S/MIME emitted and (Historical note: some early implementations of S/MIME emitted and
expected "rsa-md5" and "rsa-sha1" for the micalg parameter.) expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)
Receiving agents SHOULD be able to recover gracefully from a micalg Receiving agents SHOULD be able to recover gracefully from a micalg
parameter value that they do not recognize. parameter value that they do not recognize.
The SHA-224 algorithm [SHA224] and SHA-384 and SHA-512 algorithms The SHA-224 algorithm, SHA-384 and SHA-512 algorithms [FIPS180-3] are
[FIPS180-3] are not currently recommended in S/MIME, and are included not currently recommended in S/MIME, and are included here for
here for completeness. completeness.
3.4.3.3. Sample multipart/signed Message 3.4.3.3. Sample multipart/signed Message
Content-Type: multipart/signed; Content-Type: multipart/signed;
protocol="application/pkcs7-signature"; protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42 micalg=sha1; boundary=boundary42
--boundary42 --boundary42
Content-Type: text/plain Content-Type: text/plain
skipping to change at page 30, line 40 skipping to change at page 31, line 15
3.9. Identifying an S/MIME Message 3.9. Identifying an S/MIME Message
Because S/MIME takes into account interoperation in non-MIME Because S/MIME takes into account interoperation in non-MIME
environments, several different mechanisms are employed to carry the environments, several different mechanisms are employed to carry the
type information, and it becomes a bit difficult to identify S/MIME type information, and it becomes a bit difficult to identify S/MIME
messages. The following table lists criteria for determining whether messages. The following table lists criteria for determining whether
or not a message is an S/MIME message. A message is considered an or not a message is an S/MIME message. A message is considered an
S/MIME message if it matches any of the criteria listed below. S/MIME message if it matches any of the criteria listed below.
The file suffix in the table below comes from the "name" parameter in The file suffix in the table below comes from the "name" parameter in
the content-type header, or the "filename" parameter on the content- the Content-Type header field, or the "filename" parameter on the
disposition header. These parameters that give the file suffix are Content-Disposition header field. These parameters that give the
not listed below as part of the parameter section. file suffix are not listed below as part of the parameter section.
MIME type: application/pkcs7-mime Media type: application/pkcs7-mime
parameters: any parameters: any
file suffix: any file suffix: any
MIME type: multipart/signed Media type: multipart/signed
parameters: protocol="application/pkcs7-signature" parameters: protocol="application/pkcs7-signature"
file suffix: any file suffix: any
MIME type: application/octet-stream
Media type: application/octet-stream
parameters: any parameters: any
file suffix: p7m, p7s, p7c, p7z file suffix: p7m, p7s, p7c, p7z
4. Certificate Processing 4. Certificate Processing
A receiving agent MUST provide some certificate retrieval mechanism A receiving agent MUST provide some certificate retrieval mechanism
in order to gain access to certificates for recipients of digital in order to gain access to certificates for recipients of digital
envelopes. This specification does not cover how S/MIME agents envelopes. This specification does not cover how S/MIME agents
handle certificates, only what they do after a certificate has been handle certificates, only what they do after a certificate has been
validated or rejected. S/MIME certificate issues are covered in validated or rejected. S/MIME certificate issues are covered in
skipping to change at page 31, line 30 skipping to change at page 32, line 5
and sending agents SHOULD also provide a mechanism to allow a user to and sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way so "store and protect" certificates for correspondents in such a way so
as to guarantee their later retrieval. as to guarantee their later retrieval.
4.1. Key Pair Generation 4.1. Key Pair Generation
All generated key pairs MUST be generated from a good source of non- All generated key pairs MUST be generated from a good source of non-
deterministic random input [RANDOM] and the private key MUST be deterministic random input [RANDOM] and the private key MUST be
protected in a secure fashion. protected in a secure fashion.
If an S/MIME agent needs to generate an RSA key pair, then the S/MIME An S/MIME user agent MUST NOT generate asymmetric keys less than 512
agent or some related administrative utility or function SHOULD bits for use with the RSA or DSA signature algorithms.
generate RSA key pairs using the following guidelines. A user agent
SHOULD generate RSA key pairs at a minimum key size of 1024 bits. A
user agent MUST NOT generate RSA key pairs less than 512 bits long.
Creating keys longer than 1024 bits can cause some older S/MIME
receiving agents to not be able to verify signatures, but gives
better security and is therefore valuable.
