Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)

Network Working Group                                          V. Manral
Request for Comments: 4835                              IP Infusion Inc.
Obsoletes: 4305                                               April 2007
Category: Standards Track


        Cryptographic Algorithm Implementation Requirements for
  Encapsulating Security Payload (ESP) and Authentication Header (AH)

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 IETF Trust (2007).

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 that there is 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.

Table of Contents

   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.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Changes from RFC 2402 and RFC 2406 to RFC 4305  . . . . . . . . 7
   7.  Changes from RFC 4305 . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 9

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) [RFC4301], [RFC4302].  To
   ensure interoperability between disparate implementations, it is
   necessary to specify a set of mandatory-to-implement algorithms to
   ensure that there is 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 in 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 that we (today) believe 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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   We 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.

         SHOULD-  This term 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 that 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-
   conformant ESP and AH implementations.  The security algorithms
   actually used for any particular ESP or AH security association are
   determined by a negotiation mechanism, such as the Internet Key
   Exchange (IKE [RFC2409], [RFC4306]) 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 [RFC2410] (1)
        MUST           AES-CBC with 128-bit keys [RFC3602]
        MUST-          TripleDES-CBC [RFC2451]
        SHOULD         AES-CTR [RFC3686]
        SHOULD NOT     DES-CBC [RFC2405] (2)
        Requirement    Authentication Algorithm (notes)
        -----------    -----------------------------
        MUST           HMAC-SHA1-96 [RFC2404] (3)
        SHOULD+        AES-XCBC-MAC-96 [RFC3566]
        MAY            NULL (1)
        MAY            HMAC-MD5-96 [RFC2403] (4)

   Notes:

      (1) Since ESP encryption is optional, support for the "NULL"
      algorithm is required to maintain consistency with the way
      services are negotiated.  Note that while authentication and
      encryption can each be "NULL", they MUST NOT both be "NULL"
      [RFC4301].

      (2) DES, with its small key size and publicly demonstrated and
      open-design special-purpose cracking hardware, is of questionable
      security for general use.

      (3) Weaknesses have become apparent in SHA-1 [SHA1-COLL]; however,
      these should not affect the use of SHA1 with HMAC.

      (4) Weaknesses have become apparent in MD5 [MD5-COLL]; however,
      these should not affect the use of MD5 with HMAC.

3.1.2.  ESP Combined Mode Algorithms

   As specified in [RFC4303], combined mode algorithms are supported
   that 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 [RFC4309] and AES-GCM [RFC4106] are of interest.  AES-CCM has
   been adopted as the preferred mode in IEEE 802.11 [802.11i], and AES-
   GCM has been adopted as the preferred mode in IEEE 802.1ae [802.1ae].

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 [RFC2404] (1)
       SHOULD+        AES-XCBC-MAC-96 [RFC3566]
       MAY            HMAC-MD5-96 [RFC2403] (2)

   Note:

      (1) Weaknesses have become apparent in SHA-1 [SHA1-COLL]; however,
      these should not affect the use of SHA1 with HMAC.

      (2) Weaknesses have become apparent in MD5 [MD5-COLL]; however,
      these should not affect the use of MD5 with HMAC.

4.  Security Considerations

   The security of cryptography-based systems depends on both the
   strength of the cryptographic algorithms chosen and the strength of
   the keys used with those algorithms.  The security also depends on
   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.  Acknowledgements

   Much of the wording herein was adapted from RFC 4305, the parent
   document of this document.  RFC 4305 itself borrows text from
   [RFC4307], "Cryptographic Algorithms for Use in the Internet Key
   Exchange Version 2", by Jeffrey I. Schiller.

   Thanks to the following people for reporting or responding to reports
   of the errors in RFC 4305: Paul Hoffman, Stephen Kent, Paul Koning,
   and Lars Volker.  Helpful Last-Call comments were received from Russ
   Housley, Elwyn Davies, Nicolas Williams, and Alfred Hoenes.

6.  Changes from RFC 2402 and RFC 2406 to RFC 4305

   [RFC2402] and [RFC2406] defined the IPsec Authentication Header and
   IPsec Encapsulating Security Payload.  Each specified the
   implementation requirements for cryptographic algorithms for their
   respective protocols.  They have now been replaced with [RFC4302] and
   [RFC4303], which do not specify cryptographic algorithm
   implementation requirements, and this document, which specifies such
   requirements for both [RFC4302] and [RFC4303].

