CBOR Object Signing and Encryption B. Sipos Internet-Draft JHU/APL Intended status: Informational 29 December 2025 Expires: 2 July 2026 AES-CMAC for COSE draft-sipos-cose-cmac-00 Abstract This document registers COSE algorithm code points for using the Advanced Encryption Standard (AES) in Cipher-based Message Authentication Code (CMAC) mode for use in CBOR Object Signing and Encryption (COSE) messages. The CMAC mode of operation is an alternative to AES-CBC-MAC which is approved by US NIST FIPS 140. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 2 July 2026. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Sipos Expires 2 July 2026 [Page 1] Internet-Draft COSE CMAC December 2025 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. The AES-CMAC Family . . . . . . . . . . . . . . . . . . . . . 3 3. Security Considerations . . . . . . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 4.1. COSE Algorithms . . . . . . . . . . . . . . . . . . . . . 4 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Normative References . . . . . . . . . . . . . . . . . . 5 5.2. Informative References . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction The base CBOR Object Signing and Encryption (COSE) specification [RFC9052] defines a container for Message Authentication Code (MAC) parameters and results. This container is parameterized on an algorithm identifier used to verify the MAC result. This document defines new fully specified algorithm identifiers for the use of Advanced Encryption Standard (AES) in Cipher-based Message Authentication Code (CMAC) mode to generate an authentication tag as defined by US NIST [SP800-38B]. These COSE algorithm identifiers are "fully specified" meaning they rely on no extra parameters (_e.g._, key length or tag length) to determine their exact operation. The COSE algorithm code point along with the shared secret key is suffient 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 which relies exclusively on a block cipher instead of a cryptographic hash function. For some implementations, cipher-based MAC can enable the use of hardware acceleration of its processing. The CMAC mode of AES is approved by US NIST FIPS 140 [FIPS-140]. 1.1. Scope This document does not define any new algorithms it only defines code points in a COSE registry so that the AES-CMAC can be used in that security environment with fully specified combinations of parameters. 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]. That algorithm family is not approved by FIPS 140. Sipos Expires 2 July 2026 [Page 2] Internet-Draft COSE CMAC December 2025 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 about fully specifying what gets assigned a COSE "algorithm" code point, AES-CMAC will be treated as an _algorithm family_ with a single code point referring to the algorithm itself along with a specific set of parameter values. The parameters associated with AES-CMAC are: key length and tag length. This document restricts the allocated code points to the commonly used key lengths of 128 and 256 bits and restricts the use of a single tag length of 128 bits, which happens to be the longest possible tag length, as indicated in Table 1. These tag lengths are consistent with the COSE use of AES-CBC-MAC in Section 3.2 of [RFC9053]. Future allocations can define the use of AES-CMAC with shortened tag lengths. +============+===========+============+============+ | COSE Value | Algorithm | Key Length | Tag Length | +============+===========+============+============+ | | AES-CMAC | 128 | 128 | | // TBA1 | | | | +------------+-----------+------------+------------+ | | AES-CMAC | 256 | 128 | | // TBA3 | | | | +------------+-----------+------------+------------+ Table 1: Registered AES-CMAC combinations 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. Sipos Expires 2 July 2026 [Page 3] Internet-Draft COSE CMAC December 2025 * 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, and so does not introduce any new security considerations. All of the applicable considerations from NIST [SP800-38B] apply when the algorithm is used in COSE. 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 128/128 Value: // TBA1 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 Sipos Expires 2 July 2026 [Page 4] Internet-Draft COSE CMAC December 2025 Capabilities: [kty] Change controller: IETF Reference: [This document] Recommended: Yes // Note to IANA: The reqested COSE algorithm code points are in the // positive less-than-256 range. 5. References 5.1. Normative References [IANA-COSE] IANA, "CBOR Object Signing and Encryption (COSE)", . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, August 2022, . [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, . 5.2. Informative References [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, . Sipos Expires 2 July 2026 [Page 5] Internet-Draft COSE CMAC December 2025 [RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, August 2022, . [FIPS-140] US National Institute of Standards and Technology, "Security Requirements for Cryptographic Modules", FIPS 140-3, March 2019, . 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 Sipos Expires 2 July 2026 [Page 6]