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IETF RFC 4615
The Advanced Encryption Standard-Cipher-based Message Authentication Code-Pseudo-Random Function-128 (AES-CMAC-PRF-128) Algorithm for the Internet Key Exchange Protocol (IKE)
Last modified on Tuesday, August 8th, 2006
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Network Working Group J. Song
Request for Comments: 4615 R. Poovendran
Category: Standards Track University of Washington
J. Lee
Samsung Electronics
T. Iwata
Nagoya University
August 2006
The Advanced Encryption Standard-Cipher-based
Message Authentication Code-Pseudo-Random Function-128
(AES-CMAC-PRF-128) Algorithm for the
Internet Key Exchange Protocol (IKE)
 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
Some implementations of IP Security (IPsec) may want to use a
pseudo-random function (PRF) based on the Advanced Encryption
Standard (AES). This memo describes such an algorithm, called
AES-CMAC-PRF-128. It supports fixed and variable key sizes.
Table of Contents
1. Introduction ....................................................2
2. Basic Definitions ...............................................2
3. The AES-CMAC-PRF-128 Algorithm ..................................2
4. Test Vectors ....................................................4
5. Security Considerations .........................................4
6. IANA Considerations .............................................5
7. Acknowledgements ................................................5
8. References ......................................................5
8.1. Normative References .......................................5
8.2. Informative References .....................................5
Song, et al. Standards Track PAGE 1 
RFC 4615 AES-CMAC-PRF-128 for IKE August 2006
1. Introduction
[RFC 4493] describes a method to use the Advanced Encryption Standard
(AES) as a Message Authentication Code (MAC) that has a 128-bit
output length. The 128-bit output is useful as a long-lived pseudo-
random function (PRF). This document specifies a PRF that supports
fixed and variable key sizes for IKEv2 [RFC 4306] Key Derivation
Function (KDF) and authentication.
2. Basic Definitions
VK Variable-length key for AES-CMAC-PRF-128, denoted
by VK.
0^128 The string that consists of 128 zero-bits, which is
equivalent to 0x00000000000000000000000000000000 in
hexadecimal notation.
AES-CMAC The AES-CMAC algorithm with a 128-bit long key described
in section 2.4 of [RFC 4493].
3. The AES-CMAC-PRF-128 Algorithm
The AES-CMAC-PRF-128 algorithm is identical to AES-CMAC defined in
[RFC 4493] except that the 128-bit key length restriction is removed.
IKEv2 [RFC 4306] uses PRFs for multiple purposes, most notably for
generating keying material and authentication of the IKE_SA. The
IKEv2 specification differentiates between PRFs with fixed key sizes
and those with variable key sizes.
When using AES-CMAC-PRF-128 as the PRF described in IKEv2, AES-CMAC-
PRF-128 is considered to take fixed size (16 octets) keys for
generating keying material but it takes variable key sizes for
authentication.
That is, when generating keying material, "half the bits must come
from Ni and half from Nr, taking the first bits of each" as described
in IKEv2, section 2.14; but for authenticating with shared secrets
(IKEv2, section 2.16), the shared secret does not have to be 16
octets and the length may vary.
Song, et al. Standards Track PAGE 2
RFC 4615 AES-CMAC-PRF-128 for IKE August 2006
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ AES-CMAC-PRF-128 +
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ +
+ Input : VK (Variable-length key) +
+ : M (Message, i.e., the input data of the PRF) +
+ : VKlen (length of VK in octets) +
+ : len (length of M in octets) +
+ Output : PRV (128-bit Pseudo-Random Variable) +
+ +
+-------------------------------------------------------------------+
+ Variable: K (128-bit key for AES-CMAC) +
+ +
+ Step 1. If VKlen is equal to 16 +
+ Step 1a. then +
+ K := VK; +
+ Step 1b. else +
+ K := AES-CMAC(0^128, VK, VKlen); +
+ Step 2. PRV := AES-CMAC(K, M, len); +
+ return PRV; +
+ +
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Figure 1. The AES-CMAC-PRF-128 Algorithm
In step 1, the 128-bit key, K, for AES-CMAC is derived as follows:
o If the key, VK, is exactly 128 bits, then we use it as-is.
o If it is longer or shorter than 128 bits, then we derive the key,
K, by applying the AES-CMAC algorithm using the 128-bit all-zero
string as the key and VK as the input message. This step is
described in step 1b.