A receiving agent needs to verify signatures whose key length is 4.2. Signature Generation
chosen by the signer. For interoperability, a receiving agent MUST be
able to verify signatures whose key length is 1024 bits or shorter. The following are the requirements for an S/MIME agent generated RSA
Being able to verify signatures shorter than 1024 bits is mandatory or DSA signature:
because earlier versions of this specification required the ability
to generate signatures with shorter key lengths. Note that most 512 <= key size < 1024 : MAY (see Security Considerations)
receiving agents are likely to see signatures whose key length is 1024 <= key size <= 2048 : SHOULD (see Security Considerations)
longer than 1024 bits in the future, and those receiving agents will 2048 < key size <= 4096 : MAY (see Security Considerations)
want to be able to verify those signatures.
4.3. Signature Verification
The following are the requirements for S/MIME receiving agents during
signature verification of RSA or DSA signatures:
512 <= key size <= 2048 : MUST (see Security Considerations)
2048 < key size <= 4096 : MAY (see Security Considerations)
4.4. Encryption
The following are the requirements for an S/MIME agent when
establishing keys for content encryption using the RSA or DH
algorithms:
512 <= key size < 1024 : MAY (see Security Considerations)
1024 <= key size <= 2048 : SHOULD (see Security Considerations)
2048 < key size <= 4096 : MAY (see Security Considerations)
4.5. Decryption
The following are the requirements for an S/MIME agent when
establishing keys for content decryption using the RSA or DH
algorithms:
512 <= key size <= 2048 : MUST (see Security Considerations)
2048 < key size <= 4096 : MAY (see Security Considerations)
5. IANA Considerations 5. IANA Considerations
None: All identifiers are already registered. Please remove this None: All identifiers are already registered. Please remove this
section prior to publication as an RFC. section prior to publication as an RFC.
6. Security Considerations 6. Security Considerations
40-bit encryption is considered weak by most cryptographers. Using Cryptographic algorithms will be broken or weakened over time.
weak cryptography in S/MIME offers little actual security over Implementers and users need to check that the cryptographic
algorithms listed in this document continue to provide the expected
level of security. The IETF from time to time may issue documents
dealing with the current state of the art. For example:
- The Million Message Attack described in RFC 3218 [MMA].
- The Diffie-Hellman "small-subgroup" attacks described in
RFC 2785 [DHSUB].
- The attacks against hash algorithms described in
RFC 4270 [HASH-ATTACK]
This specification uses Public-Key Cryptography technologies. It is
assumed that the private is protected to ensure that it is not
accessed or altered by unauthorized parties.
It is impossible for most people or software to estimate the value of
a message content. Further, it is impossible for most people or
software to estimate the actual cost of recovering an encrypted
message content that is encrypted with a key of a particular size.
Further, it is quite difficult to determine the cost of a failed
decryption if a recipient cannot process a message content. Thus,
choosing between different key sizes (or choosing whether to just use
plaintext) is also impossible for most people or software. However,
decisions based on these criteria are made all the time, and
therefore this specification gives a framework for using those
estimates in choosing algorithms.
The choice of 1024 bits as the RSA, DSA, and DH asymmetric key size
in this specification is based on the desire to provide 80 bits of
security. This key size seems prudent for the Internet based on
Section 4.3 of [RFC3766]. There are other environments (e.g.,
government, financial, and medical) that may consider this key size
to be inadequate. Likewise, there are other environments that may
consider this key size to be excessive.
Larger keys are not necessarily better keys. Larger keys take more
computational resources, and this can quickly become impractical. In
fact, support for an excessively large key offers a denial of service
opportunity if the attacker can cause excessive cryptographic
processing by providing such a public key. One mitigation approach
would require that the corresponding public key certificate be
validated to a trust anchor prior to use, thus ensuring that only
trusted public keys are used. However, some implementations may
choose to perform signature verification (or key establishment for
encryption) in parallel with certificate validation, even if
certificate validation fails. In such cases, measures should be
included to limit the impact, for example by limiting cryptographic
processing time or requiring certificate validation prior to the use
of large keys.
Today, 512 bit RSA, DSA, and DH keys are considered by many experts
to be cryptographically insecure.
Using weak cryptography in S/MIME offers little actual security over
sending plaintext. However, other features of S/MIME, such as the sending plaintext. However, other features of S/MIME, such as the
specification of tripleDES and the ability to announce stronger specification of AES and the ability to announce stronger
cryptographic capabilities to parties with whom you communicate, cryptographic capabilities to parties with whom you communicate,
allow senders to create messages that use strong encryption. Using allow senders to create messages that use strong encryption. Using
weak cryptography is never recommended unless the only alternative is weak cryptography is never recommended unless the only alternative is
no cryptography. When feasible, sending and receiving agents SHOULD no cryptography. When feasible, sending and receiving agents SHOULD
inform senders and recipients of the relative cryptographic strength inform senders and recipients of the relative cryptographic strength
of messages. of messages.
