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rfc:rfc3370

Network Working Group R. Housley Request for Comments: 3370 RSA Laboratories Obsoletes: 2630, 3211 August 2002 Category: Standards Track

           Cryptographic Message Syntax (CMS) Algorithms

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 This document describes the conventions for using several
 cryptographic algorithms with the Cryptographic Message Syntax (CMS).
 The CMS is used to digitally sign, digest, authenticate, or encrypt
 arbitrary message contents.

Table of Contents

 1     Introduction ...............................................  2
 1.1   Changes Since RFC 2630 .....................................  2
 1.2   Terminology ................................................  2
 2     Message Digest Algorithms ..................................  3
 2.1   SHA-1 ......................................................  3
 2.2   MD5 ........................................................  3
 3     Signature Algorithms .......................................  4
 3.1   DSA ........................................................  4
 3.2   RSA ........................................................  5
 4     Key Management Algorithms ..................................  6
 4.1   Key Agreement Algorithms ...................................  6
 4.1.1 X9.42 Ephemeral-Static Diffie-Hellman ......................  7
 4.1.2 X9.42 Static-Static Diffie-Hellman .........................  8
 4.2   Key Transport Algorithms ...................................  9
 4.2.1 RSA (PKCS #1 v1.5) ......................................... 10
 4.3   Symmetric Key-Encryption Key Algorithms .................... 10
 4.3.1 Triple-DES Key Wrap ........................................ 11
 4.3.2 RC2 Key Wrap ............................................... 12
 4.4   Key Derivation Algorithms .................................. 12

Housley Standards Track [Page 1] RFC 3370 CMS Algorithms August 2002

 4.4.1 PBKDF2 ..................................................... 13
 5     Content Encryption Algorithms .............................. 13
 5.1   Triple-DES CBC ............................................. 14
 5.2   RC2 CBC .................................................... 14
 6     Message Authentication Code (MAC) Algorithms ............... 15
 6.1   HMAC with SHA-1 ............................................ 15
 7     ASN.1 Module ............................................... 16
 8     References ................................................. 18
 9     Security Considerations .................................... 20
 10    Acknowledgments ............................................ 22
 11    Author's Address ........................................... 23
 12    Full Copyright Statement ................................... 24

1 Introduction

 The Cryptographic Message Syntax (CMS) [CMS] is used to digitally
 sign, digest, authenticate, or encrypt arbitrary message contents.
 This companion specification describes the use of common
 cryptographic algorithms with the CMS.  Implementations of the CMS
 may support these algorithms; implementations of the CMS may also
 support other algorithms as well.  However, if an implementation
 chooses to support one of the algorithms discussed in this document,
 then the implementation MUST do so as described in this document.
 The CMS values are generated using ASN.1 [X.208-88], using BER-
 encoding [X.209-88].  Algorithm identifiers (which include ASN.1
 object identifiers) identify cryptographic algorithms, and some
 algorithms require additional parameters.  When needed, parameters
 are specified with an ASN.1 structure.  The algorithm identifier for
 each algorithm is specified, and when needed, the parameter structure
 is specified.  The fields in the CMS employed by each algorithm are
 identified.

1.1 Changes Since RFC 2630

 This document obsoletes section 12 of RFC 2630 [OLDCMS].  RFC 3369
 [CMS] obsoletes the rest of RFC 2630.  Separation of the protocol and
 algorithm specifications allows each one to be updated without
 impacting the other.  However, the conventions for using additional
 algorithms with the CMS are likely to be specified in separate
 documents.

1.2 Terminology

 In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
 SHOULD NOT, RECOMMENDED, and MAY are to be interpreted as described
 in [STDWORDS].

Housley Standards Track [Page 2] RFC 3370 CMS Algorithms August 2002

2 Message Digest Algorithms

 This section specifies the conventions employed by CMS
 implementations that support SHA-1 or MD5.
 Digest algorithm identifiers are located in the SignedData
 digestAlgorithms field, the SignerInfo digestAlgorithm field, the
 DigestedData digestAlgorithm field, and the AuthenticatedData
 digestAlgorithm field.
 Digest values are located in the DigestedData digest field and the
 Message Digest authenticated attribute.  In addition, digest values
 are input to signature algorithms.

2.1 SHA-1

 The SHA-1 message digest algorithm is defined in FIPS Pub 180-1
 [SHA1].  The algorithm identifier for SHA-1 is:
    sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
        oiw(14) secsig(3) algorithm(2) 26 }
 There are two possible encodings for the SHA-1 AlgorithmIdentifier
 parameters field.  The two alternatives arise from the fact that when
 the 1988 syntax for AlgorithmIdentifier was translated into the 1997
 syntax, the OPTIONAL associated with the AlgorithmIdentifier
 parameters got lost.  Later the OPTIONAL was recovered via a defect
 report, but by then many people thought that algorithm parameters
 were mandatory.  Because of this history some implementations encode
 parameters as a NULL element and others omit them entirely.  The
 correct encoding is to omit the parameters field; however,
 implementations MUST also handle a SHA-1 AlgorithmIdentifier
 parameters field which contains a NULL.
 The AlgorithmIdentifier parameters field is OPTIONAL.  If present,
 the parameters field MUST contain a NULL.  Implementations MUST
 accept SHA-1 AlgorithmIdentifiers with absent parameters.
 Implementations MUST accept SHA-1 AlgorithmIdentifiers with NULL
 parameters.  Implementations SHOULD generate SHA-1
 AlgorithmIdentifiers with absent parameters.

