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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.
Hoffman Standards Track [Page 1]
RFC 3664 The AES-XCBC-PRF-128 Algorithm for IKE January 2004
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
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Hoffman Standards Track [Page 2]
RFC 3664 The AES-XCBC-PRF-128 Algorithm for IKE January 2004
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
Hoffman Standards Track [Page 3]
RFC 3664 The AES-XCBC-PRF-128 Algorithm for IKE January 2004
7. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hoffman Standards Track [Page 4]