Network Working Group P. Hoffman Internet-Draft VPN Consortium Obsoletes: 3664 (if approved) October 6, 2005 Expires: April 9, 2006 The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol (IKE) draft-hoffman-rfc3664bis-05.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 9, 2006. 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. Hoffman Expires April 9, 2006 [Page 1]
Internet-Draft AES-XCBC-PRF-128 algorithm October 2005 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. 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 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 using the PRF described in this document with IKEv2, the PRF is considered to be 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 when 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 used Hoffman Expires April 9, 2006 [Page 2]
Internet-Draft AES-XCBC-PRF-128 algorithm October 2005 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. 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 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 Hoffman Expires April 9, 2006 [Page 3]
Internet-Draft AES-XCBC-PRF-128 algorithm October 2005 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. IANA Considerations Any reference to RFC 3664 needs to be updated to refer to this document when it is published. 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., Ed., "Internet Key Exchange (IKEv2) Protocol", draft-ietf-ipsec-ikev2-17 (work in progress), September 2004. Appendix A. Acknowledgments 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 Full Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS Hoffman Expires April 9, 2006 [Page 4]
Internet-Draft AES-XCBC-PRF-128 algorithm October 2005 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Hoffman Expires April 9, 2006 [Page 5]
