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 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.
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