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Versions: 00 01 02 03 04 RFC 3602

Internet Draft                                       IPsec Working Group
November 2001                                           S. Frankel, NIST
Expiration Date: May 2002                            S. Kelly, SonicWALL
                                                          R. Glenn, NIST


            The AES Cipher Algorithm and Its Use With IPsec
                 <draft-ietf-ipsec-ciph-aes-cbc-03.txt>




Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  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.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Drafts Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This document is a submission to the IETF Internet Protocol Security
   (IPsec) Working Group. Comments are solicited and should be addressed
   to the working group mailing list (ipsec@lists.tislabs.com) or to the
   editors.

   Distribution of this memo is unlimited.

Abstract

   This document describes the use of the AES Cipher Algorithm in Cipher
   Block Chaining Mode, with an explicit IV, as a confidentiality mecha-
   nism within the context of the IPsec Encapsulating Security Payload
   (ESP).













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                             Table of Contents



 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.1 Specification of Requirements  . . . . . . . . . . . . . . .   3
 2. The AES Cipher Algorithm . . . . . . . . . . . . . . . . . . . .   3
    2.1 Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
    2.2 Key Size . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    2.3 Weak Keys  . . . . . . . . . . . . . . . . . . . . . . . . .   4
    2.4 Block Size and Padding . . . . . . . . . . . . . . . . . . .   4
    2.5 Rounds . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
    2.6 Additional Information . . . . . . . . . . . . . . . . . . .   5
    2.7 Performance  . . . . . . . . . . . . . . . . . . . . . . . .   5
 3. ESP Payload  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
    3.1 ESP Algorithmic Interactions . . . . . . . . . . . . . . . .   6
    3.2 Keying Material  . . . . . . . . . . . . . . . . . . . . . .   6
 4. IKE Interactions . . . . . . . . . . . . . . . . . . . . . . . .   6
    4.1 Phase 1 Identifier . . . . . . . . . . . . . . . . . . . . .   6
    4.2 Phase 2 Identifier . . . . . . . . . . . . . . . . . . . . .   6
    4.3 Key Length Attribute . . . . . . . . . . . . . . . . . . . .   6
    4.4 Diffie-Hellman Groups  . . . . . . . . . . . . . . . . . . .   6
        4.4.1 Relative Strength  . . . . . . . . . . . . . . . . . .   7
    4.5 Hash Algorithm Considerations  . . . . . . . . . . . . . . .   8
 5. Security Considerations  . . . . . . . . . . . . . . . . . . . .   9
 6. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . .   9
 7. Intellectual Property Rights Statement . . . . . . . . . . . . .   9
 8. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  10
 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  11
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  12


























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1. Introduction

   As the culmination of a four-year competitive process, NIST (the Na-
   tional Institute of Standards and Technology) has selected the AES
   (Advanced Encryption Standard), the successor to the venerable DES.
   The competition was an open one, with public participation and com-
   ment solicited at each step of the process. The AES, formerly known
   as Rijndael, was chosen from a field of five finalists.

   The final AES selection was made on the basis of several additional
   characteristics:


     +    computational efficiency and memory requirements on a variety
          of software and hardware, including smart cards

     +    flexibility, simplicity and ease of implementation


   The AES will be the government's designated encryption cipher, and
   will be definitively described in a FIPS (Federal Information Pro-
   cessing Standard), expected to be completed by summer 2001.  The
   expectation is that the AES will suffice to protect sensitive
   (unclassified) government information at least until the next cen-
   tury.  It is also expected to be widely adopted by businesses and
   financial institutions.

   It is the intention of the IETF IPsec Working Group that AES will
   eventually be adopted as the default IPsec ESP cipher and will obtain
   the status of MUST be included in compliant IPsec implementations.

   The remainder of this document specifies the use of the AES within
   the context of IPsec ESP.  For further information on how the various
   pieces of ESP fit together to provide security services, refer to
   [ARCH], [ESP], and [ROAD].

1.1 Specification of Requirements

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" that
   appear in this document are to be interpreted as described in
   [RFC-2119].

2. The AES Cipher Algorithm

   All symmetric block cipher algorithms share common characteristics
   and variables, including mode, key size, weak keys, block size, and
   rounds.  The following sections contain descriptions of the relevant
   characteristics of the AES cipher.

