draft-ietf-smime-aes-alg-01.txt   draft-ietf-smime-aes-alg-02.txt 
S/MIME Working Group J. Schaad S/MIME Working Group J. Schaad
Internet Draft Soaring Hawk Consulting Internet Draft Soaring Hawk Consulting
Document: draft-ietf-smime-aes-alg-01.txt R. Housley Document: draft-ietf-smime-aes-alg-02.txt R. Housley
Expires: September 2, 2001 RSA Laboratories Expires: December 20, 2001 RSA Laboratories
March 2001 July 2001
Use of the Advanced Encryption Algorithm in CMS Use of the AES Encryption Algorithm and RSA-OAEP Key Transport in CMS
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1]. all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Internet-Drafts are draft documents valid for a maximum of Drafts. Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or obsoleted by other six months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet- Drafts documents at any time. It is inappropriate to use Internet- Drafts
as reference material or to cite them other than as "work in as reference material or to cite them other than as "work in
progress." progress."
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This document specifies how to incorporate the Advanced Encryption This document specifies how to incorporate the Advanced Encryption
Standard (AES) algorithm [AES] and RSAES-OAEP key transport method of Standard (AES) algorithm [AES] and RSAES-OAEP key transport method of
key management into the S/MIME Cryptographic Message Syntax [CMS] as key management into the S/MIME Cryptographic Message Syntax [CMS] as
additional algorithms. additional algorithms.
Conventions used in this document Conventions used in this document
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 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [2]. this document are to be interpreted as described in RFC 2119
[MUSTSHOULD].
1. Overview 1 Overview
This document describes the conventions for using the RSAES-OAEP key This document describes the conventions for using the RSAES-OAEP key
transport algorithm and Advanced Encryption Standard (AES) content transport algorithm and Advanced Encryption Standard (AES) content
encryption algorithm with the Cryptographic Message Syntax [CMS] encryption algorithm with the Cryptographic Message Syntax [CMS]
enveloped-data, encrypted-data and authenticated-data content types. enveloped-data and encrypted-data content types.
While this document presents the use of the two algorithms together,
that fact does not imply that they cannot be used independently.
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They are presented together simply because the initial usage of each This document presents the use of the two algorithms together, and we
will be as a matched pair. anticipate that they will be used together. However,the two
algorithms can be used independently. For example, RSA-OAEP could be
used to transport Triple-DES keys, and AES keys could be distributed
out-of-band for use with mail lists. The two algorithms are
presented together simply because the initial usage of each will be
as a matched pair.
1.1 AES
The Advanced Encryption Standard (AES) is being developed to replace
DES [DES]. The AES will be a new Federal Information Processing
Standard (FIPS) Publication that will specify a cryptographic
algorithm for use by U.S. Government organizations. However, the AES
will also be widely used by organizations, institutions, and
individuals outside of the U.S. Government.
NIST has posted the Draft FIPS for the AES (see
http://csrc.nist.gov/encryption/aes). The AES will become official
after a 90-day public comment period, NIST makes appropriate changes
to the Draft FIPS, and the Secretary of Commerce approves the FIPS.
Current estimates place this sometime in the summer of 2001. In
other words, any day now.
The two researchers who developed and submitted the Rijndael
algorithm for the AES are both cryptographers from Belgium: Dr. Joan
Daemen of Proton World International and Dr. Vincent Rijmen, a
postdoctoral researcher in the Electrical Engineering Department of
Katholieke Universiteit Leuven.
NIST selected the Rijndael algorithm for AES because it offers a
combination of security, performance, efficiency, ease of
implementation, and flexibility. Specifically, Rijndael appears to
be consistently a very good performer in both hardware and software
across a wide range of computing environments regardless of its use
in feedback or non-feedback modes. Its key setup time is excellent,
and its key agility is good. The very low memory requirements of the
Rijndael algorithm make it very well suited for restricted-space
environments, in which it also demonstrates excellent performance.
The Rijndael algorith operations are among the easiest to defend
against power and timing attacks. Additionally, it appears that some
defense can be provided against such attacks without significantly
impacting the algorithm's performance. Finally, the algorithm's
internal round structure appears to have good potential to benefit
from instruction-level parallelism.
The AES specifies three key sizes: 128, 192 and 256 bits.
1.2 RSA-OAEP
When the variant of the RSA key transport algorithm specified in PKCS When the variant of the RSA key transport algorithm specified in PKCS
#1 Version 1.5 [PKCS#1v1.5] is used for key management, it is #1 Version 1.5 [PKCS#1v1.5] is used for key management, it is
vulnerable to adaptive chosen ciphertext attacks. This attack is vulnerable to adaptive chosen ciphertext attacks. This attack is
explained in [RSALAB] and [CRYPTO98]. The use of PKCS #1 Version 1.5 described in [RSALAB] and [CRYPTO98]. The use of PKCS #1 Version 1.5
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Use of the AES Algorithm in CMS November 2000
key transport in interactive applications is especially vulnerable. key transport in interactive applications is especially vulnerable.
