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Versions: (RFC 3664) 00 01 02 03 04 05 RFC 4434

Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Obsoletes: 3664 (if approved)                             April 18, 2005
Expires: October 20, 2005


 The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol
                                 (IKE)
                    draft-hoffman-rfc3664bis-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

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



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   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.

1.1  Differences from RFC 3664

   This document specifies the same algorithm as RFC 3664 except that
   the restriction on keys having to be exactly 128 bits from [AES-XCBC-
   MAC] is removed.  Implementations of RFC 3664 will have the same
   bits-on-the-wire results as this algorithm; the only difference is
   that keys that were not equal in length to 128 bits will no loger be
   rejected, but instead will be made 128 bits in accordance with [HMAC-
   definition].

2.  The AES-XCBC-PRF-128 Algorithm

   The AES-XCBC-PRF-128 algorithm is identical to [AES-XCBC-MAC] except
   for two changes.  First, the key length restriction of exactly 128
   bits in [AES-XCBC-MAC] is eliminated, as described below; this brings
   AES-XCBC-PRF-128 in alignment with HMAC-SHA1 and HMAC-MD5 when used
   as PRFs in IKE.  Second, 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 key for AES-XCBC-PRF-128 is created as follows:

   o  If the key is exactly 128 bits long, use it as-is.

   o  If the key has fewer than 128 bits, pad it on the right with zero
      bits until it is 128 bits long.

   o  If the key is 129 bits or longer, call the first 128 bits TKEY
      (temporary key) and the remaining bits TDATA (temporary data).  If
      TDATA's length is not a multiple of 128, pad TDATA on the right
      with enough 0 bits to make its length an exact multiple of 128.
      Encrypt each 128-bit block of TDATA with TKEY using AES-128-EBC as
      the cipher.  When finished, XOR the resulting blocks in the same
      order as they were created.




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3.  Security Considerations

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

   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.  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.

   [HMAC-definition]
              Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.


Author's Address

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org

















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