2.1 Mode

   No operational modes are currently defined for the AES cipher.  NIST
   is in the process of developing a modes of operation FIPS for AES



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   [MODES].  However, the Cipher Block Chaining (CBC) mode is well-
   defined and well-understood for symmetric ciphers, and is currently
   required for all other ESP ciphers.  This document specifies the use
   of the AES cipher in CBC mode within ESP.  This mode requires an Ini-
   tialization Vector (IV) that is the same size as the block size.  Use
   of a randomly generated IV prevents generation of identical cipher-
   text from packets which have identical data that spans the first
   block of the cipher algorithm's block size.

   The IV is XOR'd with the first plaintext block before it is
   encrypted.  Then for successive blocks, the previous ciphertext block
   is XOR'd with the current plaintext, before it is encrypted.

   More information on CBC mode can be obtained in [CRYPTO-S].  For the
   use of CBC mode in ESP with 64-bit ciphers, see [CBC].

2.2 Key Size

   Some cipher algorithms allow for variable sized keys, while others
   only allow specific, pre-defined key sizes.  The length of the key
   typically correlates with the strength of the algorithm; thus larger
   keys are usually harder to break than shorter ones.

   This document specifies the default (i.e. MUST be supported) key size
   for the AES cipher algorithm.  The default key size that implementa-
   tions MUST support for IPsec is 128 bits.  In addition, implementa-
   tions MAY support key sizes of 192 and 256 bits.

2.3 Weak Keys

   At the time of writing this document there are no known weak keys for
   the AES.

   Some cipher algorithms have weak keys or keys that MUST not be used
   due to their interaction with some aspect of the cipher's definition.
   If weak keys are discovered for the AES, then weak keys SHOULD be
   checked for and discarded when using manual key management.  When
   using dynamic key management, such as [IKE], weak key checks SHOULD
   NOT be performed as they are seen as an unnecessary added code com-
   plexity that could weaken the intended security [EVALUATION].

2.4 Block Size and Padding

   The AES uses a block size of sixteen octets (128 bits).

   Padding is required by the AES to maintain a 16-octet (128-bit)
   blocksize.  Padding MUST be added, as specified in [ESP], such that
   the data to be encrypted (which includes the ESP Pad Length and Next
   Header fields) has a length that is a multiple of 16 octets.

   Because of the algorithm specific padding requirement, no additional
   padding is required to ensure that the ciphertext terminates on a
   4-octet boundary (i.e. maintaining a 16-octet blocksize guarantees
   that the ESP Pad Length and Next Header fields will be right aligned



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   within a 4-octet word).   Additional padding MAY be included, as
   specifed in [ESP], as long as the 16-octet blocksize is maintained.

2.5 Rounds

   This variable determines how many times a block is encrypted.  While
   this variable MAY be negotiated, a default value MUST always exist
   when it is not negotiated. Within IPsec, the AES MUST support 10
   rounds, corresponding to the mandatory 128-bit keysize.

   The AES's default number of rounds is 12 for a 192-bit keysize and 14
   for a 256-bit keysize.

2.6 Additional Information

   AES was invented by Joan Daemen from Banksys/PWI and Vincent Rijmen
   from ESAT-COSIC, both in Belgium, and is available world-wide on a
   royalty-free basis.  It is not covered by any patents, and the Rijn-
   dael homepage contains the following statement: "Rijndael is avail-
   able for free. You can use it for whatever purposes you want, irre-
   spective of whether it is accepted as AES or not."  AES's description
   can be found in [RIJNDAEL].  The Rijndael homepage is:
   http://www.esat.kuleuven.ac.be/~rijmen/rijndael/.

   The AES homepage, http://www.nist.gov/aes, contains a wealth of in-
   formation about the AES, including a definitive description of the
   AES algorithm, performance statistics, test vectors and intellectual
   property information.  This site also contains information on how to
   obtain an AES reference implementation from NIST.

2.7 Performance

   For a comparison table of the estimated speeds of AES and other ci-
   pher algorithms, please see [PERF-1], [PERF-2], [PERF-3], or
   [PERF-4]. The AES homepage has pointers to other analyses. The AES
   cypher document [RIJNDAEL] also contains performance statistics.

3. ESP Payload

   The ESP payload is made up of the IV followed by raw cipher-text.
   Thus the payload field, as defined in [ESP], is broken down according
   to the following diagram:

    +---------------+---------------+---------------+---------------+
    |                                                               |
    +               Initialization Vector (16 octets)               +
    |                                                               |
    +---------------+---------------+---------------+---------------+
    |                                                               |
    ~ Encrypted Payload (variable length, a multiple of 16 octets)  ~
    |                                                               |
    +---------------------------------------------------------------+

   The IV field MUST be the same size as the block size of the cipher



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   algorithm being used.  The IV MUST be chosen at random.  Common prac-
   tice is to use random data for the first IV and the last block of en-
   crypted data from an encryption process as the IV for the next en-
   cryption process.