2.2 MD5

 The MD5 digest algorithm is defined in RFC 1321 [MD5].  The algorithm
 identifier for MD5 is:
    md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) digestAlgorithm(2) 5 }

Housley Standards Track [Page 3] RFC 3370 CMS Algorithms August 2002

 The AlgorithmIdentifier parameters field MUST be present, and the
 parameters field MUST contain NULL.  Implementations MAY accept the
 MD5 AlgorithmIdentifiers with absent parameters as well as NULL
 parameters.

3 Signature Algorithms

 This section specifies the conventions employed by CMS
 implementations that support DSA or RSA (PKCS #1 v1.5).
 Signature algorithm identifiers are located in the SignerInfo
 signatureAlgorithm field of SignedData.  Also, signature algorithm
 identifiers are located in the SignerInfo signatureAlgorithm field of
 countersignature attributes.
 Signature values are located in the SignerInfo signature field of
 SignedData.  Also, signature values are located in the SignerInfo
 signature field of countersignature attributes.

3.1 DSA

 The DSA signature algorithm is defined in FIPS Pub 186 [DSS].  DSA
 MUST be used with the SHA-1 message digest algorithm.
 The algorithm identifier for DSA subject public keys in certificates
 is:
    id-dsa OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) x9-57 (10040) x9cm(4) 1 }
 DSA signature validation requires three parameters, commonly called
 p, q, and g.  When the id-dsa algorithm identifier is used, the
 AlgorithmIdentifier parameters field is optional.  If present, the
 AlgorithmIdentifier parameters field MUST contain the three DSA
 parameter values encoded using the Dss-Parms type.  If absent, the
 subject DSA public key uses the same DSA parameters as the
 certificate issuer.
    Dss-Parms ::= SEQUENCE {
      p INTEGER,
      q INTEGER,
      g INTEGER  }
 When the id-dsa algorithm identifier is used, the DSA public key,
 commonly called Y, MUST be encoded as an INTEGER.  The output of this
 encoding is carried in the certificate subject public key.
    Dss-Pub-Key ::= INTEGER  -- Y

Housley Standards Track [Page 4] RFC 3370 CMS Algorithms August 2002

 The algorithm identifier for DSA with SHA-1 signature values is:
    id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) x9-57 (10040) x9cm(4) 3 }
 When the id-dsa-with-sha1 algorithm identifier is used, the
 AlgorithmIdentifier parameters field MUST be absent.
 When signing, the DSA algorithm generates two values, commonly called
 r and s.  To transfer these two values as one signature, they MUST be
 encoded using the Dss-Sig-Value type:
    Dss-Sig-Value ::= SEQUENCE {
      r INTEGER,
      s INTEGER }

3.2 RSA

 The RSA (PKCS #1 v1.5) signature algorithm is defined in RFC 2437
 [NEWPKCS#1].  RFC 2437 specifies the use of the RSA signature
 algorithm with the SHA-1 and MD5 message digest algorithms.
 The algorithm identifier for RSA subject public keys in certificates
 is:
    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
 When the rsaEncryption algorithm identifier is used, the
 AlgorithmIdentifier parameters field MUST contain NULL.
 When the rsaEncryption algorithm identifier is used, the RSA public
 key, which is composed of a modulus and a public exponent, MUST be
 encoded using the RSAPublicKey type.  The output of this encoding is
 carried in the certificate subject public key.
    RSAPublicKey ::= SEQUENCE {
       modulus INTEGER, -- n
       publicExponent INTEGER } -- e
 CMS implementations that include the RSA (PKCS #1 v1.5) signature
 algorithm MUST also implement the SHA-1 message digest algorithm.
 Such implementations SHOULD also support the MD5 message digest
 algorithm.

Housley Standards Track [Page 5] RFC 3370 CMS Algorithms August 2002

 The rsaEncryption algorithm identifier is used to identify RSA (PKCS
 #1 v1.5) signature values regardless of the message digest algorithm
 employed.  CMS implementations that include the RSA (PKCS #1 v1.5)
 signature algorithm MUST support the rsaEncryption signature value
 algorithm identifier, and CMS implementations MAY support RSA (PKCS
 #1 v1.5) signature value algorithm identifiers that specify both the
 RSA (PKCS #1 v1.5) signature algorithm and the message digest
 algorithm.
 The algorithm identifier for RSA (PKCS #1 v1.5) with SHA-1 signature
 values is:
    sha1WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
        member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5 }
 The algorithm identifier for RSA (PKCS #1 v1.5) with MD5 signature
 values is:
    md5WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
        member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4 }
 When the rsaEncryption, sha1WithRSAEncryption, or
 md5WithRSAEncryption signature value algorithm identifiers are used,
 the AlgorithmIdentifier parameters field MUST be NULL.
 When signing, the RSA algorithm generates a single value, and that
 value is used directly as the signature value.