In step 2, we apply the AES-CMAC algorithm using K as the key and M
as the input message. The output of this algorithm is returned.
Song, et al. Standards Track PAGE 3
RFC 4615 AES-CMAC-PRF-128 for IKE August 2006
4. Test Vectors
------------------------------------------------------------
Test Case AES-CMAC-PRF-128 with 20-octet input
Key : 00010203 04050607 08090a0b 0c0d0e0f edcb
Key Length : 18
Message : 00010203 04050607 08090a0b 0c0d0e0f 10111213
PRF Output : 84a348a4 a45d235b abfffc0d 2b4da09a
Test Case AES-CMAC-PRF-128 with 20-octet input
Key : 00010203 04050607 08090a0b 0c0d0e0f
Key Length : 16
Message : 00010203 04050607 08090a0b 0c0d0e0f 10111213
PRF Output : 980ae87b 5f4c9c52 14f5b6a8 455e4c2d
Test Case AES-CMAC-PRF-128 with 20-octet input
Key : 00010203 04050607 0809
Key Length : 10
Message : 00010203 04050607 08090a0b 0c0d0e0f 10111213
PRF Output : 290d9e11 2edb09ee 141fcf64 c0b72f3d
------------------------------------------------------------
5. Security Considerations
The security provided by AES-CMAC-PRF-128 is based upon the strength
of AES and AES-CMAC. At the time of this writing, there are no known
practical cryptographic attacks against AES or AES-CMAC. However, as
is true with any cryptographic algorithm, part of its strength lies
in the secret key, VK, and the correctness of the implementation in
all of the participating systems. The key, VK, needs to be chosen
independently and randomly based on RFC 4086 [RFC 4086], and both
keys, VK and K, should be kept safe and periodically refreshed.
Section 4 presents test vectors that assist in verifying the
correctness of the AES-CMAC-PRF-128 code.
If VK is longer than 128 bits and it is shortened to meet the AES-128
key size, then some entropy might be lost. However, as long as VK is
longer than 128 bits, then the new key, K, preserves sufficient
entropy, i.e., the entropy of K is about 128 bits.
Therefore, we recommend the use of VK that is longer than or equal to
128 bits, and we discourage the use of VK that is shorter than or
equal to 64 bits, because of the small entropy.
Song, et al. Standards Track PAGE 4
RFC 4615 AES-CMAC-PRF-128 for IKE August 2006
6. IANA Considerations
IANA has allocated a value of 8 for IKEv2 Transform Type 2 (Pseudo-
Random Function) to the PRF_AES128_CMAC algorithm.
7. Acknowledgements
Portions of this text were borrowed from [RFC 3664] and [RFC 4434].
Many thanks to Russ Housley and Paul Hoffman for suggestions and
guidance. We also thank Alfred Hoenes for many useful comments.
We acknowledge support from 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; ONR YIP N00014-04-1-0479. Results do not reflect
any position of the funding agencies.
8. References
8.1. Normative References
[RFC 4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, June 2006.
[RFC 4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
[RFC 4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
8.2. Informative References
[RFC 3664] Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
Internet Key Exchange Protocol (IKE)", RFC 3664, January
2004.
[RFC 4434] Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
Internet Key Exchange Protocol (IKE)", RFC 4434, February
2006.
Song, et al. Standards Track PAGE 5
RFC 4615 AES-CMAC-PRF-128 for IKE August 2006
Authors' Addresses
JunHyuk Song
Samsung Electronics
University of Washington
Phone: (206) 853-5843
EMail: junhyuk.song@samsung.com, junhyuk.song@gmail.com
Radha Poovendran
Network Security Lab
University of Washington
Phone: (206) 221-6512
EMail: radha@ee.washington.edu
Jicheol Lee
Samsung Electronics
Phone: +82-31-279-3605
EMail: jicheol.lee@samsung.com
Tetsu Iwata
Nagoya University
EMail: iwata@cse.nagoya-u.ac.jp
Song, et al. Standards Track PAGE 6
RFC 4615 AES-CMAC-PRF-128 for IKE August 2006
Full Copyright Statement
Copyright © 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
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Acknowledgement
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Song, et al. Standards Track PAGE 7
The Advanced Encryption Standard-Cipher-based Message Authentication Code-Pseudo-Random Function-128 (AES-CMAC-PRF-128) Algorithm for the Internet Key Exchange Protocol (IKE)
RFC TOTAL SIZE: 13312 bytes
PUBLICATION DATE: Tuesday, August 8th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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