RFC 3664 (rfc3664) - Page 1 of 4
The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol (IKE)
Alternative Format: Original Text Document
Network Working Group P. Hoffman
Request for Comments: 3664 VPN Consortium
Category: Standards Track January 2004
The AES-XCBC-PRF-128 Algorithm for
the 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 (2004). All Rights Reserved.
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 (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