4 Key Management Algorithms

 CMS accommodates the following general key management techniques: key
 agreement, key transport, previously distributed symmetric key-
 encryption keys, and passwords.

4.1 Key Agreement Algorithms

 This section specifies the conventions employed by CMS
 implementations that support key agreement using X9.42 Ephemeral-
 Static Diffie-Hellman (X9.42 E-S D-H) and X9.42 Static-Static
 Diffie-Hellman (X9.42 S-S D-H).
 When a key agreement algorithm is used, a key-encryption algorithm is
 also needed.  Therefore, when key agreement is supported, a key-
 encryption algorithm MUST be provided for each content-encryption
 algorithm.  The key wrap algorithms for Triple-DES and RC2 are
 described in RFC 3217 [WRAP].

Housley Standards Track [Page 6] RFC 3370 CMS Algorithms August 2002

 For key agreement of RC2 key-encryption keys, 128 bits MUST be
 generated as input to the key expansion process used to compute the
 RC2 effective key [RC2].
 Key agreement algorithm identifiers are located in the EnvelopedData
 RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
 keyEncryptionAlgorithm fields.
 Key wrap algorithm identifiers are located in the KeyWrapAlgorithm
 parameters within the EnvelopedData RecipientInfos
 KeyAgreeRecipientInfo keyEncryptionAlgorithm and AuthenticatedData
 RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm fields.
 Wrapped content-encryption keys are located in the EnvelopedData
 RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
 encryptedKey field.  Wrapped message-authentication keys are located
 in the AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
 RecipientEncryptedKeys encryptedKey field.

4.1.1 X9.42 Ephemeral-Static Diffie-Hellman

 Ephemeral-Static Diffie-Hellman key agreement is defined in RFC 2631
 [DH-X9.42].  When using Ephemeral-Static Diffie-Hellman, the
 EnvelopedData RecipientInfos KeyAgreeRecipientInfo field is used as
 follows:
    version MUST be 3.
    originator MUST be the originatorKey alternative.  The
    originatorKey algorithm field MUST contain the dh-public-number
    object identifier with absent parameters.  The originatorKey
    publicKey field MUST contain the sender's ephemeral public key.
    The dh-public-number object identifier is:
       dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
           us(840) ansi-x942(10046) number-type(2) 1 }
    ukm may be present or absent.  CMS implementations MUST support
    ukm being absent, and CMS implementations SHOULD support ukm being
    present.  When present, the ukm is used to ensure that a different
    key-encryption key is generated when the ephemeral private key
    might be used more than once.

Housley Standards Track [Page 7] RFC 3370 CMS Algorithms August 2002

    keyEncryptionAlgorithm MUST be the id-alg-ESDH algorithm
    identifier.  The algorithm identifier parameter field for id-alg-
    ESDH is KeyWrapAlgorithm, and this parameter MUST be present.  The
    KeyWrapAlgorithm denotes the symmetric encryption algorithm used
    to encrypt the content-encryption key with the pairwise key-
    encryption key generated using the X9.42 Ephemeral-Static Diffie-
    Hellman key agreement algorithm. Triple-DES and RC2 key wrap
    algorithms are described in RFC 3217 [WRAP].  The id-alg-ESDH
    algorithm identifier and parameter syntax is:
       id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2)
           us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
           alg(3) 5 }
       KeyWrapAlgorithm ::= AlgorithmIdentifier
    recipientEncryptedKeys contains an identifier and an encrypted key
    for each recipient.  The RecipientEncryptedKey
    KeyAgreeRecipientIdentifier MUST contain either the
    issuerAndSerialNumber identifying the recipient's certificate or
    the RecipientKeyIdentifier containing the subject key identifier
    from the recipient's certificate.  In both cases, the recipient's
    certificate contains the recipient's static public key.
    RecipientEncryptedKey EncryptedKey MUST contain the
    content-encryption key encrypted with the X9.42 Ephemeral-Static
    Diffie-Hellman generated pairwise key-encryption key using the
    algorithm specified by the KeyWrapAlgorithm.

4.1.2 X9.42 Static-Static Diffie-Hellman

 Static-Static Diffie-Hellman key agreement is defined in RFC 2631
 [DH-X9.42].  When using Static-Static Diffie-Hellman, the
 EnvelopedData RecipientInfos KeyAgreeRecipientInfo and
 AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are
 used as follows:
    version MUST be 3.
    originator MUST be either the issuerAndSerialNumber or
    subjectKeyIdentifier alternative.  In both cases, the originator's
    certificate contains the sender's static public key.  RFC 3279
    [CERTALGS] specifies the AlgorithmIdentifier parameters syntax and
    values that are included in the originator's certificate.  The
    originator's certificate subject public key information field MUST
    contain the dh-public-number object identifier:
       dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
           us(840) ansi-x942(10046) number-type(2) 1 }