It is impossible for most software or people to estimate the value of Implementers SHOULD be aware that multiple active key pairs can be
a message. Further, it is impossible for most software or people to associated with a single individual. For example, one key pair can
estimate the actual cost of decrypting a message that is encrypted be used to support confidentiality, while a different key pair can be
with a key of a particular size. Further, it is quite difficult to used for digital signatures.
determine the cost of a failed decryption if a recipient cannot
decode a message. Thus, choosing between different key sizes (or
choosing whether to just use plaintext) is also impossible. However,
decisions based on these criteria are made all the time, and
therefore this specification gives a framework for using those
estimates in choosing algorithms.
If a sending agent is sending the same message using different If a sending agent is sending the same message using different
strengths of cryptography, an attacker watching the communications strengths of cryptography, an attacker watching the communications
channel might be able to determine the contents of the strongly- channel might be able to determine the contents of the strongly-
encrypted message by decrypting the weakly-encrypted version. In encrypted message by decrypting the weakly-encrypted version. In
other words, a sender SHOULD NOT send a copy of a message using other words, a sender SHOULD NOT send a copy of a message using
weaker cryptography than they would use for the original of the weaker cryptography than they would use for the original of the
message. message.
Modification of the ciphertext can go undetected if authentication is Modification of the ciphertext can go undetected if authentication is
not also used, which is the case when sending EnvelopedData without not also used, which is the case when sending EnvelopedData without
wrapping it in SignedData or enclosing SignedData within it. wrapping it in SignedData or enclosing SignedData within it.
See RFC 3218 [MMA] for more information about thwarting the adaptive
chosen ciphertext vulnerability in PKCS #1 Version 1.5
implementations.
In some circumstances the use of the Diffie-Hellman key agreement
scheme in a prime order subgroup of a large prime p is vulnerable to
certain attacks known as "small-subgroup" attacks. Methods exist,
however, to prevent these attacks. These methods are described in
RFC 2785 [DHSUB].
Some S/MIME agents created in the United States have chosen to create
512 bit keys in order to get more advantageous export licenses.
However, 512 bit keys are considered by many to be cryptographically
insecure.
Receiving agents are only required to validate signatures that are
the same length as sending agents are required to produce. Many
people feel that signatures of 1024 bits do not meet their security
requirements today, or even if they meet their requirements today,
they will not meet their requirements in the foreseeable future.
Therefore, sending and receiving agents need to decide what strength
of signature they want to produce and validate, respectively.
Further, those decisions need to be reviewed periodically in light of
decreasing cryptographic strength over time of signatures.
For receiving agents to avoid Denial of Service (DoS) attacks it is
RECOMMENDED that receiving agents not process keys larger than they
support and that the certificate containing the private key be
validated prior to use.
Implementers SHOULD be aware that multiple (active) key pairs can be
associated with a single individual. For example, one key pair can
be used to support confidentiality, while a different key pair can be
used for authentication.
Appendix A. ASN.1 Module Appendix A. ASN.1 Module
NOTE: The ASN.1 module contained herein is unchanged from [MSG3.1],
with the exception of a minor change to the prefersBinaryInside ASN.1
comment.
SecureMimeMessageV3dot1 SecureMimeMessageV3dot1
{ iso(1) member-body(2) us(840) rsadsi(113549) { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) } pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }
DEFINITIONS IMPLICIT TAGS ::= DEFINITIONS IMPLICIT TAGS ::=
BEGIN BEGIN
IMPORTS IMPORTS
skipping to change at page 35, line 18 skipping to change at page 36, line 18
issuerAndSerialNumber [0] IssuerAndSerialNumber, issuerAndSerialNumber [0] IssuerAndSerialNumber,
receipentKeyId [1] RecipientKeyIdentifier, receipentKeyId [1] RecipientKeyIdentifier,
subjectAltKeyIdentifier [2] SubjectKeyIdentifier subjectAltKeyIdentifier [2] SubjectKeyIdentifier
} }
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs9(9) 16 } rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-cap OBJECT IDENTIFIER ::= { id-smime 11 } id-cap OBJECT IDENTIFIER ::= { id-smime 11 }
-- The preferBinaryInside indicates an ability to receive messages -- The preferBinaryInside OID indicates an ability to receive
-- with binary encoding inside the CMS wrapper -- messages with binary encoding inside the CMS wrapper
id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 } id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 }
-- The following list the OIDs to be used with S/MIME V3 -- The following list the OIDs to be used with S/MIME V3
-- Signature Algorithms Not Found in [CMSALG] -- Signature Algorithms Not Found in [CMSALG]
-- --
-- md2WithRSAEncryption OBJECT IDENTIFIER ::= -- md2WithRSAEncryption OBJECT IDENTIFIER ::=
skipping to change at page 36, line 43 skipping to change at page 37, line 43
Message Syntax", work in progress. Message Syntax", work in progress.
