idnits 2.17.1 draft-hoffman-rfc3664bis-05.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** It looks like you're using RFC 3978 boilerplate. You should update this to the boilerplate described in the IETF Trust License Policy document (see https://trustee.ietf.org/license-info), which is required now. -- Found old boilerplate from RFC 3978, Section 5.1 on line 16. -- Found old boilerplate from RFC 3978, Section 5.5 on line 187. -- Found old boilerplate from RFC 3979, Section 5, paragraph 1 on line 198. -- Found old boilerplate from RFC 3979, Section 5, paragraph 2 on line 205. -- Found old boilerplate from RFC 3979, Section 5, paragraph 3 on line 211. ** This document has an original RFC 3978 Section 5.4 Copyright Line, instead of the newer IETF Trust Copyright according to RFC 4748. ** This document has an original RFC 3978 Section 5.5 Disclaimer, instead of the newer disclaimer which includes the IETF Trust according to RFC 4748. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- == No 'Intended status' indicated for this document; assuming Proposed Standard Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The draft header indicates that this document obsoletes RFC3664, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the RFC 3978 Section 5.4 Copyright Line does not match the current year -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (October 6, 2005) is 6777 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 3 errors (**), 0 flaws (~~), 2 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Hoffman 3 Internet-Draft VPN Consortium 4 Obsoletes: 3664 (if approved) October 6, 2005 5 Expires: April 9, 2006 7 The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol 8 (IKE) 9 draft-hoffman-rfc3664bis-05.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on April 9, 2006. 36 Copyright Notice 38 Copyright (C) The Internet Society (2005). 40 Abstract 42 Some implementations of IP Security (IPsec) may want to use a pseudo- 43 random function derived from the Advanced Encryption Standard (AES). 44 This document describes such an algorithm, called AES-XCBC-PRF-128. 46 1. Introduction 48 [AES-XCBC-MAC] describes a method to use the Advanced Encryption 49 Standard (AES) as a message authentication code (MAC) whose output is 50 96 bits long. While 96 bits is considered appropriate for a MAC, it 51 is too short to be useful as a long-lived pseudo-random (PRF) in 52 either IKE version 1 or version 2. Both versions of IKE use the PRF 53 to create keys in a fashion that is dependent on the length of the 54 output of the PRF. Using a PRF that has 96 bits of output creates 55 keys that are easier to attack with brute force than a PRF that uses 56 128 bits of output. 58 Fortunately, there is a very simple method to use much of [AES-XCBC- 59 MAC] as a PRF whose output is 128 bits: omit the step that truncates 60 the 128-bit value to 96 bits. 62 1.1. Differences from RFC 3664 64 This document specifies the same algorithm as RFC 3664 except that 65 the restriction on keys having to be exactly 128 bits from [AES-XCBC- 66 MAC] is removed. Implementations of RFC 3664 will have the same 67 bits-on-the-wire results as this algorithm; the only difference is 68 that keys that were not equal in length to 128 bits will no longer be 69 rejected, but instead will be made 128 bits. 71 IKEv2 [IKEv2] uses PRFs for multiple purposes, most notably for 72 generating keying material and authentication of the IKE_SA. The 73 IKEv2 specification differentiates between PRFs with fixed key sizes 74 and those with variable key sizes. 76 When using the PRF described in this document with IKEv2, the PRF is 77 considered to be fixed-length for generating keying material but 78 variable-length for authentication. That is, when generating keying 79 material, "half the bits must come from Ni and half from Nr, taking 80 the first bits of each" as described in IKEv2 section 2.14, but when 81 authenticating with shared secrets (IKEv2 section 2.16), the shared 82 secret does not have to be 128 bits long. This somewhat tortured 83 logic allows IKEv2 implementations that use the fixed-length-key 84 semantics from RFC 3664 to interoperate with implementations that use 85 the variable-length-key semantics of this document. 87 2. The AES-XCBC-PRF-128 Algorithm 89 The AES-XCBC-PRF-128 algorithm is identical to [AES-XCBC-MAC] except 90 for two changes. First, the key length restriction of exactly 128 91 bits in [AES-XCBC-MAC] is eliminated, as described below; this brings 92 AES-XCBC-PRF-128 in alignment with HMAC-SHA1 and HMAC-MD5 when used 93 as PRFs in IKE. Second, the truncation step in section 4.3 of [AES- 94 XCBC-MAC] is *not* performed; that is, there is no processing after 95 section 4.2 of [AES-XCBC-MAC]. 