Housley Standards Track [Page 8] RFC 3370 CMS Algorithms August 2002

    ukm MUST be present.  The ukm ensures that a different key-
    encryption key is generated for each message between the same
    sender and recipient.
    keyEncryptionAlgorithm MUST be the id-alg-SSDH algorithm
    identifier.  The algorithm identifier parameter field for id-alg-
    SSDH is KeyWrapAlgorihtm, and this parameter MUST be present.  The
    KeyWrapAlgorithm denotes the symmetric encryption algorithm used
    to encrypt the content-encryption key with the pairwise key-
    encryption key generated using the X9.42 Static-Static Diffie-
    Hellman key agreement algorithm.  Triple-DES and RC2 key wrap
    algorithms are described in RFC 3217 [WRAP].  The id-alg-SSDH
    algorithm identifier and parameter syntax is:
       id-alg-SSDH OBJECT IDENTIFIER ::= { iso(1) member-body(2)
           us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
           alg(3) 10 }
       KeyWrapAlgorithm ::= AlgorithmIdentifier
    recipientEncryptedKeys contains an identifier and an encrypted key
    for each recipient.  The RecipientEncryptedKey
    KeyAgreeRecipientIdentifier MUST contain either the
    issuerAndSerialNumber identifying the recipient's certificate or
    the RecipientKeyIdentifier containing the subject key identifier
    from the recipient's certificate.  In both cases, the recipient's
    certificate contains the recipient's static public key.
    RecipientEncryptedKey EncryptedKey MUST contain the content-
    encryption key encrypted with the X9.42 Static-Static Diffie-
    Hellman generated pairwise key-encryption key using the algorithm
    specified by the KeyWrapAlgortihm.

4.2 Key Transport Algorithms

 This section specifies the conventions employed by CMS
 implementations that support key transport using RSA (PKCS #1 v1.5).
 Key transport algorithm identifiers are located in the EnvelopedData
 RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm field.
 Key transport encrypted content-encryption keys are located in the
 EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey
 field.

Housley Standards Track [Page 9] RFC 3370 CMS Algorithms August 2002

4.2.1 RSA (PKCS #1 v1.5)

 The RSA key transport algorithm is the RSA encryption scheme defined
 in RFC 2313 [PKCS#1], block type is 02, where the message to be
 encrypted is the content-encryption key.  The algorithm identifier
 for RSA (PKCS #1 v1.5) is:
    rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
 The AlgorithmIdentifier parameters field MUST be present, and the
 parameters field MUST contain NULL.
 When using a Triple-DES content-encryption key, CMS implementations
 MUST adjust the parity bits for each DES key comprising the Triple-
 DES key prior to RSA encryption.
 The use of RSA (PKCS #1 v1.5) encryption, as defined in RFC 2313
 [PKCS#1], to provide confidentiality has a known vulnerability.  The
 vulnerability is primarily relevant to usage in interactive
 applications rather than to store-and-forward environments.  Further
 information and proposed countermeasures are discussed in the
 Security Considerations section of this document and RFC 3218 [MMA].
 Note that the same RSA encryption scheme is also defined in RFC 2437
 [NEWPKCS#1].  Within RFC 2437, this RSA encryption scheme is called
 RSAES-PKCS1-v1_5.

4.3 Symmetric Key-Encryption Key Algorithms

 This section specifies the conventions employed by CMS
 implementations that support symmetric key-encryption key management
 using Triple-DES or RC2 key-encryption keys.  When RC2 is supported,
 RC2 128-bit keys MUST be used as key-encryption keys, and they MUST
 be used with the RC2ParameterVersion parameter set to 58.  A CMS
 implementation MAY support mixed key-encryption and content-
 encryptionalgorithms.  For example, a 40-bit RC2 content-encryption
 key MAY be wrapped with a 168-bit Triple-DES key-encryption key or
 with a 128-bit RC2 key-encryption key.
 Key wrap algorithm identifiers are located in the EnvelopedData
 RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KEKRecipientInfo
 keyEncryptionAlgorithm fields.

Housley Standards Track [Page 10] RFC 3370 CMS Algorithms August 2002

 Wrapped content-encryption keys are located in the EnvelopedData
 RecipientInfos KEKRecipientInfo encryptedKey field.  Wrapped
 message-authentication keys are located in the AuthenticatedData
 RecipientInfos KEKRecipientInfo encryptedKey field.
 The output of a key agreement algorithm is a key-encryption key, and
 this key-encryption key is used to encrypt the content-encryption
 key.  To support key agreement, key wrap algorithm identifiers are
 located in the KeyWrapAlgorithm parameter of the EnvelopedData
 RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
 AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
 keyEncryptionAlgorithm fields.  However, only key agreement
 algorithms that inherently provide authentication ought to be used
 with AuthenticatedData.  Wrapped content-encryption keys are located
 in the EnvelopedData RecipientInfos KeyAgreeRecipientInfo
 RecipientEncryptedKeys encryptedKey field, wrapped message-
 authentication keys are located in the AuthenticatedData
 RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
 encryptedKey field.

4.3.1 Triple-DES Key Wrap

 A CMS implementation MAY support mixed key-encryption and content-
 encryption algorithms.  For example, a 128-bit RC2 content-encryption
 key MAY be wrapped with a 168-bit Triple-DES key-encryption key.
 Triple-DES key encryption has the algorithm identifier:
    id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
 The AlgorithmIdentifier parameter field MUST be NULL.
 The key wrap algorithm used to encrypt a Triple-DES content-
 encryption key with a Triple-DES key-encryption key is specified in
 section 3.1 of RFC 3217 [WRAP].  The corresponding key unwrap
 algorithm is specified in section 3.2 of RFC 3217 [WRAP].
 Out-of-band distribution of the Triple-DES key-encryption key used to
 encrypt the Triple-DES content-encryption key is beyond the scope of
 this document.