   Including the IV in each datagram ensures that decryption of each re-
   ceived datagram can be performed, even when some datagrams are
   dropped, or datagrams are re-ordered in transit.

   To avoid CBC encryption of very similar plaintext blocks in different
   packets, implementations MUST NOT use a counter or other low-Hamming
   distance source for IVs.

3.1 ESP Algorithmic Interactions

   Currently, there are no known issues regarding interactions between
   the AES and other aspects of ESP, such as use of certain authentica-
   tion schemes.

3.2 Keying Material

   The minimum number of bits sent from the key exchange protocol to the
   ESP algorithm must be greater than or equal to the key size.

   The cipher's encryption and decryption key is taken from the first
   <x> bits of the keying material, where <x> represents the required
   key size.

4. IKE Interactions

4.1 Phase 1 Identifier

   For Phase 1 negotiations, IANA has assigned an Encryption Algorithm
   ID of 7 for AES-CBC.

4.2 Phase 2 Identifier

   For Phase 2 negotiations, IANA has assigned an ESP Transform Identi-
   fier of 12 for ESP_AES.

4.3 Key Length Attribute

   Since the AES allows variable key lengths, the Key Length attribute
   MUST be specified in both a Phase 1 exchange [IKE] and a Phase 2 ex-
   change [DOI].

4.4 Diffie-Hellman Groups

   The Diffie-Hellman algorithm is the basis of cryptographic key ex-
   change within IPsec. The algorithm may be implemented using either
   "MODP" (modulus-exponent) groups or "EC" (elliptic curve) groups. The
   general procedure is as follows: the initiator chooses a random expo-
   nent x with K bits of entropy that is 2K bits in length (the K bits
   may be hashed to produce 2K bits), and then computes g^x using the



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   group operation:

                                  X = g^x


   For MODP the group operation is modular multiplication, while for EC
   the operation is point addition on the curve.  The notation "g^x"
   means "iterate the group operation x times".  X is then sent to the
   responder. The responder chooses a secret number y, and similarly
   computes

                                  Y = g^y


   which is in turn sent to the initiator. At this point, both the ini-
   tiator and responder may compute a shared secret value by combining
   their own secret value with the exponential and applying the group
   operation:

                          Z = g^(xy) = Y^x = X^y


   From Z, both derive identical cryptographic keys.

   This description is simplified in the interest of brevity, and an in-
   depth description of this mechanism is beyond the scope of this memo.
   For further details, refer to the wealth of published literature on
   this topic.

4.4.1 Relative Strength

   The relative strength of the encryption keys derived via the Diffie-
   Hellman exchange may be characterized in terms the randomness of the
   participant's exponents and the strength of the Diffie-Hellman group;
   if an exponent has at least 128 completely random bits,  it is said
   to have 128-bits of "entropy".  If the Diffie-Hellman group cannot be
   broken in less time than searching a 128-bit key space, then the de-
   rived 128-bit key is said to have 128 bits of "strength". For an in-
   depth discussion regarding relative strength of values derived from
   DH exchanges, see [KEYLEN-1].

   In some cases, one may choose to settle for an amount of entropy
   which is less than that of a completely random key of the given size.
   There are numerous reasons for making such a choice, among which
   might include a concern for the computational effort required to com-
   plete the key exchange. For example, the following table lists recom-
   mended modulus and exponent sizes for various key lengths using ei-
   ther MODP or EC groups.









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   +===========+=================+================+===============+
   | Key Size  |  Exponent Size  |  Modulus Size  |  Group Type   |
   +===========+=================+================+===============+
   | 128       |  256            |  3240          |  MODP         |
   +-----------+-----------------+----------------+---------------+
   | 192       |  384            |  7945          |  MODP         |
   +-----------+-----------------+----------------+---------------+
   | 256       |  512            |  15430         |  MODP         |
   +-----------+-----------------+----------------+---------------+
   | 128       |  248            |  248           |  EC2N         |
   +-----------+-----------------+----------------+---------------+
   | 192       |  376            |  376           |  EC2N         |
   +-----------+-----------------+----------------+---------------+
   | 256       |  504            |  504           |  EC2N         |
   +-----------+-----------------+----------------+---------------+

   NOTE: This table is based on Section 4.5 in [KEYLEN-1] and on email
   communications with Hilarie Orman [KEYLEN-2].

