RFC 9044

Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 9044                                Vigil Security
Category: Standards Track                                      June 2021
ISSN: 2070-1721

Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)


   This document specifies the conventions for using the AES-GMAC
   Message Authentication Code algorithm with the Cryptographic Message
   Syntax (CMS) as specified in RFC 5652.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Message Authentication Code Algorithms
     3.1.  AES-GMAC
   4.  Implementation Considerations
   5.  ASN.1 Module
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References

   Author's Address

1.  Introduction

   This document specifies the conventions for using the AES-GMAC [AES]
   [GCM] Message Authentication Code (MAC) algorithm with the
   Cryptographic Message Syntax (CMS) [RFC5652].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Message Authentication Code Algorithms

   This section specifies the conventions employed by CMS [RFC5652]
   implementations that support the AES-GMAC [AES] [GCM] Message
   Authentication Code (MAC) algorithm.

   MAC algorithm identifiers are located in the AuthenticatedData
   macAlgorithm field.

   MAC values are located in the AuthenticatedData mac field.

3.1.  AES-GMAC

   The AES-GMAC [AES] [GCM] Message Authentication Code (MAC) algorithm
   uses one of the following algorithm identifiers in the
   AuthenticatedData macAlgorithm field; the choice depends on the size
   of the AES key, which is either 128 bits, 192 bits, or 256 bits:

      aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
              organization(1) gov(101) csor(3) nistAlgorithm(4) 1 }

      id-aes128-GMAC OBJECT IDENTIFIER ::= { aes 9 }

      id-aes192-GMAC OBJECT IDENTIFIER ::= { aes 29 }

      id-aes256-GMAC OBJECT IDENTIFIER ::= { aes 49 }

   For all three of these algorithm identifier values, the
   AlgorithmIdentifier parameters field MUST be present, and the
   parameters MUST contain GMACParameters:

      GMACParameters ::= SEQUENCE {
         nonce        OCTET STRING, -- recommended size is 12 octets
         length       MACLength DEFAULT 12 }

      MACLength ::= INTEGER (12 | 13 | 14 | 15 | 16)

   The GMACParameters nonce field is the GMAC initialization vector.
   The nonce may have any number of bits between 8 and (2^64)-1, but it
   MUST be a multiple of 8 bits.  Within the scope of any content-
   authentication key, the nonce value MUST be unique.  A nonce value of
   12 octets can be processed more efficiently, so that length for the
   nonce value is RECOMMENDED.

   The GMACParameters length field tells the size of the message
   authentication code.  It MUST match the size in octets of the value
   in the AuthenticatedData mac field.  A length of 12 octets is

4.  Implementation Considerations

   An implementation of the Advanced Encryption Standard (AES) Galois/
   Counter Mode (GCM) authenticated encryption algorithm is specified in
   [GCM].  An implementation of AES-GCM can be used to compute the GMAC
   message authentication code by providing the content-authentication
   key as the AES key, the nonce as the initialization vector, a zero-
   length plaintext content, and the content to be authenticated as the
   additional authenticated data (AAD).  The result of the AES-GCM
   invocation is the AES-GMAC authentication code, which is called the
   "authentication tag" in some implementations.  In AES-GCM, the
   encryption step is skipped when no input plaintext is provided;
   therefore, no ciphertext is produced.

   The DEFAULT and RECOMMENDED values in GMACParameters were selected to
   align with the parameters defined for AES-GCM in Section 3.2 of

5.  ASN.1 Module

   The following ASN.1 module uses the definition for MAC-ALGORITHM from

       { iso(1) member-body(2) us(840) rsadsi(113549)
         pkcs(1) pkcs-9(9) smime(16) modules(0)
         id-mod-aes-gmac-alg-2020(72) }


   -- EXPORTS All

     AlgorithmIdentifier{}, MAC-ALGORITHM
     FROM AlgorithmInformation-2009 -- from [RFC5912]
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-algorithmInformation-02(58)} ;

   -- Object Identifiers

   aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
          organization(1) gov(101) csor(3) nistAlgorithm(4) 1 }

   id-aes128-GMAC OBJECT IDENTIFIER ::= { aes 9 }

   id-aes192-GMAC OBJECT IDENTIFIER ::= { aes 29 }

   id-aes256-GMAC OBJECT IDENTIFIER ::= { aes 49 }

   -- GMAC Parameters

   GMACParameters ::= SEQUENCE {
      nonce        OCTET STRING, -- recommended size is 12 octets
      length       MACLength DEFAULT 12 }