Housley Standards Track [Page 11] RFC 3370 CMS Algorithms August 2002

4.3.2 RC2 Key Wrap

 A CMS implementation MAY support mixed key-encryption and content-
 encryption algorithms.  For example, a 128-bit RC2 content-encryption
 key MAY be wrapped with a 168-bit Triple-DES key-encryption key.
 Similarly, a 40-bit RC2 content-encryption key MAY be wrapped with a
 128-bit RC2 key-encryption key.
 RC2 key encryption has the algorithm identifier:
    id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
 The AlgorithmIdentifier parameter field MUST be RC2wrapParameter:
    RC2wrapParameter ::= RC2ParameterVersion
    RC2ParameterVersion ::= INTEGER
 The RC2 effective-key-bits (key size) greater than 32 and less than
 256 is encoded in the RC2ParameterVersion.  For the effective-key-
 bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
 and 58 respectively.  These values are not simply the RC2 key length.
 Note that the value 160 must be encoded as two octets (00 A0),
 because the one octet (A0) encoding represents a negative number.
 RC2 128-bit keys MUST be used as key-encryption keys, and they MUST
 be used with the RC2ParameterVersion parameter set to 58.
 The key wrap algorithm used to encrypt a RC2 content-encryption key
 with a RC2 key-encryption key is specified in section 4.1 of RFC 3217
 [WRAP].  The corresponding key unwrap algorithm is specified 4.2 of
 RFC 3217 [WRAP].
 Out-of-band distribution of the RC2 key-encryption key used to
 encrypt the RC2 content-encryption key is beyond of the scope of this
 document.

4.4 Key Derivation Algorithms

 This section specifies the conventions employed by CMS
 implementations that support password-based key management using
 PBKDF2.
 Key derivation algorithms are used to convert a password into a key-
 encryption key as part of the password-based key management
 technique.

Housley Standards Track [Page 12] RFC 3370 CMS Algorithms August 2002

 Key derivation algorithm identifiers are located in the EnvelopedData
 RecipientInfos PasswordRecipientInfo keyDerivationAlgorithm and
 AuthenticatedData RecipientInfos PasswordRecipientInfo
 keyDerivationAlgorithm fields.
 The key-encryption key that is derived from the password is used to
 encrypt the content-encryption key.
 The content-encryption keys encrypted with password-derived key-
 encryption keys are located in the EnvelopedData RecipientInfos
 PasswordRecipientInfo encryptedKey field.  The message-authentication
 keys encrypted with password-derived key-encryption keys are located
 in the AuthenticatedData RecipientInfos PasswordRecipientInfo
 encryptedKey field.

4.4.1 PBKDF2

 The PBKDF2 key derivation algorithm is specified in RFC 2898
 [PKCS#5].  The KeyDerivationAlgorithmIdentifer identifies the key-
 derivation algorithm, and any associated parameters used to derive
 the key-encryption key from the user-supplied password.  The
 algorithm identifier for the PBKDF2 key derivation algorithm is:
    id-PBKDF2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) pkcs(1) pkcs-5(5) 12 }
 The AlgorithmIdentifier parameter field MUST be PBKDF2-params:
    PBKDF2-params ::= SEQUENCE {
      salt CHOICE {
        specified OCTET STRING,
        otherSource AlgorithmIdentifier },
      iterationCount INTEGER (1..MAX),
      keyLength INTEGER (1..MAX) OPTIONAL,
      prf AlgorithmIdentifier
        DEFAULT { algorithm hMAC-SHA1, parameters NULL } }
 Within the PBKDF2-params, the salt MUST use the specified OCTET
 STRING.

5 Content Encryption Algorithms

 This section specifies the conventions employed by CMS
 implementations that support content encryption using Three-Key
 Triple-DES in CBC mode, Two-Key Triple-DES in CBC mode, or RC2 in CBC
 mode.

Housley Standards Track [Page 13] RFC 3370 CMS Algorithms August 2002

 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
 field and the EncryptedData EncryptedContentInfo encryptedContent
 field.

5.1 Triple-DES CBC

 The Triple-DES algorithm is described in ANSI X9.52 [3DES].  The
 Triple-DES is composed from three sequential DES [DES] operations:
 encrypt, decrypt, and encrypt.  Three-Key Triple-DES uses a different
 key for each DES operation.  Two-Key Triple-DES uses one key for the
 two encrypt operations and a different key for the decrypt operation.
 The same algorithm identifiers are used for Three-Key Triple-DES and
 Two-Key Triple-DES.  The algorithm identifier for Triple-DES in
 Cipher Block Chaining (CBC) mode is:
    des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
        us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
 The AlgorithmIdentifier parameters field MUST be present, and the
 parameters field must contain a CBCParameter:
    CBCParameter ::= IV
    IV ::= OCTET STRING  -- exactly 8 octets

