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 © 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.

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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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Acknowledgement

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