Exploitation of this identified vulnerability, revealing the result Exploitation of this identified vulnerability, revealing the result
of a particular RSA decryption, requires access to an oracle which of a particular RSA decryption, requires access to an oracle which
will respond to hundreds of thousands of ciphertexts, which are will respond to hundreds of thousands of ciphertexts, which are
constructed adaptively in response to previously-received replies constructed adaptively in response to previously-received replies
providing information on the successes or failures of attempted providing information on the successes or failures of attempted
decryption operations. decryption operations.
The attack appears significantly less feasible in store-and-forward The attack appears significantly less feasible in store-and-forward
environments, such as S/MIME. When PKCS #1 Version 1.5 key transport environments, such as S/MIME. When PKCS #1 Version 1.5 key transport
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more feasible. However, Secure Sockets Layer (SSL) [SSL] and more feasible. However, Secure Sockets Layer (SSL) [SSL] and
Transport Layer Security (TLS) [TLS] protocol implementations could Transport Layer Security (TLS) [TLS] protocol implementations could
include countermeasures that detect and prevent Bleichenbacher's and include countermeasures that detect and prevent Bleichenbacher's and
other chosen-ciphertext attacks, without changing the way the RSA key other chosen-ciphertext attacks, without changing the way the RSA key
transport algorithm is used. These countermeasures are performed transport algorithm is used. These countermeasures are performed
within the protocol level. In the interest of long-term security within the protocol level. In the interest of long-term security
assurance, it is prudent to adopt an improved cryptographic technique assurance, it is prudent to adopt an improved cryptographic technique
rather than embedding countermeasures within protocols. rather than embedding countermeasures within protocols.
An updated version of PKCS #1 has been published, PKCS #1 Version 2.0 An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
[PKCS#1v2.0]. This new document supersedes RFC 2313. PKCS #1 [PKCS#1v2.0]. This new document supersedes RFC 2313 [PKCS#1v1.5].
Version 2.0 preserves support for the encryption padding format PKCS #1 Version 2.0 preserves support for the encryption padding
defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also defines a format defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also
new alternative. To resolve the adaptive chosen ciphertext defines a new alternative. To resolve the adaptive chosen ciphertext
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
is used to provide confidentiality, such as key transport. is used to provide confidentiality, such as key transport.
This document specifies the use of RSAES-OAEP key transport algorithm This document specifies the use of RSAES-OAEP key transport algorithm
in the Cryptographic Message Syntax (CMS) [CMS]. CMS can be used in in the Cryptographic Message Syntax (CMS) [CMS]. CMS can be used in
either a store-and-forward or an interactive request-response either a store-and-forward or an interactive request-response
environment. environment.
CMS supports variety of architectures for certificate-based key CMS supports variety of architectures for certificate-based key
management, particularly the one defined by the PKIX working group management, particularly the one defined by the PKIX working group
[PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the [PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the
same RSA public key information. Thus, a certified RSA public key same RSA public key information. Thus, a certified RSA public key
may be used with either RSA key transport technique. may be used with either RSA key transport technique.
CMS values are generated using ASN.1 [X.208-88], using the Basic CMS values are generated using ASN.1 [X.208-88], using the Basic
Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
(DER) [X.509-88]. (DER) [X.509-88].
2. Enveloped-data Conventions 2 Enveloped-data Conventions
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The CMS enveloped-data content type consists of encrypted content and The CMS enveloped-data content type consists of encrypted content and
wrapped content-encryption keys for one or more recipients. The wrapped content-encryption keys for one or more recipients. The
RSAES-OAEP key transport algorithm is used to wrap the content- RSAES-OAEP key transport algorithm is used to wrap the content-
encryption key for one recipient. The AES algorithm is used to encryption key for one recipient. The AES algorithm is used to
encrypt the content. encrypt the content.
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Compliant software MUST meet the requirements for constructing an Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 6, "Enveloped- enveloped-data content type stated in [CMS] Section 6, "Enveloped-
data Content Type". data Content Type".
A content-encryption key MUST be randomly generated for each instance A content-encryption key MUST be randomly generated for each instance
of an enveloped-data content type. The content-encryption key is of an enveloped-data content type. The content-encryption key is
used to encipher the content. used to encipher the content.
AES can be used with the enveloped-data content type using any of the AES can be used with the enveloped-data content type using any of the
following key management techniques defined in [CMS] Section 6. following key management techniques defined in [CMS] Section 6.
1) Key Transport: The AES CEK is uniquely wrapped for each recipient 1) Key Transport: The AES CEK is uniquely wrapped for each recipient
using the recipient's public RSA key and other values. Section 2.XX using the recipient's public RSA key and other values. Section 2.2
provides additional details. RSAES-OEAP is a key transport algorithm provides additional details.
and itĂs usage is described here.