97 The key for AES-XCBC-PRF-128 is created as follows: 99 o If the key is exactly 128 bits long, use it as-is. 101 o If the key has fewer than 128 bits, lengthen it to exactly 128 102 bits by padding it on the right with zero bits. 104 o If the key is 129 bits or longer, shorten it to exactly 128 bits 105 by performing the steps in AES-XCBC-PRF-128 (that is, the 106 algorithm described in this document). In that re-application of 107 this algorithm, the key is 128 zero bits; the message is the too- 108 long current key. 110 2.1. Test Vectors 112 Test Case AES-XCBC-PRF-128 with 20-byte input 113 Key : 000102030405060708090a0b0c0d0e0f 114 Key Length : 16 115 Message : 000102030405060708090a0b0c0d0e0f10111213 116 PRF Output : 47f51b4564966215b8985c63055ed308 118 Test Case AES-XCBC-PRF-128 with 20-byte input 119 Key : 00010203040506070809 120 Key Length : 10 121 Message : 000102030405060708090a0b0c0d0e0f10111213 122 PRF Output : 0fa087af7d866e7653434e602fdde835 124 Test Case AES-XCBC-PRF-128 with 20-byte input 125 Key : 000102030405060708090a0b0c0d0e0fedcb 126 Key Length : 18 127 Message : 000102030405060708090a0b0c0d0e0f10111213 128 PRF Output : 8cd3c93ae598a9803006ffb67c40e9e4 130 3. Security Considerations 132 The security provided by AES-XCBC-MAC-PRF is based upon the strengths 133 of AES and HMAC. At the time of this writing, there are no known 134 practical cryptographic attacks against AES or AES-XCBC-MAC-PRF or 135 HMACs. 137 As is true with any cryptographic algorithm, part of its strength 138 lies in the security of the key management mechanism, the strength of 139 the associated secret key, and upon the correctness of the 140 implementations in all of the participating systems. [AES-XCBC-MAC] 141 contains test vectors to assist in verifying the correctness of the 142 AES-XCBC-MAC-PRF code. The test vectors all show the full MAC value 143 before it is truncated to 96 bits. The PRF makes use of the full MAC 144 value, not the truncated one. 146 4. IANA Considerations 148 Any reference to RFC 3664 needs to be updated to refer to this 149 document when it is published. 151 5. Normative References 153 [AES-XCBC-MAC] 154 Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm 155 and Its Use With IPsec", RFC 3566, September 2003. 157 [IKEv2] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) 158 Protocol", draft-ietf-ipsec-ikev2-17 (work in progress), 159 September 2004. 161 Appendix A. Acknowledgments 163 Pasi Eronen suggested the easy method for shortening too-long keys. 164 Saroop Mathur and John Black provided and verified the test vectors. 166 Author's Address 168 Paul Hoffman 169 VPN Consortium 171 Email: paul.hoffman@vpnc.org 173 Full Copyright Statement 175 Copyright (C) The Internet Society (2005). 177 This document is subject to the rights, licenses and restrictions 178 contained in BCP 78, and except as set forth therein, the authors 179 retain all their rights. 181 This document and the information contained herein are provided on an 182 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 183 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 184 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 185 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 186 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 187 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 189 Intellectual Property 191 The IETF takes no position regarding the validity or scope of any 192 Intellectual Property Rights or other rights that might be claimed to 193 pertain to the implementation or use of the technology described in 194 this document or the extent to which any license under such rights 195 might or might not be available; nor does it represent that it has 196 made any independent effort to identify any such rights. Information 197 on the procedures with respect to rights in RFC documents can be 198 found in BCP 78 and BCP 79. 200 Copies of IPR disclosures made to the IETF Secretariat and any 201 assurances of licenses to be made available, or the result of an 202 attempt made to obtain a general license or permission for the use of 203 such proprietary rights by implementers or users of this 204 specification can be obtained from the IETF on-line IPR repository at 205 http://www.ietf.org/ipr. 207 The IETF invites any interested party to bring to its attention any 208 copyrights, patents or patent applications, or other proprietary 209 rights that may cover technology that may be required to implement 210 this standard. Please address the information to the IETF at 211 ietf-ipr@ietf.org. 213 Acknowledgment 215 Funding for the RFC Editor function is currently provided by the 216 Internet Society.