AES-CMAC for COSE
draft-ietf-cose-cmac-00

CBOR Object Signing and Encryption                              B. Sipos
Internet-Draft                                                   JHU/APL
Intended status: Informational                               6 July 2026
Expires: 7 January 2027

                           AES-CMAC for COSE
                        draft-ietf-cose-cmac-00

Abstract

   The CBOR Object Signing and Encryption (COSE) specification defines
   structures for generating, conveying, and verifying Message
   Authentication Code (MAC) tags.  This document registers code points
   for using the Advanced Encryption Standard (AES) block cipher in
   Cipher-based Message Authentication Code (CMAC) mode within those
   COSE structures.  Specifically, these uses are for computing MAC tag
   values with no additional parameters.

Status of This Memo

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   This Internet-Draft will expire on 7 January 2027.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The AES-CMAC Family . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
     3.1.  Key Overuse Limit . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  COSE Algorithms . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Document History  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The base CBOR Object Signing and Encryption (COSE) specification
   [RFC9052] defines two message types for Message Authentication Code
   (MAC) parameters and results: COSE_Mac and COSE_Mac0.  These messages
   are parameterized on an algorithm identifier used to generate and
   verify the MAC tag.  This document defines new fully specified COSE
   algorithm code points for the use of Advanced Encryption Standard
   (AES) block cipher [FIPS-197] in Cipher-based Message Authentication
   Code (CMAC) mode [SP800-38B] to compute a MAC tag.

   These COSE algorithm code points are "fully specified" in accordance
   with [RFC9864], meaning they rely on no extra parameters to determine
   their exact operation.  The COSE algorithm code point along with the
   shared secret key is sufficient to generate or verify the MAC tag.

   The use of CMAC is an alternative to the Hash-based Message
   Authentication Code (HMAC) family of algorithms registered by the
   base COSE specification in Section 3.1 of [RFC9053].  CMAC relies
   exclusively on a block cipher instead of the HMAC use of a
   cryptographic hash function.  For some implementations, cipher-based
   MAC can be hardware accelerated.

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   To avoid confusion, the AES-CMAC algorithm family specified in this
   document is distinct from the "AES-MAC" (also known as "AES-CBC-MAC")
   algorithm family from Section 3.2 of [RFC9053].

1.1.  Scope

   This document does not define any new cryptographic algorithms or
   functions.  It only defines code points in a COSE registry so that
   the AES-CMAC algorithm family can be used in COSE messages.

   This document does not address the use of CMAC for any other purposes
   than to compute a fixed-length MAC tag.  These registered code points
   are not to be used as a pseudorandom function (PRF) or key-derivation
   function (KDF).

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

2.  The AES-CMAC Family

   While the CMAC mode [SP800-38B] can be used with any underlying
   encryption block cipher, this document focuses on its use with the
   AES cipher referred to as AES-CMAC.

   For the sake of adhering to COSE best practice [RFC9864] about fully
   specifying what gets assigned a COSE "algorithm" code point, AES-CMAC
   will be treated as an _algorithm family_ with a single COSE code
   point referring to the algorithm family along with a specific set of
   parameter values.  The parameters associated with AES-CMAC family
   are: key length and tag length.

   This document registers code points for the commonly used key lengths
   of 128 and 256 bits and tag lengths of 96 and 128 bits.  The 128-bit
   tag happens to be the longest possible tag length while the 96-bit
   tag is a truncated form.  These tag lengths are consistent with the
   use cases for single-use keys and limited-use keys (see Section 3.1)
   and with the use of AES-CMAC in IPsec [RFC4494].

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    +==========+============+==================+============+========+
    | Name     | COSE Value | Algorithm Family | Key Length | Tag    |
    |          |            |                  |            | Length |
    +==========+============+==================+============+========+
    | AES-CMAC |            | AES-CMAC         | 128        | 96     |
    | 128/96   | // TBA1    |                  |            |        |
    +----------+------------+------------------+------------+--------+
    | AES-CMAC |            | AES-CMAC         | 256        | 96     |
    | 256/96   | // TBA2    |                  |            |        |
    +----------+------------+------------------+------------+--------+
    | AES-CMAC |            | AES-CMAC         | 128        | 128    |
    | 128/128  | // TBA3    |                  |            |        |
    +----------+------------+------------------+------------+--------+
    | AES-CMAC |            | AES-CMAC         | 256        | 128    |
    | 256/128  | // TBA4    |                  |            |        |
    +----------+------------+------------------+------------+--------+

                Table 1: Registered algorithm code points

   When using a COSE key for these algorithms, the following checks are
   made:

   *  The "kty" field MUST be present with a value of "Symmetric".

   *  The "k" field MUST match the key length for the algorithm being
      used.

   *  If the "alg" field is present, it MUST match the algorithm being
      used.

