Tech-invite3GPPspaceIETF RFCsSIP
in Index   Prev   Next

RFC 4494

The AES-CMAC-96 Algorithm and Its Use with IPsec

Pages: 8
Proposed Standard

ToP   noToC   RFC4494 - Page 1
Network Working Group                                           JH. Song
Request for Comments: 4494                                 R. Poovendran
Category: Standards Track                       University of Washington
                                                                  J. Lee
                                                     Samsung Electronics
                                                               June 2006

            The AES-CMAC-96 Algorithm and Its Use with IPsec

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


The National Institute of Standards and Technology (NIST) has recently specified the Cipher-based Message Authentication Code (CMAC), which is equivalent to the One-Key CBC-MAC1 (OMAC1) algorithm submitted by Iwata and Kurosawa. OMAC1 efficiently reduces the key size of Extended Cipher Block Chaining mode (XCBC). This memo specifies the use of CMAC mode on the authentication mechanism of the IPsec Encapsulating Security Payload (ESP) and the Authentication Header (AH) protocols. This new algorithm is named AES-CMAC-96.
ToP   noToC   RFC4494 - Page 2

1. Introduction

The National Institute of Standards and Technology (NIST) has recently specified the Cipher-based Message Authentication Code (CMAC). CMAC [NIST-CMAC] is a message authentication code that is based on a symmetric key block cipher such as the Advanced Encryption Standard [NIST-AES]. CMAC is equivalent to the One-Key CBC MAC1 (OMAC1) submitted by Iwata and Kurosawa [OMAC1a, OMAC1b]. OMAC1 is an improvement of the eXtended Cipher Block Chaining mode (XCBC) submitted by Black and Rogaway [XCBCa, XCBCb], which itself is an improvement of the basic CBC-MAC. XCBC efficiently addresses the security deficiencies of CBC-MAC, and OMAC1 efficiently reduces the key size of XCBC. This memo specifies the usage of CMAC on the authentication mechanism of the IPsec Encapsulating Security Payload [ESP] and Authentication Header [AH] protocols. This new algorithm is named AES-CMAC-96. For further information on AH and ESP, refer to [AH] and [ROADMAP].

2. Basic Definitions

CBC Cipher Block Chaining mode of operation for message authentication code. MAC Message Authentication Code. A bit string of a fixed length, computed by the MAC generation algorithm, that is used to establish the authority and, hence, the integrity of a message. CMAC Cipher-based MAC based on an approved symmetric key block cipher, such as the Advanced Encryption Standard. Key (K) 128-bit (16-octet) key for AES-128 cipher block. Denoted by K. Message (M) Message to be authenticated. Denoted by M. Length (len) The length of message M in octets. Denoted by len. The minimum value is 0. The maximum value is not specified in this document. truncate(T,l) Truncate T (MAC) in most-significant-bit-first (MSB-first) order to a length of l octets. T The output of AES-CMAC.
ToP   noToC   RFC4494 - Page 3
   Truncated T     The truncated output of AES-CMAC-128 in MSB-first

   AES-CMAC        CMAC generation function based on AES block cipher
                   with 128-bit key.

   AES-CMAC-96     IPsec AH and ESP MAC generation function based on
                   AES-CMAC, which truncates the 96 most significant
                   bits of the 128-bit output.


The core of AES-CMAC-96 is the AES-CMAC [AES-CMAC]. The underlying algorithms for AES-CMAC are the Advanced Encryption Standard cipher block [NIST-AES] and the recently defined CMAC mode of operation [NIST-CMAC]. AES-CMAC provides stronger assurance of data integrity than a checksum or an error detecting code. The verification of a checksum or an error detecting code detects only accidental modifications of the data, while CMAC is designed to detect intentional, unauthorized modifications of the data, as well as accidental modifications. The output of AES-CMAC can validate the input message. Validating the message provides assurance of the integrity and authenticity over the message from the source. According to [NIST-CMAC], at least 64 bits should be used against guessing attacks. AES-CMAC achieves the similar security goal of HMAC [RFC-HMAC]. Since AES-CMAC is based on a symmetric key block cipher (AES), while HMAC is based on a hash function (such as SHA-1), AES-CMAC is appropriate for information systems in which AES is more readily available than a hash function. Detailed information about AES-CMAC is available in [AES-CMAC] and [NIST-CMAC].
ToP   noToC   RFC4494 - Page 4

