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IETF RFC 4494
The AES-CMAC-96 Algorithm and Its Use with IPsec
Last modified on Monday, June 19th, 2006
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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 © 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.
Song, et al. Standards Track PAGE 1 
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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.
Song, et al. Standards Track PAGE 2
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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].
Song, et al. Standards Track PAGE 3
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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
Song, et al. Standards Track PAGE 4
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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.
Song, et al. Standards Track PAGE 5
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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.
Song, et al. Standards Track PAGE 6
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
[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: songlee@ee.washington.edu, junhyuk.song@samsung.com
Jicheol Lee
Samsung Electronics
Phone: +82-31-279-3605
EMail: jicheol.lee@samsung.com
Radha Poovendran
Network Security Lab (NSL)
Dept. of Electrical Engineering
University of Washington
Phone: (206) 221-6512
EMail: radha@ee.washington.edu
Song, et al. Standards Track PAGE 7
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
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Song, et al. Standards Track PAGE 8
The AES-CMAC-96 Algorithm and Its Use with IPsec
RFC TOTAL SIZE: 14992 bytes
PUBLICATION DATE: Monday, June 19th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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