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Versions: 00 01 02 RFC 4305

INTERNET-DRAFT                                    Donald E. Eastlake 3rd
Obsoletes RFCs 2402 and 2406                       Motorola Laboratories
Expires: February 2005                                       August 2004



   Cryptographic Algorithm Implementation Requirements For ESP And AH
   ------------- --------- -------------- ------------ --- --- --- --
              <draft-ietf-ipsec-esp-ah-algorithms-02.txt>


Status of This Document

   Distribution of this draft is unlimited. Comments should be sent to
   the authors.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

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   and may be updated, replaced, or obsoleted by other documents at any
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   Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract

   The IPsec series of protocols makes use of various cryptographic
   algorithms in order to provide security services. The Encapsulating
   Security Payload (ESP) and the Authentication Header (AH) provide two
   mechanisms for protecting data being sent over an IPsec Security
   Association (SA).  To ensure interoperability between disparate
   implementations it is necessary to specify a set of mandatory-to-
   implement algorithms to ensure at least one algorithm that all
   implementations will have available. This document defines the
   current set of mandatory-to-implement algorithms for ESP and AH as
   well as specifying algorithms that should be implemented because they
   may be promoted to mandatory at some future time.







D. Eastlake 3rd                                                 [Page 1]

INTERNET-DRAFT    Cryptographic Algorithms For ESP & AH      August 2004


Acknowledgement

   Much of the wording herein was adapted from "Cryptographic Algorithms
   for use in the Internet Key Exchange Version 2" <draft-ietf-ipsec-
   ikev2-algorithms-*.txt> by Jeffrey I. Schiller.


Table of Contents

      Status of This Document....................................1
      Abstract...................................................1

      Acknowledgement............................................2
      Table of Contents..........................................2

      1. Introduction............................................3
      2. Requirements Terminology................................3
      3. Algorithm Selection.....................................4
      3.1 Encapsulating Security Payload.........................4
      3.1.1 ESP Encryption and Authentication Algorithms.........4
      3.1.2 ESP Combined Mode Algorithms.........................5
      3.2 Authentication Header..................................5
      4. Security Considerations.................................6
      5. IANA Considerations.....................................6
      6. Changes from RFC 2402 and 2406..........................6

      Normative References.......................................8
      Informative References.....................................8

      Authors Address...........................................10

      Full Copyright Statement..................................11
      Expiration and File Name..................................11



















D. Eastlake 3rd                                                 [Page 2]

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1. Introduction

   The Encapsulating Security Payload (ESP) and the Authentication
   Header (AH) provide two mechanisms for protecting data being sent
   over an IPsec Security Association (SA) [IPsec, ESP, AH].  To ensure
   interoperability between disparate implementations it is necessary to
   specify a set of mandatory-to-implement algorithms to ensure at least
   one algorithm that all implementations will have available. This
   document defines the current set of mandatory-to-implement algorithms
   for ESP and AH as well as specifying algorithms that should be
   implemented because they may be promoted to mandatory at some future
   time.

   The nature of cryptography is that new algorithms surface
   continuously and existing algorithms are continuously attacked. An
   algorithm believed to be strong today may be demonstrated to be weak
   tomorrow.  Given this, the choice of mandatory-to-implement algorithm
   should be conservative so as to minimize the likelihood of it being
   compromised quickly. Thought should also be given to performance
   considerations as many uses of IPsec will be in environments where
   performance is a concern.

   Finally we need to recognize that the mandatory-to-implement
   algorithm(s) may need to change over time to adapt to the changing
   world. For this reason the selection of mandatory-to-implement
   algorithms is not included the main IPsec, ESP, or AH specifications.
   It is instead placed in this document. As the choice of algorithm
   changes, only this document should need to be updated.

   Ideally the mandatory-to-implement algorithm of tomorrow should
   already be available in most implementations of IPsec by the time it
   is made mandatory. To facilitate this we will attempt to identify
   such algorithms as they are known today in this document. There is no
   guarantee that the algorithms we believe today may be mandatory in
   the future will in fact become so. All algorithms known today are
   subject to cryptographic attack, and may be broken in the future.



2. Requirements Terminology

   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
   "MAY" that appear in this document are to be interpreted as described
   in [RFC 2119].

   In addition we will define some additional terms here:

   SHOULD+     This term means the same as SHOULD. However it is likely
               that an algorithm marked as SHOULD+ will be promoted at
               some future time to be a MUST.


D. Eastlake 3rd                                                 [Page 3]

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   SHOULD-     This terms means the same as SHOULD. However it is likely
               that an algorithm marked as SHOULD- will be deprecated to
               a MAY or worse in a future version of this document.
   MUST-       This term means the same as MUST. However we expect at
               some point in the future this algorithm will no longer be
               a MUST.