2) Key Agreement: The AES CEK is uniquely wrapped for each recipient 2) Key Agreement: The AES CEK is uniquely wrapped for each recipient
using a pairwise symmetric key-encryption key (KEK) generated using using a pairwise symmetric key-encryption key (KEK) generated using
DH-ES using the a randomly generated private key value for the DH-ES using the a randomly generated private key value for the
originator, the recipient's public DH key and other values. Section originator, the recipient's public DH key and other values. Section
2.XX provides additional details. 2.3 provides additional details.
3) "Previously Distributed" Symmetric KEK: The AES CEK is wrapped 3) "Previously Distributed" Symmetric KEK: The AES CEK is wrapped
using a "previously distributed" symmetric KEK (such as a Mail List using a "previously distributed" symmetric KEK (such as a Mail List
Key). The methods by which the symmetric KEK is generated and Key). The methods by which the symmetric KEK is generated and
distributed are beyond the scope of this document. Section 2.XXX distributed are beyond the scope of this document. Section 2.4
provides more details. provides additional details.
4) Password Encryption: The AES CEK is wrapped using a KEK derived
from a password or other shared-secret value. Section 2.5 provides
additional details.
2.1 EnvelopedData Fields 2.1 EnvelopedData Fields
The enveloped-data content type is ASN.1 encoded using the The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax must be EnvelopedData syntax. The fields of the EnvelopedData syntax must be
populated as follows: populated as follows:
The EnvelopedData version MUST be either 0 or 2. The EnvelopedData version is determined based on a number of factors.
See [CMS] section 6.1 for the algorithm to determine this value.
The EnvelopedData originatorInfo field is not used for the RSAES-OAEP The EnvelopedData originatorInfo field is not used for the RSAES-OAEP
key transport algorithm. However, this field MAY be present to key transport algorithm. However, this field MAY be present to
support recipients using other key management algorithms. support recipients using other key management algorithms.
The EnvelopedData recipientInfos CHOICE is dependent on the key The EnvelopedData recipientInfos CHOICE is dependent on the key
managment technique used. Section 2.2, 2.3 and 2.4 provide more management technique used. Section 2.2, 2.3 and 2.4 provide
inforamtion. additional information.
The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
field MUST specify a symmetric encryption algorithm. Implementations field MUST specify a symmetric encryption algorithm. Implementations
MUST support the encryption of AES keys, but implementations MAY MUST support the encryption of AES keys, but implementations MAY
support other algorithms as well. support other algorithms as well.
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The EnvelopedData unprotectedAttrs MAY be present. The EnvelopedData unprotectedAttrs MAY be present.
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2.2 KeyTransRecipientInfo Fields 2.2 KeyTransRecipientInfo Fields
The enveloped-data content type is ASN.1 encoded using the The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax must be EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be
populated as follows: populated as follows:
The KeyTransRecipientInfo version MUST be either 0 or 2. If the The KeyTransRecipientInfo version MUST be either 0 or 2. If the
RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
version MUST be 0. If the RecipientIdentifier is version MUST be 0. If the RecipientIdentifier is
subjectKeyIdentifier, then the version MUST be 2. subjectKeyIdentifier, then the version MUST be 2.
The KeyTransRecipientInfo RecipientIdentifier provides two The KeyTransRecipientInfo RecipientIdentifier provides two
alternatives for specifying the recipient's certificate, and thereby alternatives for specifying the recipient's certificate, and thereby
the recipient's public key. The recipient's certificate must contain the recipient's public key. The recipient's certificate must contain
a RSA public key. The content-encryption key is encrypted with the a RSA public key. The content-encryption key is encrypted with the
recipient's RSA public key. The issuerAndSerialNumber alternative recipient's RSA public key. The issuerAndSerialNumber alternative
identifies the recipient's certificate by the issuer's distinguished identifies the recipient's certificate by the issuer's distinguished
name and the certificate serial number; the subjectKeyIdentifier name and the certificate serial number; the subjectKeyIdentifier
identifies the recipient's certificate by the X.509 identifies the recipient's certificate by the X.509
subjectKeyIdentifier extension value. subjectKeyIdentifier extension value.
The KeyTransRecipientInfo keyEncryptionAlgorithm specifies that the The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the
RSAES-OAEP algorithm, and its associated parameters, was used to RSAES-OAEP algorithm, and the associated parameters used to encrypt
encrypt the content-encryption key for the recipient. The key- the content-encryption key for the recipient. The key-encryption
encryption process is described in [PKCS#1v2.0]. See section 3 of process is described in [PKCS#1v2.0]. See section 4.1 of this
this document for the algorithm identifier and the parameter syntax. document for the algorithm identifier and the parameter syntax.