   *  If the "key_ops" field is present, it MUST include "MAC create"
      when creating an authentication tag.

   *  If the "key_ops" field is present, it MUST include "MAC verify"
      when verifying an authentication tag.

3.  Security Considerations

   This document does not define any new behavior of the AES-CMAC
   family, so all of the applicable considerations of AES [FIPS-197] and
   CMAC [SP800-38B] apply when the algorithm family is used in COSE.

   The CMAC mode of AES is approved by US NIST FIPS 140 [FIPS-140].  The
   pre-existing uses of AES-CBC-MAC in COSE [RFC9053] are not approved
   by FIPS 140.

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3.1.  Key Overuse Limit

   From analysis of Appendix B of [SP800-38B] performed in 2024
   [Ericsson], an "effective tag length" in bits can be computed for the
   128-bit AES block length as

   T_eff = 128 - 2 * log2(q)

   where the factor "q" is the message span of each key (number of
   messages for which a tag is generated).

   This means that only for single-use keys is the effective tag length
   is the actual tag length.  For the NIST recommended limit of q =
   2^48, the effective tag length becomes shortened to only 48 bits.

   The analysis itself [Ericsson] recommends an effective tag length no
   less than 64 bits (_i.e._, equivalent to a 64-bit ideal MAC).  This
   translates to a message span limit of q < 2^32, meaning a single key
   is limited to generate MAC tags for fewer than 2^32 messages.  For a
   96-bit effective tag length, the limit becomes fewer than 2^16
   messages.

4.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of code points in accordance
   with BCP 26 [RFC8126].

4.1.  COSE Algorithms

   A new set of entries have been added to the "COSE Algorithms"
   registry [IANA-COSE] with the following parameters:

   Name:  AES-CMAC 128/96

   Value:
      // TBA1

   Description:  AES-CMAC with 128-bit key and 96-bit tag

   Capabilities:  [kty]

   Change controller:  IETF

   Reference:  [This document]

   Recommended:  Yes

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   Name:  AES-CMAC 256/96

   Value:
      // TBA2

   Description:  AES-CMAC with 256-bit key and 96-bit tag

   Capabilities:  [kty]

   Change controller:  IETF

   Reference:  [This document]

   Recommended:  Yes

   Name:  AES-CMAC 128/128

   Value:
      // TBA3

   Description:  AES-CMAC with 128-bit key and 128-bit tag

   Capabilities:  [kty]

   Change controller:  IETF

   Reference:  [This document]

   Recommended:  Yes

   Name:  AES-CMAC 256/128

   Value:
      // TBA3

   Description:  AES-CMAC with 256-bit key and 128-bit tag

   Capabilities:  [kty]

   Change controller:  IETF

   Reference:  [This document]

   Recommended:  Yes

   // Note to IANA: The requested COSE algorithm code points are in the
   // positive less-than-256 range.

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5.  References

5.1.  Normative References

   [FIPS-197] US National Institute of Standards and Technology, "The
              Advanced Encryption Standard (AES)", FIPS 197, 26 November
              2001, <http://csrc.nist.gov/publications/fips/fips197/
              fips-197.pdf>.

   [IANA-COSE]
              IANA, "CBOR Object Signing and Encryption (COSE)",
              <https://www.iana.org/assignments/cose/>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

   [SP800-38B]
              US National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation: The
              CMAC Mode for Authentication", NIST SP 800-38B, May 2005,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-38b.pdf>.

5.2.  Informative References

   [Ericsson] Mattsson, J. P., "Comments on SP 800-38B and SP 800-38C",
              14 September 2024, <https://emanjon.github.io/NIST-
              comments/2024%20-%20SP%20800-38B%20and%20800-38C.pdf>.

   [FIPS-140] US National Institute of Standards and Technology,
              "Security Requirements for Cryptographic Modules",
              FIPS 140-3, March 2019,
              <https://doi.org/10.6028/NIST.FIPS.140-3>.

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   [RFC4494]  Song, JH., Poovendran, R., and J. Lee, "The AES-CMAC-96
              Algorithm and Its Use with IPsec", RFC 4494,
              DOI 10.17487/RFC4494, June 2006,
              <https://www.rfc-editor.org/info/rfc4494>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/info/rfc9053>.

   [RFC9864]  Jones, M.B. and O. Steele, "Fully-Specified Algorithms for
              JSON Object Signing and Encryption (JOSE) and CBOR Object
              Signing and Encryption (COSE)", RFC 9864,
              DOI 10.17487/RFC9864, October 2025,
              <https://www.rfc-editor.org/info/rfc9864>.

Document History

   This section is to be removed before publishing as an RFC.

   Version history of this document:

   -00  Initial working group draft based on draft-sipos-cose-cmac-02.

Author's Address

   Brian Sipos
   The Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Rd.
   Laurel, MD 20723
   United States of America
   Email: brian.sipos+ietf@gmail.com

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