[CONTDISP] Troost, R., Dorner, S., and K. Moore, "Communicating [CONTDISP] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August Content-Disposition Header Field", RFC 2183, August
1997. 1997.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME", [ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999. RFC 2634, June 1999.
[FIPS180-3] "Secure Hash Signature Standard (SHS)", National [FIPS180-3] National Institute of Standards and Technology (NIST),
Institute of Standards and Technology (NIST). FIPS "Secure Hash Standard (SHS)", FIPS Publication 180-3,
Publication 180-3. June 2007.
[MIME-SPEC] Freed, N. and N. Borenstein, "Multipurpose Internet [MIME-SPEC] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996. Message Bodies", RFC 2045, November 1996.
Freed, N. and N. Borenstein, "Multipurpose Internet Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types", RFC Mail Extensions (MIME) Part Two: Media Types", RFC
2046, November 1996. 2046, November 1996.
Moore, K., "MIME (Multipurpose Internet Mail Moore, K., "MIME (Multipurpose Internet Mail
skipping to change at page 37, line 29 skipping to change at page 38, line 33
Mail Extensions (MIME) Part Five: Conformance Criteria Mail Extensions (MIME) Part Five: Conformance Criteria
and Examples", RFC 2049, November 1996. and Examples", RFC 2049, November 1996.
[MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed, [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and "Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995. Multipart/Encrypted", RFC 1847, October 1995.
[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
[RSAPSS] Schaad, J., "Use of RSASA-PSS Signature Algorithm in [RSAPSS] Schaad, J., "Use of RSASA-PSS Signature Algorithm in
Cryptographic Message Syntax (CMS)", RFC 4056, June Cryptographic Message Syntax (CMS)", RFC 4056, June
2005. 2005.
[X.208-88] CCITT. Recommendation X.208: Specification of [X.208-88] CCITT. Recommendation X.208: Specification of
Abstract Syntax Notation One (ASN.1). 1988. Abstract Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic [X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One Encoding Rules for Abstract Syntax Notation One
(ASN.1). 1988. (ASN.1). 1988.
skipping to change at page 38, line 5 skipping to change at page 39, line 15
B.2. Informative References B.2. Informative References
[DHSUB] Zuccherato, R., "Methods for Avoiding the "Small- [DHSUB] Zuccherato, R., "Methods for Avoiding the "Small-
Subgroup" Attacks on the Diffie-Hellman Key Agreement Subgroup" Attacks on the Diffie-Hellman Key Agreement
Method for S/MIME", RFC 2785, March 2000. Method for S/MIME", RFC 2785, March 2000.
[MMA] Rescorla, E., "Preventing the Million Message Attack [MMA] Rescorla, E., "Preventing the Million Message Attack
on Cryptographic Message Syntax", RFC 3218, January on Cryptographic Message Syntax", RFC 3218, January
2002. 2002.
[MSG3.1] Ramsdell, B., "S/MIME Version 3.1 Message
Specification", RFC 3851, July 2004.
[PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax [PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998. Version 1.5", RFC 2315, March 1998.
[RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller,
"Randomness Requirements for Security", RFC 4086, June
2005.
[SHA224] Housley, R., "A 224-bit One-way Hash Function: SHA-
224", RFC 3874, September 2004.
[SMIMEV2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., [SMIMEV2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
and L. Repka, "S/MIME Version 2 Message and L. Repka, "S/MIME Version 2 Message
Specification", RFC 2311, March 1998. Specification", RFC 2311, March 1998.
[HASH-ATTACK] Hoffman, P., Schneier, B., "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November
2005.
Appendix C. Acknowledgements Appendix C. Acknowledgements
Many thanks go out to the other authors of the S/MIME Version 2 Many thanks go out to the other authors of the S/MIME Version 2
Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
Lundblade and Lisa Repka. Lundblade and Lisa Repka.
A number of the members of the S/MIME Working Group have also worked A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is very hard and contributed to this document. Any list of people is
doomed to omission, and for that I apologize. In alphabetical order, doomed to omission, and for that I apologize. In alphabetical order,
the following people stand out in my mind due to the fact that they the following people stand out in my mind due to the fact that they
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