The KeyTransRecipientInfo encryptedKey is the result of encrypting The KeyTransRecipientInfo encryptedKey is the result of encrypting
the content-encryption key in the recipient's RSA public key using the content-encryption key in the recipient's RSA public key using
the RSAES-OAEP algorithm. When using a Triple-DES content-encryption the RSAES-OAEP algorithm.
key, implementations MUST adjust the parity bits for each DES key
comprising the Triple-DES key prior to RSAES-OAEP encryption. Note: When using a Triple-DES content-encryption key, implementations
MUST adjust the parity bits for each DES key comprising the Triple-
DES key prior to RSAES-OAEP encryption.
2.3 KeyAgreeRecipientInfo Fields 2.3 KeyAgreeRecipientInfo Fields
This section describes the conventions for using ES-DH and AES with This section describes the conventions for using ES-DH and AES with
the CMS enveloped-data content type to support key agreement. When the CMS enveloped-data content type to support key agreement. When
key agreement is used, then the RecipientInfo keyAgreeRecipientInfo key agreement is used, then the RecipientInfo keyAgreeRecipientInfo
CHOICE MUST be used. CHOICE MUST be used.
The KeyAgreeRecipient version MUST be 3. The KeyAgreeRecipient version MUST be 3.
The EnvelopedData originatorInfo field must be the originatorKey The EnvelopedData originatorInfo field must be the originatorKey
alternative. The originatoryKey algorithm fields MUST contain the alternative. The originatoryKey algorithm fields MUST contain the
dh-public-number object identifier with absent parameters. The dh-public-number object identifier with absent parameters. The
originatorKey publicKey MUST contain the senderĂs ephemeral public originatorKey publicKey MUST contain the senderĂs ephemeral public
key. key.
The EnvelopedData ukm MAY be absent. The EnvelopedData ukm MAY be absent.
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The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH
algorithm identifier. algorithm identifier.
2.3.1 ES-DH/AES Key Derivation 2.3.1 ES-DH/AES Key Derivation
Generation of the an AES key used in doing AES-KeyWrap is done using Generation of the an AES key used in doing AES-KeyWrap is done using
the method in [DH] with the following modifications: the method in [DH] with the following modifications:
The Hash function H will be [SHA-256] rather than SHA-1. The Hash function H will be [SHA-256] rather than SHA-1.
NOTE: 2 examples to be provided at this location. 2.3.1.1 Example 1
ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13
The key wrap algorithm is AES-128 wrap, so we need 128 bits (20
bytes) of keying material.
No partyAInfo is used.
Consequently, the input to the first invocation of SHA-256 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 1d
30 13
06 0b TBS ; AES-128 wrap OID
04 04
00 00 00 01 ; Counter
a2 06
04 04
00 00 00 80 ; key length
And the output is the 32 bytes:
TBS
Consenquently,
K=TBS
2.3.1.2 Example 2
ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13
The key wrap algorithm is AES-256 key wrap, so we need 256 bits (32
bytes) of keying material.
The partyAInfo used is the 64 bytes
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
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Consequently, the input to SHA-256 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 61
30 13
06 0b TBS ; AES-256 wrap OID
04 04
00 00 00 01 ; Counter
a0 42
04 40
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
a2 06
04 04
00 00 01 00 ; key length
And the output is the 32 bytes:
TBS
Consequently,
K=TBS
2.3.2 AES CEK Wrap Process 2.3.2 AES CEK Wrap Process
To be supplied. To be supplied.
2.4 KEKRecipientInfo Fields 2.4 KEKRecipientInfo Fields
This section describes the conventions for using AES with the CMS This section describes the conventions for using AES with the CMS
enveloped-data content type to support "previously distributed" enveloped-data content type to support previously distributed
symmetric KEKs. When a "previously distributed" symmetric KEK is symmetric KEKs. When a previously distributed symmetric KEK is used
used to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE
CHOICE MUST be used. The methods used to generate and distribute the MUST be used. The methods used to generate and distribute the
symmetric KEK are beyond the scope of this document. symmetric KEK are beyond the scope of this document. One possible
method of distributing keys is documented in [SYMKEYDIST].
The KEKRecipientInfo fields MUST be populated as specified in [CMS] The KEKRecipientInfo fields MUST be populated as specified in [CMS]
Section 6.2.3, "KEKRecipientInfo Type". The KEKRecipientInfo Section 6.2.3, KEKRecipientInfo Type.
keyEncryptionAlgorithm algorithm field MUST be the id-NIST-AES-KEY-
WRAP OID indicating that the AES wrap function is used to wrap the The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be
AES CEK. The KEKRecipientInfo keyEncryptionAlgorithm parameters field the id-NIST-AES-KEY-WRAP OID indicating that the AES wrap function is
MUST be absent. The KEKRecipientInfo encryptedKey field MUST include used to wrap the AES CEK. The KEKRecipientInfo keyEncryptionAlgorithm
the AES CEK wrapped using the "previously distributed" symmetric KEK parameters field MUST be absent.
as input to the AES wrap function.