   Note that the sizes of the moduli and exponents for the MODP groups
   in the table above are very large, and the computational effort re-
   quired to complete the exponentiation and modulo operations with such
   large values is quite significant using hardware commonly available
   in the year 2001. If such considerations are deemed important, then
   keys larger than 128 bits SHOULD NOT be used. Further, if it is de-
   termined that less than 128 bits of strength will suffice for the se-
   curity requirements of the given application, then smaller exponents
   and moduli may be used.

   [GROUPS] defines four additional Diffie-Hellman MODP groups for IKE.
   Two of these groups, a 3072-bit MODP group and a 4096-bit MODP group,
   could be used to establish 128-bit AES keys. [IKE-ECC] defines four
   additional Diffie-Hellman ECC groups for IKE.  Two of these groups,
   Group 8 and 9, both of which are 283-bit ECC groups, could be used to
   establish 128-bit AES keys.  Additional information about the rela-
   tionship between the group governing a Diffie-Hellman exchange and
   the symmetric keys derived from the exchange can be found in
   [KEYLEN-1].

4.5 Hash Algorithm Considerations

   A companion competition, to select the successor to SHA-1, the wide-
   ly-used hash algorithm, recently concluded.  The resulting hashes,
   called SHA-256, SHA-384 and SHA-512 [SHA2-1] are capable of producing
   output of three different lengths (256, 384 and 512 bits), sufficient
   for the generation (within IKE) and authentication (within ESP) of
   the three AES key sizes (128, 192 and 256 bits).  IANA has already
   assigned Phase 1 Hash Algorithm values of 4, 5 and 6 to SHA2-256,
   SHA2-384, and SHA2-512.  IANA has also assigned AH Transform Identi-
   fiers of 5, 6 and 7 to AH_SHA2-256, AH_SHA2-384, and AH_SHA2-512.)

   However, HMAC-SHA-1 [HMAC-SHA] and HMAC-MD5 [HMAC-MD5] are currently
   considered of sufficient strength to serve both as IKE generators of
   128-bit AES keys and as ESP authenticators for AES encryption using



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   128-bit keys.

5. Security Considerations

   Implementations are encouraged to use the largest key sizes they can
   when taking into account performance considerations for their partic-
   ular hardware and software configuration.  Note that encryption nec-
   essarily impacts both sides of a secure channel, so such considera-
   tion must take into account not only the client side, but the server
   as well. However, a key size of 128 bits is considered secure for the
   foreseeable future.

   Because the AES algorithm is relatively new and has only undergone
   limited cryptographic analysis, its use in IPsec implementations
   should be considered experimental.  Once NIST has published the AES
   FIPS, and at the recommendation of cryptographic experts, AES should
   become a default and mandatory-to-implement cipher algorithm for
   IPsec.

   For more information regarding the necessary use of random IV values,
   see [CRYPTO-B].

   For further security considerations, the reader is encouraged to read
   [RIJNDAEL].

6. IANA Considerations

   IANA has assigned Encryption Algorithm ID 7 to AES-CBC.
   IANA has assigned ESP Transform Identifier 12 to ESP_AES.

7. Intellectual Property Rights Statement


   Pursuant to the provisions of [RFC-2026], the authors represent that
   they have disclosed the existence of any proprietary or intellectual
   property rights in the contribution that are reasonably and personal-
   ly known to the authors.  The authors do not represent that they per-
   sonally know of all potentially pertinent proprietary and intellectu-
   al property rights owned or claimed by the organizations they repre-
   sent or third parties.

   The IETF takes no position regarding the validity or scope of any in-
   tellectual property or other rights that might be claimed to pertain
   to the implementation or use of the technology described in this doc-
   ument or the extent to which any license under such rights might or
   might not be available; neither does it represent that it has made
   any effort to identify any such rights.  Information on the IETF's
   procedures with respect to rights in standards-track and standards-
   related documentation can be found in BCP-11.  Copies of claims of
   rights made available for publication 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 Secretariat.



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8. Acknowledgments

   Portions of this text, as well as its general structure, were un-
   abashedly lifted from [CBC].

   The authors want to thank Hilarie Orman for providing expert advice
   (and a sanity check) on key sizes, requirements for Diffie-Hellman
   groups, and IKE interactions.

