Cryptographic Message Syntax (CMS) Algorithms

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
   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].

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 }

   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
   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.

   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].

   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.

      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 }

      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.

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.

   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.

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.

   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.

   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

   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.

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

   -- EXPORTS All
   -- The types and values defined in this module are exported for use
   -- in the other ASN.1 modules.  Other applications may use them for
   -- 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) } ;

   -- 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 }

   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 }

   -- Public Key Types

   Dss-Pub-Key ::= INTEGER  -- Y

   RSAPublicKey ::= SEQUENCE {
     modulus INTEGER,  -- n
     publicExponent INTEGER }  -- e

   DHPublicKey ::= INTEGER  -- y = g^x mod p


   -- Signature Value Types

   Dss-Sig-Value ::= SEQUENCE {
     r INTEGER,
     s INTEGER }

   -- Algorithm Identifier Parameter Types

   Dss-Parms ::= SEQUENCE {
     p INTEGER,
     q INTEGER,
     g INTEGER }
   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.

   [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.

   [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].

   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
   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.

11 Author Address

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

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