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Network Working Group
Request for Comments: 4494
Category: Standards Track
JH. Song
R. Poovendran
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 © The Internet Society (2006).

Abstract

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.

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.
   Truncated T     The truncated output of AES-CMAC-128 in MSB-first
                   order.
   
   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.

3. AES-CMAC

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].

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

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.

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, http://csrc.nist.gov/publications/fips/
               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
               http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/
               omac/omac-spec.pdf.
   
   [RFC-HMAC]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
               Hashing for Message Authentication", RFC 2104, February
               1997.
   
   [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 http://csrc.nist.gov/CryptoToolkit/modes/
               proposedmodes/xcbc-mac/xcbc-mac-spec.pdf.
   
   [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: [email protected], [email protected]

Jicheol Lee
Samsung Electronics 

   Phone: +82-31-279-3605
   EMail: [email protected]

Radha Poovendran
Network Security Lab (NSL)
Dept. of Electrical Engineering
University of Washington 

   Phone: (206) 221-6512
   EMail: [email protected]

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