The KEKRecipientInfo encryptedKey field MUST include the AES CEK
wrapped using the previously distributed symmetric KEK as input to
the AES wrap function.
To Be Decided ű Do we have multiple sizes of key wrap algorithms. To Be Decided ű Do we have multiple sizes of key wrap algorithms.
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3. Encrypted-data Conventions 2.5 PasswordRecipientInfo Fields
To be supplied. To Be Provided
4. Authenticated-data Conventions 3 Encrypted-data Conventions
To be supplied. The encrypted-data content type is ASN.1 encoded using the
EncryptededData syntax. The fields of the EncryptedData syntax MUST
be populated as follows:
5. Algorithm Identifiers and Parameters The EncryptedData version is determined based on a number of factors.
See [CMS] section 9.1 for the algorithm to determine this value.
5.1 RSAES-OAEP Algorithm Identifiers and Parameters The EncryptedData encryptedContentInfo contentEncryptionAlgorithm
field MUST specify a symmetric encryption algorithm. Implementations
MUST support the encryption of AES keys, but implementations MAY
support other algorithms as well.
The EncryptedData unprotectedAttrs MAY be present.
4 Algorithm Identifiers and Parameters
4.1 AES Algorithm Identifiers and Parameters
The AES algorithm is defined in [AES]. RSA #1 v1.5 MUST NOT be used
to transport AES keys.
AES is added to the set of symmetric content encryption algorithms in
CMS. The AES content-encryption algorithm in Cipher Block Chaining
(CBC) mode for the three different key sizes are identified by the
OID:
id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain a AES-IV associated with this OID
contains the initialization vector IV:
AES-IV ::= OCTET STRING (SIZE(16))
Content encryption algorithm identifiers are located in the
EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.
Content encryption algorithms are used to encipher the content
located in the EnvelopedData EncryptedContentInfo encryptedContent
and the EncryptedData EncryptedContentInfo encryptedContent fields.
4.2 RSAES-OAEP Algorithm Identifiers and Parameters
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The RSAES-OAEP key transport algorithm is the RSA encryption scheme The RSAES-OAEP key transport algorithm is the RSA encryption scheme
defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted
is the content-encryption key. The algorithm identifier for RSAES- is the content-encryption key.
OAEP is:
id-RSAES-OAEP OBJECT IDENTIFIER ::= The RSA key is identified in a certificate using the OID
rsaEncryption.
Schaad, Housley 5 pkcs-1 OBJECT IDENTIFIER ::=
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iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) }
7 }
The AlgorithmIdentifier parameters field must be present, and the rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1 }
parameters field must contain RSAES-OAEP-params. RSAES-OAEP-params
Note: This is the same algorithm identifier used by RSAES-PKCS1-v1_5.
This means that the existence of an RSA key in a certificate cannot
be used to infer that a recipient can decrypt an RSAES-OAEP encrypted
content-encryption key.
The algorithm identifier for RSAES-OAEP is:
id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain RSAES-OAEP-params. RSAES-OAEP-params
have the following syntax: have the following syntax:
RSAES-OAEP-params ::= SEQUENCE RSAES-OAEP-params ::= SEQUENCE
hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
pSourceFunc [2] AlgorithmIdentifier DEFAULT pSourceFunc [2] AlgorithmIdentifier DEFAULT
pSpecifiedEmptyIdentifier } pSpecifiedEmptyIdentifier }
sha1Identifier ::= AlgorithmIdentifier sha1Identifier ::= AlgorithmIdentifier
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pSpecifiedEmptyIdentifier ::= AlgorithmIdentifier pSpecifiedEmptyIdentifier ::= AlgorithmIdentifier
id-pSpecified, OCTET STRING SIZE (0) } id-pSpecified, OCTET STRING SIZE (0) }
id-sha1 OBJECT IDENTIFIER ::= id-sha1 OBJECT IDENTIFIER ::=
iso(1) identified-organization(3) oiw(14) secsig(3) iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) 26 } algorithms(2) 26 }
id-mgf1 OBJECT IDENTIFIER ::= id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
8 }
id-pSpecified OBJECT IDENTIFIER ::=
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 }
9 }
The fields of type RSAES-OAEP-params have the following meanings: The fields of type RSAES-OAEP-params have the following meanings:
hashFunc identifies the one-way hash function. Implementations hashFunc identifies the one-way hash function. Implementations MUST
MUST support SHA-1 [SHA1]. The SHA-1 algorithm identifier is support SHA-1 [SHA1]. The SHA-1 algorithm identifier is comprised of
comprised of the id-sha1 object identifier and a parameter of NULL. the id-sha1 object identifier and a parameter of NULL.
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Use of the AES Algorithm in CMS November 2000
Implementations that perform encryption MUST omit the hashFunc field Implementations that perform encryption MUST omit the hashFunc field
when SHA-1 is used, indicating that the default algorithm was used. when SHA-1 is used, indicating that the default algorithm was used.
