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 (C) The Internet Society (2006).


   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

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RFC 4615                AES-CMAC-PRF-128 for IKE             August 2006

1.  Introduction

   [RFC4493] 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 [RFC4306] 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 [RFC4493].

3.  The AES-CMAC-PRF-128 Algorithm

   The AES-CMAC-PRF-128 algorithm is identical to AES-CMAC defined in
   [RFC4493] except that the 128-bit key length restriction is removed.

   IKEv2 [RFC4306] 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

   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.

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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.

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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 [RFC4086], 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.

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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 [RFC3664] and [RFC4434].
   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

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, June 2006.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
              4306, December 2005.

   [RFC4086]  Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              June 2005.

8.2.  Informative References

   [RFC3664]  Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
              Internet Key Exchange Protocol (IKE)", RFC 3664, January

   [RFC4434]  Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
              Internet Key Exchange Protocol (IKE)", RFC 4434, February

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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

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RFC 4615                AES-CMAC-PRF-128 for IKE             August 2006

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