   The implementation requirements are compared below:

      Old    Old             New
      Req.   RFC(s)       Requirement     Algorithm (notes)
      ----   ------       -----------     -----------------
      MUST   2406         SHOULD NOT      DES-CBC [RFC2405] (1)
      MUST   2402 2406    MAY             HMAC-MD5-96 [RFC2403]
      MUST   2402 2406    MUST            HMAC-SHA1-96 [RFC2404]


   Note:

      (1) The IETF deprecated the use of single DES years ago and has
      not included it in any new standard for some time (see IESG note
      on the first page of [RFC2407]).  [RFC4305] represented 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
      [DES-WDRAW] proposing to withdraw it as a US Government Standard.
      Triple DES remains approved by both the IETF and NIST.

7.  Changes from RFC 4305

   This document obsoletes [RFC4305].  The document incorporates changes
   for the support for the NULL Authentication Algorithm making the
   support from a MUST to a MAY.  This change is made to make this
   document consistent with [RFC4301].  Text for SHA-1 collision attacks
   as well as the future use of AES-GCM and AES-CCM is added.

   The changed implementation requirement resulting from the above
   changes is listed below:

      Old      Old         New
      Req.     RFC(s)      Requirement  Algorithm (notes)
      ----     ------      -----------  -----------------
      MUST     2406        MAY          NULL Authentication
      MUST     2406        MUST         NULL Encryption
      SHOULD+  4305        MUST         AES-CBC Encryption

8.  References

8.1.  Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP14,  RFC2119, March 1997.

   [RFC2403]    Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within
                ESP and AH", RFC 2403, November 1998.

   [RFC2404]    Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
                within ESP and AH", RFC 2404, November 1998.

   [RFC2405]    Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher
                Algorithm With Explicit IV", RFC 2405, November 1998.

   [RFC2410]    Glenn, R. and S. Kent, "The NULL Encryption Algorithm
                and Its Use With IPsec", RFC 2410, November 1998.

   [RFC2451]    Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
                Algorithms", RFC 2451, November 1998.

   [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.

   [RFC3686]    Housley, R., "Using Advanced Encryption Standard (AES)
                Counter Mode With IPsec Encapsulating Security Payload
                (ESP)", RFC 3686, January 2004.

   [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
                Internet Protocol", RFC 4301, December 2005.

   [RFC4302]    Kent, S., "IP Authentication Header", RFC 4302,

                December 2005.

   [RFC4303]    Kent, S., "IP Encapsulating Security Payload (ESP)",
                RFC 4303, December 2005.

   [RFC4305]    Eastlake, D., "Cryptographic Algorithm Implementation
                Requirements for Encapsulating Security Payload (ESP)
                and Authentication Header (AH)", RFC 4305,
                December 2005.

8.2.  Informative References

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

   [802.1ae]    "Media Access Control (MAC) Security", IEEE
                Standard Medium Access Control (MAC) Security, IEEE Std
                802.1ae, June 2006.

   [DES-WDRAW]  "Announcing Proposed Withdrawal of Federal Information
                Processing Standard (FIPS) for the Data Encryption
                Standard (DES) and Request for Comments", FIPS
                Notice Docket No. 040602169-4169-01, July 2004.

   [MD5-COLL]   Klima, V., "Finding MD5 Collisions - a Toy For a
                Notebook", Cryptology ePrint Archive Medium Report 2005/
                075, March 2005.

   [RFC2402]    Kent, S. and R. Atkinson, "IP Authentication Header",
                RFC 2402, November 1998.

   [RFC2406]    Kent, S. and R. Atkinson, "IP Encapsulating Security
                Payload (ESP)", RFC 2406, November 1998.

   [RFC2407]    Piper, D., "The Internet IP Security Domain of
                Interpretation for ISAKMP", RFC 2407, November 1998.

   [RFC2409]    Harkins, D. and D. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.

   [RFC4106]    Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
                (GCM) in IPsec Encapsulating Security Payload (ESP)",
                RFC 4106, June 2005.

   [RFC4306]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
                RFC 4306, December 2005.

   [RFC4307]    Schiller, J., "Cryptographic Algorithms for Use in the
                Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
                December 2005.

   [RFC4309]    Housley, R., "Using Advanced Encryption Standard (AES)
                CCM Mode with IPsec Encapsulating Security Payload
                (ESP)", RFC 4309, December 2005.

   [SHA1-COLL]  Rijmen, V. and E. Oswald, "Update on SHA-1", Cryptology
                ePrint Archive Report 2005/010, January 2005.

Author's Address

   Vishwas Manral
   IP Infusion Inc.
   Bamankhola, Bansgali,
   Almora, Uttarakhand  263601
   India

   Phone: +91-98456-61911
   EMail: vishwas@ipinfusion.com

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