Network Working Group                                         P. Hoffman
Request for Comments: 4109                                VPN Consortium
Updates: 2409                                                   May 2005
Category: Standards Track

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

Copyright (C) The Internet Society (2005).

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.

Hoffman                     Standards Track                     [Page 1]


RFC 4109                  Algorithms for IKEv1                  May 2005
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.

Hoffman                     Standards Track                     [Page 2]


RFC 4109                  Algorithms for IKEv1                  May 2005

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.

Hoffman                     Standards Track                     [Page 3]


RFC 4109                  Algorithms for IKEv1                  May 2005
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

Hoffman                     Standards Track                     [Page 4]


RFC 4109                  Algorithms for IKEv1                  May 2005

Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org.

Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society.

Translate documents to 日本語, svenska, Nederlands, Deutsch, français, русский, italiano, español, Tiếng Việt, polski, português, 中文, українська, català, norsk, فارسی, suomi, Bahasa Indonesia, العربية, čeština, 한국어, Bahasa Melayu, magyar, română, српски and other languages.