AES-CMAC-96 for IPsec: The AES-CMAC-96 Algorithm and Its Use with IPsec
How to use the AES block cipher to stamp IPsec packets with a 96-bit authentication tag.
Song, JH., Poovendran, R., and J. Lee, "The AES-CMAC-96 Algorithm and Its Use with IPsec", RFC 4494, DOI 10.17487/RFC4494, June 2006, <https://www.rfc-editor.org/info/rfc4494>.
@misc{rfc4494,
series = {Request for Comments},
number = 4494,
howpublished = {RFC 4494},
publisher = {RFC Editor},
doi = {10.17487/RFC4494},
url = {https://www.rfc-editor.org/info/rfc4494},
author = {JH. Song and R. Poovendran and J. Lee},
title = {{The AES-CMAC-96 Algorithm and Its Use with IPsec}},
pagetotal = 8,
year = 2006,
month = jun,
} RFC text
Contents
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.
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: 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
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On this page
§1Introduction
RFC 4494 specifies how to use CMAC, a Cipher-based Message Authentication Code from NIST, in the authentication mechanism of the IPsec Encapsulating Security Payload (ESP) and Authentication Header (AH) protocols. The new algorithm is named AES-CMAC-96.
The RFC traces the family tree. CMAC is equivalent to OMAC1, which improves on XCBC, which in turn improves on the basic CBC-MAC. XCBC addresses security deficiencies of CBC-MAC, and OMAC1 reduces the key size of XCBC.
Grown-up computers need a way to sign every packet. This memo picks a signing method built on a scrambler called AES and says how to plug it into secure tunnels.
§2Basic Definitions
The RFC defines a handful of symbols. The key K is a 128-bit (16-octet) key for the AES-128 cipher. The message M is what gets authenticated, and len is its length in octets. The minimum length is 0, and this document does not specify a maximum.
truncate(T,l) keeps the first l octets of the MAC T, taking bits in most-significant-bit-first order. AES-CMAC-96 is the IPsec MAC function that keeps the 96 most significant bits of the 128-bit AES-CMAC output.
Before the recipe, the memo names its ingredients: a secret key, the message, and a rule for chopping the signature down to size.
§3What AES-CMAC Provides
AES-CMAC is built on the AES block cipher and the CMAC mode of operation. The RFC says it gives stronger assurance of integrity than a checksum or error detecting code. A checksum detects only accidental changes, while CMAC is designed to detect intentional, unauthorized modifications as well as accidental ones.
The RFC says AES-CMAC achieves a similar security goal to HMAC. The difference is the building block: AES-CMAC uses a symmetric key block cipher, while HMAC uses a hash function such as SHA-1. So AES-CMAC suits systems where AES is more readily available than a hash function. It also notes that, according to NIST, at least 64 bits should be used against guessing attacks.
A plain checksum only notices honest mistakes. This signature also catches someone sneaky changing the message on purpose.
§4The AES-CMAC-96 Algorithm
For IPsec message authentication on AH and ESP, AES-CMAC-96 should be used. It is AES-CMAC with the output truncated to 96 bits in MSB-first order. The resulting 96-bit MAC meets the default authenticator length specified in the AH RFC.
The algorithm has two steps. In step 1, AES-CMAC is applied to the message M of length len with key K, giving T. In step 2, T is truncated to 12 octets and that truncated value is returned.
First make a long signature. Then keep only the first part of it, like using just the first twelve letters of a long secret code.
Input : K (128-bit key)
: M (message to be authenticated)
: len (length of message in octets)
Output : Truncated T (12 octets)
Step 1. T := AES-CMAC (K,M,len);
Step 2. TT := truncate (T, 12);
return TT;§5Test Vectors
The RFC gives four test cases so implementers can check their work. They are the same as the ones in the NIST CMAC document, except that the outputs are truncated to 96 bits. All use the key 2b7e1516 28aed2a6 abf71588 09cf4f3c.
The messages have lengths 0, 16, 40 and 64 octets. For example, the empty message gives bb1d6929 e9593728 7fa37d12, and the 16-octet message gives 070a16b4 6b4d4144 f79bdd9d. The RFC also lists the two subkeys, K1 and K2, derived from the key.
The memo includes answer-key examples, so you can check that your own signing program gets the same answers.
§6ESP Interaction, Security and IANA
On interaction with the ESP cipher mechanism, the RFC says that as of writing there are no known issues that preclude using AES-CMAC-96 with any specific cipher algorithm.
Its security considerations section simply points to the security considerations of the AES-CMAC document, RFC 4493. For IANA, value 8 of the IKEv2 Transform Type 3 (Integrity Algorithm) registry was allocated to AUTH_AES_CMAC_96.
The memo says it knows of no problems mixing this signature with any scrambler, and it borrows its safety notes from the parent document. It also got its own number in the official list.