5.2 RC2 CBC

 The RC2 algorithm is described in RFC 2268 [RC2].  The algorithm
 identifier for RC2 in CBC mode is:
    rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
        rsadsi(113549) encryptionAlgorithm(3) 2 }
 The AlgorithmIdentifier parameters field MUST be present, and the
 parameters field MUST contain a RC2CBCParameter:
    RC2CBCParameter ::= SEQUENCE {
      rc2ParameterVersion INTEGER,
      iv OCTET STRING  }  -- exactly 8 octets

Housley Standards Track [Page 14] RFC 3370 CMS Algorithms August 2002

 The RC2 effective-key-bits (key size) greater than 32 and less than
 256 is encoded in the rc2ParameterVersion.  For the effective-key-
 bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
 and 58 respectively.  These values are not simply the RC2 key length.
 Note that the value 160 must be encoded as two octets (00 A0), since
 the one octet (A0) encoding represents a negative number.

6 Message Authentication Code Algorithms

 This section specifies the conventions employed by CMS
 implementations that support the HMAC with SHA-1 message
 authentication code (MAC).
 MAC algorithm identifiers are located in the AuthenticatedData
 macAlgorithm field.
 MAC values are located in the AuthenticatedData mac field.

6.1 HMAC with SHA-1

 The HMAC with SHA-1 algorithm is described in RFC 2104 [HMAC].  The
 algorithm identifier for HMAC with SHA-1 is:
    hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1)
       identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) 8 1 2 }
 There are two possible encodings for the HMAC with SHA-1
 AlgorithmIdentifier parameters field.  The two alternatives arise
 from the fact that when the 1988 syntax for the AlgorithmIdentifier
 type was translated into the 1997 syntax, the OPTIONAL associated
 with the AlgorithmIdentifier parameters got lost.  Later the OPTIONAL
 was recovered via a defect report, but by then many people thought
 that algorithm parameters were mandatory.  Because of this history
 some implementations may encode parameters as a NULL while others
 omit them entirely.
 The AlgorithmIdentifier parameters field is OPTIONAL.  If present,
 the parameters field MUST contain a NULL.  Implementations MUST
 accept HMAC with SHA-1 AlgorithmIdentifiers with absent parameters.
 Implementations MUST accept HMAC with SHA-1 AlgorithmIdentifiers with
 NULL parameters.  Implementations SHOULD generate HMAC with SHA-1
 AlgorithmIdentifiers with absent parameters.

Housley Standards Track [Page 15] RFC 3370 CMS Algorithms August 2002

7 ASN.1 Module

 CryptographicMessageSyntaxAlgorithms
     { iso(1) member-body(2) us(840) rsadsi(113549)
       pkcs(1) pkcs-9(9) smime(16) modules(0) cmsalg-2001(16) }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS All
  2. - The types and values defined in this module are exported for use
  3. - in the other ASN.1 modules. Other applications may use them for
  4. - their own purposes.
 IMPORTS
   -- Imports from RFC 3280 [PROFILE], Appendix A.1
         AlgorithmIdentifier
            FROM PKIX1Explicit88 { iso(1)
                 identified-organization(3) dod(6) internet(1)
                 security(5) mechanisms(5) pkix(7) mod(0)
                 pkix1-explicit(18) } ;
  1. - Algorithm Identifiers
 sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
     oiw(14) secsig(3) algorithm(2) 26 }
 md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
     rsadsi(113549) digestAlgorithm(2) 5 }
 id-dsa OBJECT IDENTIFIER ::=  { iso(1) member-body(2) us(840)
     x9-57(10040) x9cm(4) 1 }
 id-dsa-with-sha1 OBJECT IDENTIFIER ::=  { iso(1) member-body(2)
     us(840) x9-57(10040) x9cm(4) 3 }
 rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
 md5WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
     member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4 }
 sha1WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
     member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5 }
 dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) ansi-x942(10046) number-type(2) 1 }

Housley Standards Track [Page 16] RFC 3370 CMS Algorithms August 2002

 id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
     rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }
 id-alg-SSDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
     rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 10 }
 id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
 id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
 des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
     us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
 rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
     rsadsi(113549) encryptionAlgorithm(3) 2 }
 hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
     dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }
 id-PBKDF2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
     rsadsi(113549) pkcs(1) pkcs-5(5) 12 }
  1. - Public Key Types
 Dss-Pub-Key ::= INTEGER  -- Y
 RSAPublicKey ::= SEQUENCE {
   modulus INTEGER,  -- n
   publicExponent INTEGER }  -- e
 DHPublicKey ::= INTEGER  -- y = g^x mod p
  1. - Signature Value Types
 Dss-Sig-Value ::= SEQUENCE {
   r INTEGER,
   s INTEGER }
  1. - Algorithm Identifier Parameter Types
 Dss-Parms ::= SEQUENCE {
   p INTEGER,
   q INTEGER,
   g INTEGER }