   MACLength ::= INTEGER (12 | 13 | 14 | 15 | 16)

   -- Algorithm Identifiers

   maca-aes128-GMAC MAC-ALGORITHM ::= {
      IDENTIFIER id-aes128-GMAC
      PARAMS TYPE GMACParameters ARE required

   maca-aes192-GMAC MAC-ALGORITHM ::= {
      IDENTIFIER id-aes192-GMAC
      PARAMS TYPE GMACParameters ARE required

   maca-aes256-GMAC MAC-ALGORITHM ::= {
      IDENTIFIER id-aes256-GMAC
      PARAMS TYPE GMACParameters ARE required

   END -- of CryptographicMessageSyntaxGMACAlgorithms

6.  IANA Considerations

   IANA has registered the object identifier shown in Table 1 in the
   "SMI Security for S/MIME Module Identifier (1.2.840.113549."

            | Decimal | Description              | References |
            | 72      | id-mod-aes-gmac-alg-2020 | RFC 9044   |

                                  Table 1

7.  Security Considerations

   The CMS provides a method for authenticating data.  This document
   identifies the conventions for using the AES-GMAC algorithm with the

   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.

   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.

   Within the scope of any content-authentication key, the AES-GMAC
   nonce value MUST be unique.  Use of a nonce value more than once
   allows an attacker to generate valid AES-GMAC authentication codes
   for arbitrary messages, resulting in the loss of authentication as
   described in Appendix A of [GCM].

   Within the scope of any content-authentication key, the
   authentication tag length (MACLength) MUST be fixed.

   If AES-GMAC is used as a building block in another algorithm (e.g.,
   as a pseudorandom function), AES-GMAC MUST be used only one time by
   that algorithm.  For instance, AES-GMAC MUST NOT be used as the
   pseudorandom function for PBKDF2.

   When initialization vector (IV) lengths other than 96 bits are used,
   the GHASH function is used to process the provided IV, which
   introduces a potential for IV collisions.  However, IV collisions are
   not a concern with CMS AuthenticatedData because a fresh content-
   authentication key is usually generated for each message.

   The probability of a successful forgery is close to 2^(-t), where t
   is the number of bits in the authentication tag length (MACLength*8).
   This nearly ideal authentication protection is achieved for CMS
   AuthenticatedData when a fresh content-authentication key is
   generated for each message.  However, the strength of GMAC degrades
   slightly as a function of the length of the message being
   authenticated [F2005] [MV2005].  Implementations SHOULD use 16-octet
   authentication tags for messages over 2^64 octets.

   Implementations must randomly generate message-authentication keys.
   The use of inadequate pseudorandom number generators (PRNGs) to
   generate keys 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.  [RFC4086] offers important
   guidance in this area.

   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.
   More information is available in BCP 201 [RFC7696].

8.  References

8.1.  Normative References

   [AES]      National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", FIPS PUB 197,
              DOI 10.6028/NIST.FIPS.197, November 2001,

   [GCM]      Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC", NIST
              Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D,
              November 2007, <https://doi.org/10.6028/NIST.SP.800-38D>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [F2005]    Ferguson, N., "Authentication weaknesses in GCM", May
              2005, <https://csrc.nist.gov/csrc/media/projects/block-

   [MV2005]   McGrew, D. and J. Viega, "GCM Update", May 2005,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC5084]  Housley, R., "Using AES-CCM and AES-GCM Authenticated
              Encryption in the Cryptographic Message Syntax (CMS)",
              RFC 5084, DOI 10.17487/RFC5084, November 2007,

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,


   Many thanks to Hans Aschauer, Hendrik Brockhaus, Quynh Dang, Roman
   Danyliw, Tim Hollebeek, Ben Kaduk, Mike Ounsworth, and Magnus
   Westerlund for their careful review and thoughtful improvements.

Author's Address

   Russ Housley
   Vigil Security, LLC
   516 Dranesville Road
   Herndon, VA 20170
   United States of America

   Email: housley@vigilsec.com