Implementations that perform decryption MUST recognize both the id- Implementations that perform decryption MUST recognize both the id-
sha1 object identifier and an absent hashFunc field as an indication sha1 object identifier and an absent hashFunc field as an indication
that SHA-1 was used. that SHA-1 was used.
maskGenFunc identifies the mask generation function. maskGenFunc identifies the mask generation function. Implementations
Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a one- MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a one-way hash
way hash function, and it is identified in the parameter field of the function, and it is identified in the parameter field of the MFG1
MFG1 algorithm identifier. Implementations MUST support SHA-1 algorithm identifier. Implementations MUST support SHA-1 [SHA1].
[SHA1]. The MFG1 algorithm identifier is comprised of the id-mgf1 The MFG1 algorithm identifier is comprised of the id-mgf1 object
object identifier and a parameter that contains the algorithm identifier and a parameter that contains the algorithm identifier of
identifier of the one-way hash function employed with MFG1. The SHA- the one-way hash function employed with MFG1. The SHA-1 algorithm
1 algorithm identifier is comprised of the id-sha1 object identifier identifier is comprised of the id-sha1 object identifier and a
and a parameter of NULL. Implementations that perform encryption parameter of NULL. Implementations that perform encryption MUST omit
Schaad, Housley 6 the maskGenFunc field when MFG1 with SHA-1 is used, indicating that
Use of the AES Algorithm in CMS November 2000 the default algorithm was used. Implementations that perform
decryption MUST recognize both the id-mgf1 and id-sha1 object
MUST omit the maskGenFunc field when MFG1 with SHA-1 is used,
indicating that the default algorithm was used. Implementations that
perform decryption MUST recognize both the id-mgf1 and id-sha1 object
identifiers as well as an absent maskGenFunc field as an indication identifiers as well as an absent maskGenFunc field as an indication
that MFG1 with SHA-1 was used. that MFG1 with SHA-1 was used.
pSourceFunc identifies the source (and possibly the value) of the pSourceFunc identifies the source (and possibly the value) of the
encoding parameters, commonly called P. Implementations MUST encoding parameters, commonly called P. Implementations MUST
represent P by an algorithm identifier, id-pSpecified, indicating represent P by an algorithm identifier, id-pSpecified, indicating
that P is explicitly provided as an OCTET STRING in the parameters. that P is explicitly provided as an OCTET STRING in the parameters.
The default value for P is an empty string. In this case, pHash in The default value for P is an empty string. In this case, pHash in
EME-OAEP contains the hash of a zero length string. Implementations EME-OAEP contains the hash of a zero length string. Implementations
MUST support a zero length P value. Implementations that perform MUST support a zero length P value. Implementations that perform
encryption MUST omit the pSourceFunc field when a zero length P value encryption MUST omit the pSourceFunc field when a zero length P value
is used, indicating that the default value was used. Implementations is used, indicating that the default value was used. Implementations
that perform decryption MUST recognize both the id-pSpecified object that perform decryption MUST recognize both the id-pSpecified object
identifier and an absent pSourceFunc field as an indication that a identifier and an absent pSourceFunc field as an indication that a
zero length P value was used. zero length P value was used.
5.2 AES Algorithm Identifiers and Parameters 5 SMIMECapabilities Attribute Conventions
AES is added to the set of symmetric content encryption algorithms in
CMS. The AES content-encryption algorithm in Cipher Block Chaining
(CBC) mode for the three different key sizes are identified by the
OID:
id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain a AES-IV associated with this OID
contains the initialization vector IV:
AES-IV ::= OCTET STRING (SIZE(16))
6 SMIMECapabilities Attribute Conventions
An S/MIME client SHOULD announce the set of cryptographic functions An S/MIME client SHOULD announce the set of cryptographic functions
it supports by using the S/MIME capabilities attribute. This it supports by using the S/MIME capabilities attribute. This
attribute provides a partial list of OIDs of cryptographic functions attribute provides a partial list of object identifiers of
and MUST be signed by the client. The algorithm OIDs SHOULD be cryptographic functions and MUST be signed by the client. The
logically separated in functional categories and MUST be ordered with algorithm OIDs SHOULD be logically separated in functional categories
respect to their preference. and MUST be ordered with respect to their preference.
RFC 2633, Section 2.5.2 defines the SMIMECapabilities signed RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
used to specify a partial list of algorithms that the software used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support. announcing the SMIMECapabilities can support.
6.1 RSAES-OEAP SMIMECapability Attribute 5.1 RSAES-OEAP SMIMECapability Attribute
Schaad, Housley 7
Use of the AES Algorithm in CMS November 2000
When constructing a signedData object, compliant software MAY include When constructing a signedData object, compliant software MAY include
the SMIMECapabilities signed attribute announcing that it supports the SMIMECapabilities signed attribute announcing that it supports
the RSAES-OAEP algorithm. the RSAES-OAEP algorithm.