Housley Standards Track [Page 17] RFC 3370 CMS Algorithms August 2002

 DHDomainParameters ::= SEQUENCE {
   p INTEGER,  -- odd prime, p=jq +1
   g INTEGER,  -- generator, g
   q INTEGER,  -- factor of p-1
   j INTEGER OPTIONAL,  -- subgroup factor
   validationParms ValidationParms OPTIONAL }
 ValidationParms ::= SEQUENCE {
   seed BIT STRING,
   pgenCounter INTEGER }
 KeyWrapAlgorithm ::= AlgorithmIdentifier
 RC2wrapParameter ::= RC2ParameterVersion
 RC2ParameterVersion ::= INTEGER
 CBCParameter ::= IV
 IV ::= OCTET STRING  -- exactly 8 octets
 RC2CBCParameter ::= SEQUENCE {
   rc2ParameterVersion INTEGER,
   iv OCTET STRING  }  -- exactly 8 octets
 PBKDF2-params ::= SEQUENCE {
   salt CHOICE {
     specified OCTET STRING,
     otherSource AlgorithmIdentifier },
   iterationCount INTEGER (1..MAX),
   keyLength INTEGER (1..MAX) OPTIONAL,
   prf AlgorithmIdentifier
     DEFAULT { algorithm hMAC-SHA1, parameters NULL } }
 END -- of CryptographicMessageSyntaxAlgorithms

8 References

 [3DES]      American National Standards Institute.  ANSI X9.52-1998,
             Triple Data Encryption Algorithm Modes of Operation.
             1998.
 [CERTALGS]  Bassham, L., Housley, R. and W. Polk, "Algorithms and
             Identifiers for the Internet X.509 Public Key
             Infrastructure Certificate and Certificate Revocation
             List (CRL) Profile", RFC 3279, April 2002.

Housley Standards Track [Page 18] RFC 3370 CMS Algorithms August 2002

 [CMS]       Housley, R., "Cryptographic Message Syntax", RFC 3269,
             August 2002.
 [DES]       American National Standards Institute.  ANSI X3.106,
             "American National Standard for Information Systems -
             Data Link Encryption".  1983.
 [DH-X9.42]  Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
             2631, June 1999.
 [DSS]       National Institute of Standards and Technology.  FIPS Pub
             186: Digital Signature Standard.  19 May 1994.
 [HMAC]      Krawczyk, H., "HMAC: Keyed-Hashing for Message
             Authentication", RFC 2104, February 1997.
 [MD5]       Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992.
 [MMA]       Rescorla, E., "Preventing the Million Message Attack on
             CMS", RFC 3218, January 2002.
 [MODES]     National Institute of Standards and Technology.  FIPS Pub
             81: DES Modes of Operation.  2 December 1980.
 [NEWPKCS#1] Kaliski, B. and J. Staddon, "PKCS #1: RSA Encryption,
             Version 2.0, RFC 2437, October 1998.
 [OLDCMS]    Housley, R., "Cryptographic Message Syntax", RFC 2630,
             June 1999.
 [PKCS#1]    Kaliski, B, "PKCS #1: RSA Encryption, Version 2.0", RFC
             2437, October, 1998.
 [PKCS#5]    Kaliski, B., "PKCS #5: Password-Based Cryptography
             Specification", RFC 2898, September 2000.
 [PROFILE]   Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
             X.509 Public Key Infrastructure Certificate and
             Certificate Revocation List (CRL) Profile", RFC 3280,
             April 2002.
 [RANDOM]    Eastlake, D., Crocker, S. and J. Schiller, "Randomness
             Recommendations for Security, RFC 1750, December 1994.
 [RC2]       Rivest, R., "A Description of the RC2 (r) Encryption
             Algorithm", RFC 2268, March 1998.

Housley Standards Track [Page 19] RFC 3370 CMS Algorithms August 2002

 [SHA1]      National Institute of Standards and Technology.  FIPS Pub
             180-1: Secure Hash Standard.  17 April 1995.
 [STDWORDS]  Bradner, S., "Key Words for Use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [WRAP]      Housley, R., "Triple-DES and RC2 Key Wrapping", RFC 3217,
             December 2001.
 [X.208-88]  CCITT.  Recommendation X.208: Specification of Abstract
             Syntax Notation One (ASN.1).  1988.
 [X.209-88]  CCITT.  Recommendation X.209: Specification of Basic
             Encoding Rules for Abstract Syntax Notation One (ASN.1).
             1988.

9 Security Considerations

 The CMS provides a method for digitally signing data, digesting data,
 encrypting data, and authenticating data.  This document identifies
 the conventions for using several cryptographic algorithms for use
 with the CMS.
 Implementations must protect the signer's private key.  Compromise of
 the signer's private key permits masquerade.
 Implementations must protect the key management private key, the
 key-encryption key, and the content-encryption key.  Compromise of
 the key management private key or the key-encryption key may result
 in the disclosure of all contents protected with that key.
 Similarly, compromise of the content-encryption key may result in
 disclosure of the associated encrypted content.
 Implementations must protect the key management private key and the
 message-authentication key.  Compromise of the key management private
 key permits masquerade of authenticated data.  Similarly, compromise
 of the message-authentication key may result in undetectable
 modification of the authenticated content.
 The key management technique employed to distribute message-
 authentication keys must itself provide authentication, otherwise the
 content is delivered with integrity from an unknown source.  Neither
 RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static Diffie-Hellman [DH-
 X9.42] provide the necessary data origin authentication.  Static-
 Static Diffie-Hellman [DH-X9.42] does provide the necessary data
 origin authentication when both the originator and recipient public
 keys are bound to appropriate identities in X.509 certificates
 [PROFILE].

