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).
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.
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RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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].
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.
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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.
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].
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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
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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.
[NIST-CMAC] NIST, Special Publication 800-38B Draft, "Recommendation for Block Cipher Modes of Operation: The CMAC Method for Authentication", March 9, 2005.
[RFC-HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997.
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[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
RFC 4494 The AES-CMAC Algorithm and IPsec June 2006
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