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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-hoffman-ipsec-aes-prf-00.txt VPN Consortium 3 June 16, 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 generally considered sufficient 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]. Of course, the test vectors in section 4.6 can be used, 55 but only those listed as "AES-XCBC-MAC", not "AES-XCBC-MAC-96". 57 3. Security considerations 59 The security considerations are the same as those in [AES-XCBC-MAC]. 61 4. References 63 4.1 Normative references 65 [AES-XCBC-MAC] "The AES-XCBC-MAC-96 Algorithm and Its Use With IPsec", 66 draft-ietf-ipsec-ciph-aes-xcbc-mac. This document is currently in the 67 RFC Editor's queue for publication. 69 5. Author's address 71 Paul Hoffman 72 VPN Consortium 73 127 Segre Place 74 Santa Cruz, CA 95060 USA 75 paul.hoffman@vpnc.org