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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Paul Hoffman 2 draft-ietf-ipsec-aes-xcbc-prf-01.txt VPN Consortium 3 October 7, 2003 4 Expires in six months 6 The AES-XCBC-PRF-128 algorithm for IKE 8 Status of this Memo 10 This document is an Internet-Draft and is in full conformance with all 11 provisions of Section 10 of RFC2026. 13 Internet-Drafts are working documents of the Internet Engineering Task 14 Force (IETF), its areas, and its working groups. Note that other groups 15 may also distribute working documents as Internet-Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference material 20 or to cite them other than as "work in progress." 22 The list of current Internet-Drafts can be accessed at 23 http://www.ietf.org/ietf/1id-abstracts.txt 25 The list of Internet-Draft Shadow Directories can be accessed at 26 http://www.ietf.org/shadow.html. 28 Abstract 30 Some implementations of IPsec may want to use a pseudo-random function 31 derived from AES. This document describes such an algorithm, called 32 AES-XCBC-PRF-128. 34 1. Introduction 36 [AES-XCBC-MAC] describes a method to use AES (the Advanced Encryption 37 Standard) as a message authentication code (MAC) whose output is 96 bits 38 long. While 96 bits is considered appropriate for a MAC, it is 39 too short to be useful as a long-lived pseudo-random (PRF) in either 40 IKE version 1 or version 2. Both versions of IKE use the PRF to create 41 keys in a fashion that is dependent on the length of the output of the 42 PRF. Using a PRF that has 96 bits of output creates keys that are easier 43 to attack with brute force than a PRF that uses 128 bits of output. 45 Fortunately, there is a very simple method to use much of [AES-XCBC-MAC] 46 as a PRF whose output is 128 bits: omit the step that truncates the 47 128-bit value to 96 bits. 49 2. The AES-XCBC-PRF-128 algorithm 51 The AES-XCBC-PRF-128 algorithm is identical to [AES-XCBC-MAC] except 52 that the truncation step in section 4.3 of [AES-XCBC-MAC] is *not* 53 performed. That is, there is no processing after section 4.2 of 54 [AES-XCBC-MAC]. 56 The test vectors in section 4.6 can be used for AES-XCBC-PRF-128, 57 but only those listed as "AES-XCBC-MAC", not "AES-XCBC-MAC-96". 59 3. Security considerations 61 The security provided by AES-XCBC-MAC-PRF is based upon the strength of 62 AES. At the time of this writing, there are no known practical 63 cryptographic attacks against AES or AES-XCBC-MAC-PRF. 65 As is true with any cryptographic algorithm, part of its strength lies 66 in the security of the key management mechanism, the strength of the 67 associated secret key, and upon the correctness of the implementations 68 in all of the participating systems. [AES-XCBC-MAC] contains test 69 vectors to assist in verifying the correctness of AES-XCBC-MAC-PRF code. 70 The test vectors all show the full MAC value before it is truncated to 71 96 bits. The PRF makes use of the full MAC value, not the truncated 72 one. 74 4. References 76 4.1 Normative references 78 [AES-XCBC-MAC] "The AES-XCBC-MAC-96 Algorithm and Its Use With IPsec", 79 RFC 3566. 81 5. Author's address 83 Paul Hoffman 84 VPN Consortium 85 127 Segre Place 86 Santa Cruz, CA 95060 USA 87 paul.hoffman@vpnc.org