4. AES-CMAC-96

For IPsec message authentication on AH and ESP, AES-CMAC-96 should be used. AES-CMAC-96 is a AES-CMAC with 96-bit truncated output in MSB-first order. The output is a 96-bit MAC that will meet the default authenticator length as specified in [AH]. The result of truncation is taken in MSB-first order. For further information on AES-CMAC, refer to [AES-CMAC] and [NIST-CMAC]. Figure 1 describes AES-CMAC-96 algorithm: In step 1, AES-CMAC is applied to the message M in length len with key K. In step 2, the output block T is truncated to 12 octets in MSB-first order, and Truncated T (TT) is returned. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + Algorithm AES-CMAC-96 + +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + + + Input : K (128-bit Key described in Section 4.1) + + : M (message to be authenticated) + + : len (length of message in octets) + + Output : Truncated T (truncated output to length 12 octets) + + + +-------------------------------------------------------------------+ + + + Step 1. T := AES-CMAC (K,M,len); + + Step 2. TT := truncate (T, 12); + + return TT; + +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Figure 1: Algorithm AES-CMAC-96
ToP   noToC   RFC4494 - Page 5

5. Test Vectors

These test cases are the same as those defined in [NIST-CMAC], with the exception of 96-bit truncation. -------------------------------------------------- K 2b7e1516 28aed2a6 abf71588 09cf4f3c Subkey Generation AES_128(key,0) 7df76b0c 1ab899b3 3e42f047 b91b546f K1 fbeed618 35713366 7c85e08f 7236a8de K2 f7ddac30 6ae266cc f90bc11e e46d513b Test Case 1: len = 0 M <empty string> AES_CMAC_96 bb1d6929 e9593728 7fa37d12 Test Case 2: len = 16 M 6bc1bee2 2e409f96 e93d7e11 7393172a AES_CMAC_96 070a16b4 6b4d4144 f79bdd9d Test Case 3: len = 40 M 6bc1bee2 2e409f96 e93d7e11 7393172a ae2d8a57 1e03ac9c 9eb76fac 45af8e51 30c81c46 a35ce411 AES_CMAC_96 dfa66747 de9ae630 30ca3261 Test Case 4: len = 64 M 6bc1bee2 2e409f96 e93d7e11 7393172a ae2d8a57 1e03ac9c 9eb76fac 45af8e51 30c81c46 a35ce411 e5fbc119 1a0a52ef f69f2445 df4f9b17 ad2b417b e66c3710 AES_CMAC_96 51f0bebf 7e3b9d92 fc497417 --------------------------------------------------

6. Interaction with the ESP Cipher Mechanism

As of this writing, there are no known issues that preclude the use of AES-CMAC-96 with any specific cipher algorithm.

7. Security Considerations

See the security considerations section of [AES-CMAC].

8. IANA Considerations

The IANA has allocated value 8 for IKEv2 Transform Type 3 (Integrity Algorithm) to the AUTH_AES_CMAC_96 algorithm.
ToP   noToC   RFC4494 - Page 6

9. Acknowledgements

Portions of this text were borrowed from [NIST-CMAC] and [XCBCa]. We would like to thank to Russ Housley for his useful comments. We acknowledge the support from the the following grants: Collaborative Technology Alliance (CTA) from US Army Research Laboratory, DAAD19-01-2-0011; Presidential Award from Army Research Office, W911NF-05-1-0491; NSF CAREER, ANI-0093187. Results do not reflect any position of the funding agencies.

10. References

10.1. Normative References

[AES-CMAC] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, June 2006. [AH] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [NIST-AES] NIST, FIPS 197, "Advanced Encryption Standard (AES)", November 2001, fips197/fips-197.pdf. [NIST-CMAC] NIST, Special Publication 800-38B Draft, "Recommendation for Block Cipher Modes of Operation: The CMAC Method for Authentication", March 9, 2005.

10.2. Informative References

[OMAC1a] Tetsu Iwata and Kaoru Kurosawa, "OMAC: One-Key CBC MAC", Fast Software Encryption, FSE 2003, LNCS 2887, pp. 129- 153, Springer-Verlag, 2003. [OMAC1b] Tetsu Iwata and Kaoru Kurosawa, "OMAC: One-Key CBC MAC", Submission to NIST, December 2002. Available from omac/omac-spec.pdf. [RFC-HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997.
ToP   noToC   RFC4494 - Page 7
   [ROADMAP]   Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
               Document Roadmap", RFC 2411, November 1998.

   [XCBCa]     John Black and Phillip Rogaway, "A Suggestion for
               Handling Arbitrary-Length Messages with the CBC MAC",
               NIST Second Modes of Operation Workshop, August 2001.
               Available from

   [XCBCb]     John Black and Phillip Rogaway, "CBC MACs for Arbitrary-
               Length Messages: The Three-Key Constructions", Journal of
               Cryptology, Vol. 18, No. 2, pp. 111-132, Springer-Verlag,
               Spring 2005.

Authors' Addresses

Junhyuk Song University of Washington Samsung Electronics Phone: (206) 853-5843 EMail:, Jicheol Lee Samsung Electronics Phone: +82-31-279-3605 EMail: Radha Poovendran Network Security Lab (NSL) Dept. of Electrical Engineering University of Washington Phone: (206) 221-6512 EMail:
ToP   noToC   RFC4494 - Page 8
Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at


   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).