CBOR Object Signing and Encryption B. Sipos Internet-Draft JHU/APL Intended status: Standards Track 23 January 2025 Expires: 27 July 2025 AES-GMAC for COSE draft-sipos-cose-gmac-00 Abstract This document registers COSE algorithm code points for using the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) to generate a Message Authentication Code (AES-GMAC). The security strength provided by these registrations is identical to existing COSE registrations for AES-GCM authenticated encryption. 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/. 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Sipos Expires 27 July 2025 [Page 1] Internet-Draft COSE GMAC January 2025 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. The AES-GMAC Family . . . . . . . . . . . . . . . . . . . . . 3 3. Security Considerations . . . . . . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4.1. COSE Algorithms . . . . . . . . . . . . . . . . . . . . . 5 4.1.1. AES-GMAC 128/128 . . . . . . . . . . . . . . . . . . 5 4.1.2. AES-GMAC 192/128 . . . . . . . . . . . . . . . . . . 5 4.1.3. AES-GMAC 256/128 . . . . . . . . . . . . . . . . . . 6 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. Normative References . . . . . . . . . . . . . . . . . . 6 5.2. Informative References . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction The base 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 Galois/Counter Mode (GCM) to generate a MAC (AES-GMAC) as defined in [SP800-38D]. Unlike the use of AES-GMAC in CMS [RFC9044] and IPsec [RFC4543], the COSE algorithm identifiers are "fully specified" meaning they rely on no extra parameters (_e.g._, tag length) to determine their exact operation. 1.1. Scope This document does not define any new algorithms it only defines code points in a COSE registry so that the AES-GMAC can be used in that security environment with specific combinations of parameters. To avoid confusion, the AES-GMAC algorithm family specified in this document is unrelated to the "AES-MAC" algorithm family from Section 3.2 of [RFC9053]. Sipos Expires 27 July 2025 [Page 2] Internet-Draft COSE GMAC January 2025 1.2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD 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-GMAC Family While the general GMAC algorithm can be used with any underlying authenticated encryption with additional data (AEAD) block cipher, this document focuses on its use with the AES-GCM cipher. The AES-GMAC defined in [SP800-38D] has a set of parameters associated with its use. For the sake of adhering to COSE best practice about fully specifying what gets assigned an "algorithm" code point, AES-GMAC 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-GMAC are: key length and tag length. This document restricts the allocated code points to the commonly used key lengths of 128, 192, and 256-bits, a fixed IV length of 96 bits (the recommended default), 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. Future allocations can define the use of AES-GMAC with shortened tag lengths. One required input for AES-GMAC is an initialization vector (IV) which is already provided by the header parameter "IV" from the "COSE Header Parameters" registry of [IANA-COSE]. The use of the AES-GMAC algorithms in COSE SHALL be combined with the IV header parameter in the same COSE layer. A valid IV for these algorithms SHALL be exactly 96 bits (12 octets) in length. The combination of key and IV SHALL be unique for each created MAC. The IV generation mechanism SHALL be deterministic (not random). The specifics of that mechanism are left to an implementation. These IV and tag lengths are consistent with the COSE use of AES-GCM encryption in Section 4.1 of [RFC9053]. Sipos Expires 27 July 2025 [Page 3] Internet-Draft COSE GMAC January 2025 +============+===========+============+===========+============+ | COSE Value | Algorithm | Key Length | IV Length | Tag Length | +============+===========+============+===========+============+ | | AES-GMAC | 128 | 96 | 128 | | // TBA1 | | | | | +------------+-----------+------------+-----------+------------+ | | AES-GMAC | 192 | 96 | 128 | | // TBA2 | | | | | +------------+-----------+------------+-----------+------------+ | | AES-GMAC | 256 | 96 | 128 | | // TBA3 | | | | | +------------+-----------+------------+-----------+------------+ Table 1: Registered AES-GMAC combinations Implementations creating and validating AES-GMAC values SHALL validate that the key type, key length, and algorithm are correct and appropriate for the entities involved. When using a COSE key for these algorithms, the following checks are made: * The "kty" field MUST be present. * The "kty" field MUST be "Symmetric". * 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 modes of operation for the GMAC algorithm, and so does not introduce any new security considerations. All of the applicable considerations from [SP800-38D] apply when the algorithm is used in COSE. The requirement to use non-random IV generation in Section 2 is meant to satisfy the critical constraint on GMAC (and nonce-based MACs generally) described in Chapter 10 of [PR2011] to guarantee the uniqueness of the combination of key and IV. Whether the mechanism is a simple counter, a determistic PRF, or something else does not affect the constraint to be non-random. Sipos Expires 27 July 2025 [Page 4] Internet-Draft COSE GMAC January 2025 4. IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of code points in accordance with BCP 26 [RFC1155]. 4.1. COSE Algorithms A new set of entries have been added to the "COSE Algorithms" registry [IANA-COSE] with the following parameters. 4.1.1. AES-GMAC 128/128 Name: AES-GMAC 128/128 Value: // TBA1 Description: AES-GMAC with 128-bit key and 128-bit tag Capabilities: [kty] Change controller: IESG Reference: [This document] Recommended: Yes 4.1.2. AES-GMAC 192/128 Name: AES-GMAC 192/128 Value: // TBA2 Description: AES-GMAC with 192-bit key and 128-bit tag Capabilities: [kty] Change controller: IESG Reference: [This document] Recommended: Yes Sipos Expires 27 July 2025 [Page 5] Internet-Draft COSE GMAC January 2025 4.1.3. AES-GMAC 256/128 Name: AES-GMAC 256/128 Value: // TBA3 Description: AES-GMAC with 256-bit key and 128-bit tag Capabilities: [kty] Change controller: IESG Reference: [This document] Recommended: Yes 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, . [SP800-38D] National Institute of Standards and Technology, "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", November 2007, . 5.2. Informative References Sipos Expires 27 July 2025 [Page 6] Internet-Draft COSE GMAC January 2025 [PR2011] Rogaway, P., "Evaluation of Some Blockcipher Modes of Operation", 10 February 2011, . [RFC1155] Rose, M. and K. McCloghrie, "Structure and identification of management information for TCP/IP-based internets", STD 16, RFC 1155, DOI 10.17487/RFC1155, May 1990, . [RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543, DOI 10.17487/RFC4543, May 2006, . [RFC9044] Housley, R., "Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)", RFC 9044, DOI 10.17487/RFC9044, June 2021, . [RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, August 2022, . 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 27 July 2025 [Page 7]