3. Algorithm Selection

   For IPsec implementations to interoperate, they must support one or
   more security algorithms in common.  This section specifies the
   security algorithm implementation requirements for standards
   comformant ESP and AH implementations. The security algorithms
   actually used for any particular ESP or AH security association is
   determined by a negotiation mechahism, such as the Internet Key
   Exchange (IKE [RFC 2409, IKEv2]), or pre-establishment.

   Of course, additional standard and proprietary algorithms beyond
   those listed below can be implemented.



3.1 Encapsulating Security Payload

   The implementation conformance requirements for security algorithms
   for ESP are given in the tables below.  See section 2 for definitions
   of the values in the "Requirement" column.



3.1.1 ESP Encryption and Authentication Algorithms

   These tables list encryption and authentication algorithms for the
   IPsec Encapsulating Security Payload protocol.

      Requirement    Encryption Algorithm (notes)
      -----------    --------------------
      MUST           NULL (1)
      MUST-          TripleDES-CBC [RFC 2451]
      SHOULD+        AES-CBC with 128-bit keys [RFC 3602]
      SHOULD         AES-CTR [RFC 3686]
      SHOULD NOT     DES-CBC [RFC 2405] (3)

      Requirement    Authentication Algorithm (notes)
      -----------    ------------------------
      MUST           HMAC-SHA1-96 [RFC 2404]
      MUST           NULL (1)
      SHOULD+        AES-XCBC-MAC-96 [RFC 3566]


D. Eastlake 3rd                                                 [Page 4]

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      MAY            HMAC-MD5-96 [RFC 2403] (2)

   Notes:

   1. Since ESP encryption and authentication are optional, support for
      the two "NULL" algorithms is required to maintain consistency with
      the way these services are negotiated. NOTE that while
      authentication and encryption can each be "NULL", they MUST NOT
      both be "NULL".
   2. Weaknesses have become apparent in MD5, however these should not
      effect the use of MD5 with HMAC.
   3. DES, with its small key size and publicly demonstrated and open
      design special purpose cracking hardware, is of questionable
      security for general use.




3.1.2 ESP Combined Mode Algorithms

   As specified in [ESP], combined mode algorithms are supported which
   provide both confidentiality and authentication services.  Support of
   such algorithms will require proper structuring of ESP
   implementations. Under many circumstances, combined mode algorithms
   provide significant efficiency and throughput advantages.  Although
   there are no suggested or required combined algorithms at this time,
   AES-CCM [CCM], which has been adopted as the prefered mode for
   security in IEEE 802.11 [802.11i], is expected to be of interest in
   the near future.



3.2 Authentication Header

   The implementation conformance requirements for security algorithms
   for AH are given below. See section 2 for definitions of the values
   in the "Requirement" column. As you would suspect, all of these
   algorithms are authentication algorithms.

      Requirement    Algorithm (notes)
      -----------    ---------
      MUST           HMAC-SHA1-96 [RFC 2404]
      SHOULD+        AES-XCBC-MAC-96 [RFC 3566]
      MAY            HMAC-MD5-96 [RFC 2403] (1)

   Notes:

   1. Weaknesses have become apparent in MD5, however these should not
      effect the use of MD5 with HMAC.



D. Eastlake 3rd                                                 [Page 5]

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4. Security Considerations

   The security of cryptographic based systems depends on both the
   strength of the cryptographic algorithms chosen, the strength of the
   keys used with those algorithms and the engineering and
   administration of the protocol used by the system to ensure that
   there are no non-cryptographic ways to bypass the security of the
   overall system.

   This document concerns itself with the selection of cryptographic
   algorithms for the use of ESP and AH, specifically with the selection
   of "Mandatory-to-Implement" algorithms. The algorithms identified in
   this document as MUST implement or SHOULD implement are not known to
   be broken at the current time and cryptographic research so far leads
   us to believe that they will likely remain secure into the
   foreseeable future. However, this is not necessarily forever. We
   would therefore expect that new revisions of this document will be
   issued from time to time that reflect the current best practice in
   this area.



5. IANA Considerations

   This document does not define any new registries nor elements in
   existing registries.



6. Changes from RFC 2402 and 2406

   [RFC 2402] and [RFC 2406] defined the IPsec Authentication Header and
   IPsec Encapsulating Security Payload. Each specified the
   implementation requirements for cryptogrpahic algorithms for their
   respectively protocols. They have now been replaced with [AH] and
   [ESP], which do not specify cryptographic algorithm implementation
   requirements, and this document which specifies such requirements for
   both [AH] and [ESP].