Housley Standards Track [Page 20] RFC 3370 CMS Algorithms August 2002

 When more than two parties share the same message-authentication key,
 data origin authentication is not provided.  Any party that knows the
 message-authentication key can compute a valid MAC, therefore the
 content could originate from any one of the parties.
 Implementations must randomly generate content-encryption keys,
 message-authentication keys, initialization vectors (IVs), one-time
 values (such as the k value when generating a DSA signature), and
 padding.  Also, the generation of public/private key pairs relies on
 a random numbers.  The use of inadequate pseudo-random number
 generators (PRNGs) to generate cryptographic such values can result
 in little or no security.  An attacker may find it much easier to
 reproduce the PRNG environment that produced the keys, searching the
 resulting small set of possibilities, rather than brute force
 searching the whole key space.  The generation of quality random
 numbers is difficult.  RFC 1750 [RANDOM] offers important guidance in
 this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
 PRNG technique.
 When using key agreement algorithms or previously distributed
 symmetric key-encryption keys, a key-encryption key is used to
 encrypt the content-encryption key.  If the key-encryption and
 content-encryption algorithms are different, the effective security
 is determined by the weaker of the two algorithms.  If, for example,
 content is encrypted with 168-bit Triple-DES and the Triple-DES
 content-encryption key is wrapped with a 40-bit RC2 key, then at most
 40 bits of protection is provided.  A trivial search to determine the
 value of the 40-bit RC2 key can recover Triple-DES key, and then the
 Triple-DES key can be used to decrypt the content.  Therefore,
 implementers must ensure that key-encryption algorithms are as strong
 or stronger than content-encryption algorithms.
 RFC 3217 [WRAP] specifies key wrap algorithms used to encrypt a
 Triple-DES content-encryption key with a Triple-DES key-encryption
 key [3DES] or to encrypt a RC2 content-encryption key with a RC2
 key-encryption key [RC2].  The key wrap algorithms makes use of CBC
 mode [MODES].  These key wrap algorithms have been reviewed for use
 with Triple-DES and RC2.  They have not been reviewed for use with
 other cryptographic modes or other encryption algorithms.  Therefore,
 if a CMS implementation wishes to support ciphers in addition to
 Triple-DES or RC2, then additional key wrap algorithms need to be
 defined to support the additional ciphers.
 Implementers should be aware that cryptographic algorithms become
 weaker with time.  As new cryptanalysis techniques are developed and
 computing performance improves, the work factor to break a particular
 cryptographic algorithm will reduce.  Therefore, cryptographic

Housley Standards Track [Page 21] RFC 3370 CMS Algorithms August 2002

 algorithm implementations should be modular allowing new algorithms
 to be readily inserted.  That is, implementers should be prepared to
 regularly update the set of algorithms in their implementations.
 Users of the CMS, particularly those employing the CMS to support
 interactive applications, should be aware that RSA (PKCS #1 v1.5), as
 specified in RFC 2313 [PKCS#1], is vulnerable to adaptive chosen
 ciphertext attacks when applied for encryption purposes.
 Exploitation of this identified vulnerability, revealing the result
 of a particular RSA decryption, requires access to an oracle which
 will respond to a large number of ciphertexts (based on currently
 available results, hundreds of thousands or more), which are
 constructed adaptively in response to previously-received replies
 providing information on the successes or failures of attempted
 decryption operations.  As a result, the attack appears significantly
 less feasible to perpetrate for store-and-forward S/MIME environments
 than for directly interactive protocols.  Where the CMS constructs
 are applied as an intermediate encryption layer within an interactive
 request-response communications environment, exploitation could be
 more feasible.
 An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
 [NEWPKCS#1].  This updated document supersedes RFC 2313.  PKCS #1
 Version 2.0 preserves support for the encryption padding format
 defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new
 alternative.  To resolve the adaptive chosen ciphertext
 vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
 of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
 is used to provide confidentiality.  Designers of protocols and
 systems employing CMS for interactive environments should either
 consider usage of OAEP, or should ensure that information which could
 reveal the success or failure of attempted PKCS #1 Version 1.5
 decryption operations is not provided.  Support for OAEP will likely
 be added to a future version of the CMS algorithm specification.
 See RFC 3218 [MMA] for more information about thwarting the adaptive
 chosen ciphertext vulnerability in PKCS #1 Version 1.5
 implementations.

10 Acknowledgments

 This document is the result of contributions from many professionals.
 I appreciate the hard work of all members of the IETF S/MIME Working
 Group.  I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
 Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
 Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
 Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
 Jim Schaad, and Dave Solo for their efforts and support.

Housley Standards Track [Page 22] RFC 3370 CMS Algorithms August 2002

11 Author Address

 Russell Housley
 RSA Laboratories
 918 Spring Knoll Drive
 Herndon, VA 20170
 EMail: rhousley@rsasecurity.com

Housley Standards Track [Page 23] RFC 3370 CMS Algorithms August 2002

12. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Housley Standards Track [Page 24]

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