AES-CMAC for COSE The Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Rd. Laurel MD 20723 United States of America brian.sipos+ietf@gmail.com
Security CBOR Object Signing and Encryption CMAC This document registers code points for using the Advanced Encryption Standard (AES) block cipher in Cipher-based Message Authentication Code (CMAC) mode within CBOR Object Signing and Encryption (COSE) messages. Specifically, these uses are for computing authentication tag values with no additional parameters.
Introduction The base CBOR Object Signing and Encryption (COSE) specification 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 in Cipher-based Message Authentication Code (CMAC) mode to compute a MAC tag. These COSE algorithm code points are "fully specified" in accordance with , 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 . 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. 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 .
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).
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 when, and only when, they appear in all capitals, as shown here.
The AES-CMAC Family While the CMAC mode 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 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 128 and 64 bits. The 128-bit tag happens to be the longest possible tag length while the 64-bit tag is a truncated form. These tag lengths are consistent with the COSE use of AES-CBC-MAC in . Registered algorithm code points
Name COSE Value Algorithm Family Key Length Tag Length
AES-CMAC 128/64 TBA1 AES-CMAC 128 64
AES-CMAC 256/64 TBA2 AES-CMAC 256 64
AES-CMAC 128/128 TBA3 AES-CMAC 128 128
AES-CMAC 256/128 TBA4 AES-CMAC 256 128
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.
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 apply when the algorithm family is used in COSE. The CMAC mode of AES is approved by US NIST FIPS 140 . The pre-existing uses of AES-CBC-MAC in COSE are not approved by FIPS 140.
IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of code points in accordance with BCP 26 .
COSE Algorithms A new set of entries have been added to the "COSE Algorithms" registry with the following parameters:
Name:
AES-CMAC 128/64
Value:
TBA1
Description:
AES-CMAC with 128-bit key and 64-bit tag
Capabilities:
[kty]
Change controller:
IETF
Reference:
[This document]
Recommended:
Yes
Name:
AES-CMAC 256/64
Value:
TBA2
Description:
AES-CMAC with 256-bit key and 64-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.
References Normative References CBOR Object Signing and Encryption (COSE) IANA Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication US National Institute of Standards and Technology The Advanced Encryption Standard (AES) US National Institute of Standards and Technology Informative References Security Requirements for Cryptographic Modules US National Institute of Standards and Technology