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rfc:rfc4835

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.

Manral Standards Track [Page 1] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

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

Manral Standards Track [Page 2] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

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].

Manral Standards Track [Page 3] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

 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)

Manral Standards Track [Page 4] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

      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.

Manral Standards Track [Page 5] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

     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.

Manral Standards Track [Page 6] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

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.

Manral Standards Track [Page 7] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

 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,

Manral Standards Track [Page 8] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

              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.

Manral Standards Track [Page 9] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

 [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

Manral Standards Track [Page 10] RFC 4835 Cryptographic Algorithms ESP and AH April 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 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.
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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Manral Standards Track [Page 11]

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