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Network Working Group                                         P. HoffmanRequest for Comments: 4434                                VPN ConsortiumObsoletes:3664                                            February 2006Category: Standards TrackThe AES-XCBC-PRF-128 Algorithm forthe 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.Hoffman                     Standards Track                     [Page 1]

RFC 4434               AES-XCBC-PRF-128 Algorithm          February 20061.1.  Differences fromRFC 3664   This document specifies the same algorithm asRFC 3664 except that   the restriction that keys be exactly 128 bits from [AES-XCBC-MAC] is   removed.  Implementations ofRFC 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 fromRFC 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.Hoffman                     Standards Track                     [Page 2]

RFC 4434               AES-XCBC-PRF-128 Algorithm          February 20062.1.  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 : 8cd3c93ae598a9803006ffb67c40e9e43.  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 toRFC 3664 needs to be updated to refer to this   document when it is published.Hoffman                     Standards Track                     [Page 3]

RFC 4434               AES-XCBC-PRF-128 Algorithm          February 20065.  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.Hoffman                     Standards Track                     [Page 4]

RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006Appendix 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.orgHoffman                     Standards Track                     [Page 5]

RFC 4434               AES-XCBC-PRF-128 Algorithm          February 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Hoffman                     Standards Track                     [Page 6]