The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol (IKE)

Network Working Group                                         P. Hoffman
Request for Comments: 3664                                VPN Consortium
Category: Standards Track                                   January 2004


                   The AES-XCBC-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 (2004).  All Rights Reserved.

Abstract

   Some implementations of IP Security (IPsec) may want to use a
   pseudo-random function derived from the Advanced Encryption Standard
   (AES).  This document describes such an algorithm, called AES-XCBC-
   PRF-128.

1.  Introduction

   [AES-XCBC-MAC] describes a method to use the Advanced Encryption
   Standard (AES) as a message authentication code (MAC) whose output is
   96 bits long.  While 96 bits is considered appropriate for a MAC, it
   is too short to be useful as a long-lived pseudo-random (PRF) in
   either IKE version 1 or version 2.  Both versions of IKE use the PRF
   to create keys in a fashion that is dependent on the length of the
   output of the PRF.  Using a PRF that has 96 bits of output creates
   keys that are easier to attack with brute force than a PRF that uses
   128 bits of output.

   Fortunately, there is a very simple method to use much of [AES-XCBC-
   MAC] as a PRF whose output is 128 bits: omit the step that truncates
   the 128-bit value to 96 bits.

2.  The AES-XCBC-PRF-128 Algorithm

   The AES-XCBC-PRF-128 algorithm is identical to [AES-XCBC-MAC] except
   that the truncation step in section 4.3 of [AES-XCBC-MAC] is *not*
   performed.  That is, there is no processing after section 4.2 of
   [AES-XCBC-MAC].

   The test vectors in section 4.6 can be used for AES-XCBC-PRF-128, but
   only those listed as "AES-XCBC-MAC", not "AES-XCBC-MAC-96".

3.  Security Considerations

   The security provided by AES-XCBC-MAC-PRF is based upon the strength
   of AES.  At the time of this writing, there are no known practical
   cryptographic attacks against AES or AES-XCBC-MAC-PRF.

   As is true with any cryptographic algorithm, part of its strength
   lies in the security of the key management mechanism, the strength of
   the associated secret key, and upon the correctness of the
   implementations in all of the participating systems.  [AES-XCBC-MAC]
   contains test vectors to assist in verifying the correctness of the
   AES-XCBC-MAC-PRF code.  The test vectors all show the full MAC value
   before it is truncated to 96 bits.  The PRF makes use of the full MAC
   value, not the truncated one.

4.  Intellectual Property Statement

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5.  References

5.1.  Normative References

   [AES-XCBC-MAC] Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96
                  Algorithm and Its Use With IPsec", RFC 3566, September
                  2003.

6.  Author's Address

   Paul Hoffman
   VPN Consortium
   127 Segre Place
   Santa Cruz, CA  95060  USA

   EMail: paul.hoffman@vpnc.org

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