   The implementation requirements are compared below:

   Old   Old         New
   Req.  RFC(s)      Requirement  Algorithm (notes)
   ---   ------      -----------  ---------
   MUST  2406        SHOULD NOT   DES-CBC [RFC 2405] (1)
   MUST  2402 2406   MAY          HMAC-MD5-96 [RFC 2403]
   MUST  2402 2406   MUST         HMAC-SHA1-96 [RFC 2404]

   Notes:
   1. The IETF deprecated the use of single DES years ago and has not


D. Eastlake 3rd                                                 [Page 6]

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      included it in any new standard for some time (see IESG note on
      the first page of [RFC 2407]). But this document represents the
      first standards track recognition of that deprecation by
      specifying that implementations SHOULD NOT provide single DES. The
      US Government National Institute of Standards and Technology
      (NIST) has formally recognized the weakness of single DES by a
      notice published in the 26 July 2004 US Government Federal
      Register (Docket No. 040602169-4169-01) proposing to withdraw it
      as a US Government Standard. Triple DES remains approved by both
      the IETF and NIST.










































D. Eastlake 3rd                                                 [Page 7]

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Normative References

   [AH] - "IP Authentication Header", draft-ietf-ipsec-rfc2402bis-*.txt,
   S. Kent, work in progress.

   [ESP] - "IP Encapsulating Security Payload (ESP)", draft-ietf-ipsec-
   esp-v3-*.txt, S. Kent, work in progress.

   [IPsec] - "Security Architecture for the Internet Protocol", draft-
   ietf-ipsec-rfc2401bis-*.txt, S. Kent, work in progress.

   [RFC 2119] - "Key words for use in RFCs to Indicate Requirement
   Levels", S. Bradner, March 1997.

   [RFC 2403] - "The Use of HMAC-MD5-96 within ESP and AH", C. Madson,
   and R. Glenn, November 1998.

   [RFC 2404] - "The Use of HMAC-SHA-1-96 within ESP and AH", C. Madson,
   and R. Glenn, November 1998.

   [RFC 2405] - "The ESP DES-CBC Cipher Algorithm With Explicit IV", C.
   Madson, and N. Doraswamy, November 1998.

   [RFC 3566] - "The AES-XCBC-MAC-96 Algorithm and Its Use With IPSec",
   S. Frankel. H. Herbert, September 2003.

   [RFC 3602] - "The AES-CBC Cipher Algorithm and Its Use with IPsec",
   S. Frankel, R. Glenn, S. Kelly, September 2003.

   [RFC 3686] - "Using Advanced Encryption Standard (AES) Counter Mode
   With IPsec Encapsulating Security Payload (ESP)", R. Housley, July
   2003.



Informative References

   [802.11i] - LAN/MAN Specific Requirements - Part 11: Wireless Medium
   Access Control (MAC) and physical layer (PHY) specifications: Medium
   Access Control (MAC) Security Enhancements, IEEE Std 802.11i, June
   2004.

   [AES CCM] - "Using AES CCM Mode With IPsec ESP", draft-ietf-ipsec-
   ciph-aes-ccm-05.txt which is in the RFC Editor Queue, R. Housley,
   November 2003.

   [IKEv2] - "Internet Key Exchange (IKEv2) Protocol", draft-ietf-ipsec-
   ikev2-*.txt, C. Kaufman, October 2003.

   [RFC 791] - "Internet Protocol", J. Postel, September 1981.


D. Eastlake 3rd                                                 [Page 8]

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   [RFC 2402] - "IP Authentication Header", S. Kent, R. Atkinson,
   November 1998.

   [RFC 2406] - "IP Encapsulating Security Payload (ESP)", S. Kent, R.
   Atkinson, November 1998.

   [RFC 2407] - "The Internet IP Security Domain of Interpretation for
   ISAKMP", D. Piper, november 1998.

   [RFC 2409] - "The Internet Key Exchange (IKE)", D. Harkins, D.
   Carrel, November 1998.









































D. Eastlake 3rd                                                 [Page 9]

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Authors Address

   Donald E. Eastlake 3rd
   Motorola Laboratories
   155 Beaver Street
   Milford, MA 01757 USA

   Telephone:   +1-508-786-7554 (w)
                +1-508-634-2066 (h)
   EMail:       Donald.Eastlake@Motorola.com










































D. Eastlake 3rd                                                [Page 10]

INTERNET-DRAFT    Cryptographic Algorithms For ESP & AH      August 2004


Full Copyright Statement

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

   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.




Expiration and File Name

   This draft expires February 2005.

   Its file name is draft-ietf-ipsec-esp-ah-algorithms-02.txt.




































D. Eastlake 3rd                                                [Page 11]


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