Schaad, Housley 10
Use of the AES Algorithm in CMS November 2000
The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the
id-RSAES-OAEP object identifier in the capabilityID field and MUST id-RSAES-OAEP object identifier in the capabilityID field and MUST
include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters
field. field.
RSAES-OAEP-Default-Identifier ::= AlgorithmIdentifier RSAES-OAEP-Default-Identifier ::= AlgorithmIdentifier
id-RSAES-OAEP, id-RSAES-OAEP,
sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier
} } } }
When all of the default settings are selected, the SMIMECapability When all of the default settings are selected, the SMIMECapability
SEQUENCE representing RSAES-OAEP MUST be DER-encoded as: SEQUENCE representing RSAES-OAEP MUST be DER-encoded as:
30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00 30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00
6.2 AES S/MIME Capability Attributes 5.2 AES S/MIME Capability Attributes
If an S/MIME client is required to support symmetric encryption with If an S/MIME client is required to support symmetric encryption with
AES, the capabilities attribute MUST contain the AES OID specified AES, the capabilities attribute MUST contain the AES object
above in the category of symmetric algorithms. The parameter identifier specified above in the category of symmetric algorithms.
associated with this OID MUST is AESSMimeCapability. The parameter associated with this object identifier MUST is
AESSMimeCapability.
AESSMimeCapabilty ::= NULL AESSMimeCapabilty ::= NULL
The encodings for the mandatory key sizes are: The encodings for the mandatory key sizes are:
Key Size Capability Key Size Capability
128 30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00 128 30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00
196 30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00 196 30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00
256 30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00 256 30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00
When a sending agent creates an encrypted message, it has to decide When a sending agent creates an encrypted message, it has to decide
which type of encryption algorithm to use. In general the decision which type of encryption algorithm to use. In general the decision
process involves information obtained from the capabilities lists process involves information obtained from the capabilities lists
included in messages received from the recipient, as well as other included in messages received from the recipient, as well as other
information such as private agreements, user preferences, legal information such as private agreements, user preferences, legal
restrictions, and so on. If users require AES for symmetric restrictions, and so on. If users require AES for symmetric
encryption, the S/MIME clients on both the sending and receiving side encryption, the S/MIME clients on both the sending and receiving side
MUST support it, and it MUST be set in the user preferences. MUST support it, and it MUST be set in the user preferences.
7. Security Considerations 6 Security Considerations
Note on mix of OEAP and v1.5 RSA encryption from RFC 2437 If RSA-OAEP and RSA #1 v1.5 are both used to transport the same
content encryption key, then an attacker can still use the
Bleichenbacher attack against the RSA #1 v1.5 encrypted key. It is
generally unadvisable to mix both RSA-OAEP and RSA #1 v1.5 in the
same set of recipients.
Implementations must protect the RSA private key and the content- Implementations must protect the RSA private key and the content-
encryption key. Compromise of the RSA private key may result in the encryption key. Compromise of the RSA private key may result in the
disclosure of all messages protected with that key. Compromise of disclosure of all messages protected with that key. Compromise of
Schaad, Housley 11
Use of the AES Algorithm in CMS November 2000
the content-encryption key may result in disclosure of the associated the content-encryption key may result in disclosure of the associated
encrypted content. encrypted content.
Schaad, Housley 8
Use of the AES Algorithm in CMS November 2000
Implementations must protect the key management private key and the Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Compromise of the key permits masquerade of authenticated data. Compromise of the
message-authentication key may result in undetectable modification of message-authentication key may result in undetectable modification of
the authenticated content. the authenticated content.
The generation of RSA public/private key pairs relies on a random The generation of RSA public/private key pairs relies on a random
numbers. The use of inadequate pseudo-random number generators numbers. The use of inadequate pseudo-random number generators
(PRNGs) to generate cryptographic keys can result in little or no (PRNGs) to generate cryptographic keys can result in little or no
security. An attacker may find it much easier to reproduce the PRNG security. An attacker may find it much easier to reproduce the PRNG
environment that produced the keys, searching the resulting small set environment that produced the keys, searching the resulting small set
of possibilities, rather than brute force searching the whole key of possibilities, rather than brute force searching the whole key
space. The generation of quality random numbers is difficult. RFC space. The generation of quality random numbers is difficult. RFC
1750 [RANDOM] offers important guidance in this area. 1750 [RANDOM] offers important guidance in this area.
8. Open Issues 7 Open Issues
- Key wrap algorithm is undetermined. - Key wrap algorithm is undetermined.
- Mandatory key sizes for Key Wrap - Mandatory key sizes for Key Wrap
- Mandatory key sizes for AES algorithm - Mandatory key sizes for AES algorithm
- Supplying any patent and licensing statements.
