Network Working Group
Request for Comments: 4109
Updates: 2409
Category: Standards Track

P. Hoffman
VPN Consortium
May 2005

Algorithms for Internet Key Exchange version 1 (IKEv1)

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

Abstract

The required and suggested algorithms in the original Internet Key Exchange version 1 (IKEv1) specification do not reflect the current reality of the IPsec market requirements. The original specification allows weak security and suggests algorithms that are thinly implemented. This document updates RFC 2409, the original specification, and is intended for all IKEv1 implementations deployed today.

1. Introduction

The original IKEv1 definition, [RFC2409], has a set of MUST-level and SHOULD-level requirements that do not match the needs of IPsec users. This document updates RFC 2409 by changing the algorithm requirements defined there.

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119].

2. Old Algorithm Requirements

RFC 2409 has the following MUST-level and SHOULD-level requirements:

   o  DES for encryption MUST be supported.
   o  MD5 and SHA-1 for hashing and HMAC functions MUST be supported.
   o  Pre-shared secrets for authentication MUST be supported.
   o  Diffie-Hellman MODP group 1 (discrete log 768 bits) MUST be
      supported.
   o  TripleDES for encryption SHOULD be supported.
   o  Tiger for hashing SHOULD be supported.
   o  DSA and RSA for authentication with signatures SHOULD be
      supported.
   o  RSA for authentication with encryption SHOULD be supported.
   o  Diffie-Hellman MODP group 2 (discrete log 1024 bits) SHOULD be
      supported.

RFC 2409 gives two conflicting requirement levels for Diffie-Hellman MODP groups with elliptic curves. Section 4 of that specification says that "IKE implementations ... MAY support ECP and EC2N groups", but Sections 6.3 and 6.4 say that MODP groups 3 and 4 for EC2N groups SHOULD be supported.

3. New Algorithm Requirements

The new requirements for IKEv1 are listed here. Note that some of the requirements are the same as those in RFC 2409, whereas others are changed.

   o  TripleDES for encryption MUST be supported.
   o  AES-128 in CBC mode [RFC3602] for encryption SHOULD be supported.
   o  SHA-1 for hashing and HMAC functions MUST be supported.
   o  Pre-shared secrets for authentication MUST be supported.
   o  AES-128 in XCBC mode for PRF functions ([RFC3566] and [RFC3664])
      SHOULD be supported.
   o  Diffie-Hellman MODP group 2 (discrete log 1024 bits) MUST be
      supported.
   o  Diffie-Hellman MODP group 14 (discrete log 2048 bits) [RFC3526]
      SHOULD be supported.
   o  RSA for authentication with signatures SHOULD be supported.

If additional updates are made to IKEv1 in the future, then it is very likely that implementation of AES-128 in CBC mode for encryption will become mandatory.

The other algorithms that were listed at MUST-level and SHOULD-level in RFC 2409 are now MAY-level. This includes DES for encryption, MD5 and Tiger for hashing, Diffie-Hellman MODP group 1, Diffie-Hellman MODP groups with elliptic curves, DSA for authentication with signatures, and RSA for authentication with encryption.

DES for encryption, MD5 for hashing, and Diffie-Hellman MODP group 1 are dropped to MAY due to cryptographic weakness.

Tiger for hashing, Diffie-Hellman MODP groups with elliptic curves, DSA for authentication with signatures, and RSA for authentication with encryption are dropped due to lack of any significant deployment and interoperability.

4. Summary

      Algorithm                     RFC 2409    This document
      ------------------------------------------------------------------
      DES for encryption            MUST        MAY (crypto weakness)
      TripleDES for encryption      SHOULD      MUST
      AES-128 for encryption        N/A         SHOULD
      MD5 for hashing and HMAC      MUST        MAY (crypto weakness)
      SHA1 for hashing and HMAC     MUST        MUST
      Tiger for hashing             SHOULD      MAY (lack of deployment)
      AES-XCBC-MAC-96 for PRF       N/A         SHOULD
      Pre-shared secrets            MUST        MUST
      RSA with signatures           SHOULD      SHOULD
      DSA with signatures           SHOULD      MAY (lack of deployment)
      RSA with encryption           SHOULD      MAY (lack of deployment)
      D-H Group 1 (768)             MUST        MAY (crypto weakness)
      D-H Group 2 (1024)            SHOULD      MUST
      D-H Group 14 (2048)           N/A         SHOULD
      D-H elliptic curves           SHOULD      MAY (lack of deployment)

5. Security Considerations

This document is all about security. All the algorithms that are either MUST-level or SHOULD-level in the "new algorithm requirements" section of this document are believed to be robust and secure at the time of this writing.

6. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.
   
   [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, May 2003.
   
   [RFC3566]  Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm
              and Its Use With IPsec", RFC 3566, September 2003.
   
   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602, September
              2003.
   
   [RFC3664]  Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
              Internet Key Exchange Protocol (IKE)", RFC 3664, January
              2004.

Author's Address

   Paul Hoffman
   VPN Consortium
   127 Segre Place
   Santa Cruz, CA  95060
   US
   
   EMail: paul.hoffman@vpnc.org

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