The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol (IKE)
RFC 4434

Network Working Group                                         P. Hoffman
Request for Comments: 4434                                VPN Consortium
Obsoletes: 3664                                            February 2006
Category: Standards Track

                  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 (2006).

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 function
   (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.

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RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006

1.1.  Differences from RFC 3664

   This document specifies the same algorithm as RFC 3664 except that
   the restriction that keys 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 longer be
   rejected but instead will be made 128 bits.

   IKEv2 [IKEv2] 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 the PRF described in this document is used with IKEv2, the PRF
   is considered fixed-length for generating keying material but
   variable-length 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 128 bits long.  This somewhat tortured
   logic allows IKEv2 implementations that use the fixed-length-key
   semantics from RFC 3664 to interoperate with implementations that use
   the variable-length-key semantics of this document.

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 they
   are 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, lengthen it to exactly 128
      bits by padding it on the right with zero bits.

   o  If the key is 129 bits or longer, shorten it to exactly 128 bits
      by performing the steps in AES-XCBC-PRF-128 (that is, the
      algorithm described in this document).  In that re-application of
      this algorithm, the key is 128 zero bits; the message is the
      too-long current key.

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RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006

2.1.  Test Vectors

   Test Case AES-XCBC-PRF-128 with 20-byte input
   Key        : 000102030405060708090a0b0c0d0e0f
   Key Length : 16
   Message    : 000102030405060708090a0b0c0d0e0f10111213
   PRF Output : 47f51b4564966215b8985c63055ed308

   Test Case AES-XCBC-PRF-128 with 20-byte input
   Key        : 00010203040506070809
   Key Length : 10
   Message    : 000102030405060708090a0b0c0d0e0f10111213
   PRF Output : 0fa087af7d866e7653434e602fdde835

   Test Case AES-XCBC-PRF-128 with 20-byte input
   Key        : 000102030405060708090a0b0c0d0e0fedcb
   Key Length : 18
   Message    : 000102030405060708090a0b0c0d0e0f10111213
   PRF Output : 8cd3c93ae598a9803006ffb67c40e9e4

3.  Security Considerations

   The security provided by AES-XCBC-MAC-PRF is based on the strengths
   of AES and HMAC.  At the time of this writing, there are no known
   practical cryptographic attacks against AES, 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 the correctness of the implementations
   in all 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.  IANA Considerations

   Any reference to RFC 3664 needs to be updated to refer to this
   document when it is published.

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RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006

5.  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.

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

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RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006

Appendix A.  Acknowledgements

   Pasi Eronen suggested the easy method for shortening too-long keys.
   Saroop Mathur and John Black provided and verified the test vectors.

Author's Address

   Paul Hoffman
   VPN Consortium

   EMail: paul.hoffman@vpnc.org

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RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006

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   Copyright (C) The Internet Society (2006).

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