- References to each algorithm that would be acceptable to the RFC - References to each algorithm that would be acceptable to the RFC
editor. editor.
- Does the oid for key derivation need to be changed since we are - Does the oid for key derivation need to be changed since we are
using SHA-256 not SHA-1? using SHA-256 not SHA-1?
References References
AES J. Daemen, V. Rijmen, "The Rijndael Block Cipher"
http://csrc.nist.gov/encryption/aes/rijndael/Rijndael.pdf
3rd September 1999.
CMS Housley, R. Cryptographic Message Syntax. RFC 2630. CMS Housley, R. Cryptographic Message Syntax. RFC 2630.
June 1999. June 1999.
CRYPTO98 Bleichenbacher, D. "Chosen Ciphertext Attacks Against CRYPTO98 Bleichenbacher, D. "Chosen Ciphertext Attacks Against
Protocols Based on the RSA Encryption Standard PKCS #1," Protocols Based on the RSA Encryption Standard PKCS #1,"
in H. Krawczyk (editor), Advances in Cryptology - CRYPTO in H. Krawczyk (editor), Advances in Cryptology - CRYPTO
'98 '98
Proceedings, Lecture Notes in Computer Science 1462 Proceedings, Lecture Notes in Computer Science 1462 (1998),
(1998),
Springer-Verlag, pp. 1-12. Springer-Verlag, pp. 1-12.
DES National Institute of Standards and Technology.
FIPS Pub 46: Data Encryption Standard. 15 January 1977.
DH E. Rescorla, ˘Diffie-Hellman Key Agreement Method÷, RFC DH E. Rescorla, ˘Diffie-Hellman Key Agreement Method÷, RFC
2631, June 1999. 2631, June 1999.
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.
MSG Ramsdell, B., Editor. S/MIME Version 3 Message
Schaad, Housley 12
Use of the AES Algorithm in CMS November 2000
Specification. RFC 2633. June 1999.
PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5. PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
RFC 2313. March 1998. RFC 2313. March 1998.
PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0. PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0.
RFC 2437. October 1998. RFC 2437. October 1998.
PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet
Schaad, Housley 9
Use of the AES Algorithm in CMS November 2000
X.509 Public Key Infrastructure: Certificate and CRL X.509 Public Key Infrastructure: Certificate and CRL
Profile. RFC 2459. January 1999. Profile. RFC 2459. January 1999.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994. Recommendations for Security. RFC 1750. December 1994.
RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon. RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon.
Recent Results on PKCS #1: RSA Encryption Standard. Recent Results on PKCS #1: RSA Encryption Standard.
RSA Laboratories' Bulletin No. 7, June 26, 1998. RSA Laboratories' Bulletin No. 7, June 26, 1998.
[Available at [Available at http://www.rsasecurity.com/rsalabs/bulletins]
http://www.rsasecurity.com/rsalabs/bulletins]
SHA1 National Institute of Standards and Technology. SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995. FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
SSL Freier, A., P. Karlton, and P. Kocher. The SSL SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol,
Protocol,
Version 3.0. Netscape Communications. November 1996. Version 3.0. Netscape Communications. November 1996.
[Available at http://draft-freier-ssl-version3-02.txt] [Available at http://draft-freier-ssl-version3-02.txt]
SYMKEYDIST TBS
TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0. TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0.
RFC 2246. January 1999. RFC 2246. January 1999.
X.208-88 CCITT. Recommendation X.208: Specification of Abstract X.208-88 CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988. 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 Encoding Rules for Abstract Syntax Notation One (ASN.1).
Rules for Abstract Syntax Notation One (ASN.1). 1988. 1988.
X.509-88 CCITT. Recommendation X.509: The Directory - X.509-88 CCITT. Recommendation X.509: The Directory -
Authentication Authentication Framework. 1988.
Framework. 1988.
Acknowledgements Acknowledgements
This document is the result of contributions from many This document is the result of contributions from many
professionals. We appreciate the hard work of all members of the professionals. We appreciate the hard work of all members of the
IETF S/MIME Working Group. We wish to extend a special thanks to IETF S/MIME Working Group. We wish to extend a special thanks to
Burt Kaliski. Burt Kaliski.
Author's Addresses Author's Addresses
Jim Schaad Jim Schaad
Soaring Hawk Consulting Soaring Hawk Consulting
Schaad, Housley 13
Use of the AES Algorithm in CMS November 2000
Email: jimsch@exmsft.com Email: jimsch@exmsft.com
Russell Housley Russell Housley
RSA Laboratories RSA Laboratories
918 Spring Knoll Drive 918 Spring Knoll Drive
Herndon, VA 20170 Herndon, VA 20170
USA USA
Schaad, Housley 10 Email: rhousley@rsasecurity.com
Use of the AES Algorithm in CMS November 2000
rhousley@rsasecurity.com
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
Schaad, Housley 11 Schaad, Housley 14
 End of changes. 

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