9. References


     [ARCH]      Kent, S. and R. Atkinson, "Security Architecture for
                 the Internet Protocol", RFC 2401, November 1998.

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

     [CRYPTO-B]  Bellovin, S., "Probable Plaintext Cryptanalysis of the
                 IP Security Protocols", Proceedings of the Symposium on
                 Network and Distributed System Security, San Diego, CA,
                 pp. 155-160, February 1997.
                 http://www.research.att.com/~smb/probtxt.{ps, pdf})

     [CRYPTO-S]  B. Schneier, "Applied Cryptography Second Edition",
                 John Wiley & Sons, New York, NY, 1995, ISBN
                 0-471-12845-7.

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

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

     [EVALUATION]
                 Ferguson, N. and B. Schneier, "A Cryptographic
                 Evaluation of IPsec," Counterpane Internet Security,
                 Inc., January 2000.

     [GROUPS]    Kivinen, T. and M. Kojo, "More MODP Diffie-Hellman
                 groups for IKE," draft-ietf-ipsec-ike-modp-
                 groups-00.txt, October 2000.

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

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

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

     [IKE-ECC]   Panjwani, P. and Y. Poeluev, "Additional ECC Groups For
                 IKE," draft-ietf-ipsec-ike-ecc-groups-02.txt, May 2000.



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     [KEYLEN-1]  Orman, H. and P. Hoffman, "Determining Strengths For
                 Public Keys Used For Exchanging Symmetric Keys," draft-
                 orman-public-key-lengths-01.txt, August 2000.

     [KEYLEN-2]  Orman, H., email communications, February 2000.

     [MODES]     "Symmetric Key Block Cipher Modes of Operation."
                 http://www.nist.gov/modes

     [PERF-1]    Bassham, L. III, "Efficiency Testing of ANSI C
                 Implementations of Round1 Candidate Algorithms for the
                 Advanced Encryption Standard."
                 http://csrc.nist.gov/encryption/aes/round1/r1-ansic.pdf

     [PERF-2]    Lipmaa, Helger, "Efficiency Testing Table."
                 http://home.cyber.ee/helger/aes

     [PERF-3]    Nechvetal, J., E. Barker, D. Dodson, M. Dworkin, J.
                 Foti and E. Roback, "Status Report on the First Round
                 of the Development of the Advanced Encryption
                 Standard."
                 http://csrc.nist.gov/encryption/aes/round1/r1report.pdf

     [PERF-4]    Schneier, B., J. Kelsey, D. Whiting, D. Wagner, C.
                 Hall, and N. Ferguson, "Performance Comparison of the
                 AES Submissions."
                 http://www.counterpane.com/AES-performance.html

     [RFC-2026]  Bradner, S., "The Internet Standards Process --
                 Revision 3", RFC2026, October 1996.

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

     [RIJNDAEL]  Daemen, J. and V. Rijman, "AES Proposal: Rijndael,"
                 NIST AES Proposal, Jun 1998.
http://csrc.nist.gov/encryption/aes/round2/AESAlgs/Rijndael/Rijndael.pdf

     [ROAD]      Thayer, R., N. Doraswamy and R. Glenn, "IP Security
                 Document Roadmap", RFC 2411, November 1998.

     [SHA2-1]    "Descriptions of SHA-256, SHA-384, and SHA-512."
                 http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf.


10. Authors' Addresses


        Sheila Frankel
        NIST
        820 West Diamond Ave.
        Room 680
        Gaithersburg, MD 20899
        Phone: +1 (301) 975-3297



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        Email: sheila.frankel@nist.gov

        Scott Kelly
        SonicWALL, Inc.
        1160 Bordeaux Dr.
        Sunnyvale, CA 94089
        Phone: +1 (408) 745-9600
        Email: skelly@sonicwall.com

        Rob Glenn
        NIST
        820 West Diamond Ave.
        Room 605
        Gaithersburg, MD 20899
        Phone: +1 (301) 975-3667
        Email: rob.glenn@nist.gov

   The IPsec working group can be contacted through the chairs:

        Barbara Fraser
        Cisco Systems Inc.
        Email: byfraser@cisco.com

        Theodore T'so
        Massachusetts Institute of Technology
        Email: tytso@mit.edu

11. Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this doc-
   ument itself may not be modified in any way, such as by removing the
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Frankel,Glenn,Kelly                                            [Page 12]


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