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The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange Protocol (IKE) (RFC3664)

IP.com Disclosure Number: IPCOM000021455D
Original Publication Date: 2004-Jan-01
Included in the Prior Art Database: 2004-Jan-20
Document File: 5 page(s) / 7K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

P. Hoffman: AUTHOR

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.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 49% of the total text.

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 [Page 1]

RFC 3664 The AES-XCBC-PRF-128 Algorithm for IKE January 2004

2. The AES-XCBC-PRF-128 Algorithm

The AES-XCBC-PRF-128 algorithm is identical to [AES-XCBC-MAC] except

that 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 test vectors in section 4.6 can be used for AES-XCBC-PRF-128, but

only those listed as "AES-XCBC-MAC", not "AES-XCBC-MAC-96".

3. Security Considerations

The security provided by AES-XCBC-MAC-PRF is based upon the strength

of AES. At the time of this writing, there are no known practical

cryptographic attacks against AES or AES-XCBC-MAC-PRF.

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 upon the correctness of the

implementations in all of 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. Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any

intellectual property or other rights that might be claimed to

pertain to the implementation or use of the technology des...