Network Working Group                                IPsec Working Group
INTERNET DRAFT                                          S. Frankel, NIST
March 2000                                                R. Glenn, NIST
Expiration Date: September 2000                       S. Kelly, RedCreek

      The Candidate AES Cipher Algorithms and Their Use With IPsec

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
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Drafts Shadow Directories can be accessed at

   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 ( or to the editors.

   Distribution of this memo is unlimited.


   This document describes the use of the AES Cipher Algorithms in Ci-
   pher Block Chaining Mode, with an explicit IV, as a confidentiality
   mechanism within the context of the IPsec Encapsulating Security Pay-
   load (ESP).

   This Internet Draft specifies the use of each of the 5 AES finalist
   candidates in the ESP Header. Once the AES cipher is chosen, this
   document will be changed to reflect that choice.

Frankel,Glenn,Kelly                                             [Page 1]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

                             Table of Contents

1.   Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .   3
1.1  Specification of Requirements . . . . . . . . . . . . . . . . .   4
2.   The Candidate AES Cipher Algorithms . . . . . . . . . . . . . .   4
2.1  Mode  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
2.2  Key Size  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
2.3  Weak Keys . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
2.4  Block Size and Padding  . . . . . . . . . . . . . . . . . . . .   6
2.5  Rounds  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
2.6  Cipher-specific Information . . . . . . . . . . . . . . . . . .   6
2.7  Performance . . . . . . . . . . . . . . . . . . . . . . . . . .   7
3.   ESP Payload . . . . . . . . . . . . . . . . . . . . . . . . . .   8
3.1  ESP Algorithmic Interactions  . . . . . . . . . . . . . . . . .   8
3.2  Keying Material . . . . . . . . . . . . . . . . . . . . . . . .   8
3.3  IKE Interactions  . . . . . . . . . . . . . . . . . . . . . . .   8
4.   Security Considerations . . . . . . . . . . . . . . . . . . . .  10
5.   Intellectual Property Rights Statement  . . . . . . . . . . . .  11
6.   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .  11
7.   References  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
8.   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . .  14
9.   Full Copyright Statement  . . . . . . . . . . . . . . . . . . .  14

Frankel,Glenn,Kelly                                             [Page 2]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

1.   Introduction

   Recognizing that the venerable DES cipher was reaching the end of its
   useful life, in January 1997 NIST (the National Institute of Stan-
   dards and Technology) announced a plan to select its successor, the
   AES (Advanced Encryption Standard).  The AES will be the government's
   designated encryption cipher, and will be definitively described in a
   FIPS (Federal Information Processing Standard).  The expectation is
   that the AES will suffice to protect sensitive government information
   at least until the next century.  It is also expected to be widely
   adopted by businesses and financial institutions.

   The initial call for AES candidates specified the following require-

     +    unclassified

     +    publicly disclosed

     +    available royalty-free worldwide

     +    capable of handling a block size of at least 128 bits

     +    at a minimum, capable of handling key sizes of 128, 192, and
          256 bits

   The distinguishing characteristics on which the final AES cipher will
   be selected are:

     +    security

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

     +    flexibility and simplicity

   Of the 15 ciphers that were submitted as AES candidates in August
   1998, 5 were designated as finalists. Analysis and discussion of the
   candidates continues.  Either 1 or 2 of the finalists will be
   selected as the AES cipher; the AES FIPS is expected to be completed
   by summer 2001.

   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.
   However, until 1 or 2 of the finalists are selected and until there
   is more experience with regard to the cryptographic strengths and
   weaknesses of the algorithms, this document should be used to experi-
   ment with the AES candidates and determine how they can best be used
   in IPsec implementations.  This document should be considered experi-

Frankel,Glenn,Kelly                                             [Page 3]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   The remainder of this document specifies the use of the five finalist
   AES candidate ciphers 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",
   appear in this document are to be interpreted as described in

2.   The Candidate AES Cipher Algorithms

   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 candidate AES ciphers.

   Some of the candidate AES ciphers are covered by copyrights, patents
   or patent applications.  Each submitter has sworn that, if selected
   as the AES cipher algorithm, the algorithm will be made available
   world-wide on a royalty-free basis.

   The AES homepage,, contains a wealth of in-
   formation about the 5 finalists, including definitive descriptions of
   each algorithm, comparative analyses, performance statistics, test
   vectors and intellectual property information.  This site also con-
   tains information on how to obtain reference implementations from
   NIST for each of the candidate algorithms.

2.1  Mode

   No operational modes are currently defined for the AES ciphers.  How-
   ever, 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 ci-
   phers in CBC mode within ESP.  This mode requires an Initialization
   Vector (IV) that is the same size as the block size.  Use of a ran-
   domly generated IV prevents generation of identical ciphertext 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 encrypt-
   ed.  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].

   [AUTHORS' NOTE: Should we require CBC mode using the ciphertext from
   the previously generated block? On the AES discussion list, it has
   been suggested that a Counter Feedback Mode be defined, which allows
   parallel encryption of blocks. Should we stick with CBC, use some

Frankel,Glenn,Kelly                                             [Page 4]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   variant of a Counter Feedback Mode, or wait for the AES FIPS to de-

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 stipulates that all key sizes MUST be a multiple of 8

   This document specifies the default (i.e. MUST be supported) key size
   for all of the AES cipher algorithms.  All of the candidate ciphers
   were required to accept key sizes of 128, 192 and 256 bits. The de-
   fault key size that implementations MUST support for IPsec is 128

   | Algorithm  |  Key Sizes (bits)       |  Default  |
   | MARS       |  128 - 448*             |  128      |
   | RC6        |  variable up to 2040    |  128      |
   | Rijndael   |  128, 192, 256          |  128      |
   | Serpent    |  variable up to 256**   |  128      |
   | Twofish    |  variable up to 256***  |  128      |

   *NOTE1: MARS key lengths must be multiples of 32 bits.
   **NOTE2: Serpent keys are always padded to 256 bits. The padding con-
   sists of a "1" bit followed by "0" bits.
   ***NOTE3: Twofish keys, other than the default sizes, are always
   padded with "0" bits up to the next default size.

2.3  Weak Keys

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

   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 any of the AES ciphers, then weak
   keys SHOULD be checked for and discarded when using manual key man-
   agement.  When using dynamic key management, such as [IKE], weak key
   checks SHOULD NOT be performed as they are seen as an unnecessary
   added code complexity that could weaken the intended security [EVALU-

Frankel,Glenn,Kelly                                             [Page 5]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

2.4  Block Size and Padding

   All of the algorithms described in this document use a block size of
   sixteen octets (128 bits), as required in the AES specifications.
   Some of the algorithms can handle larger block sizes as well.

   Padding is required by the candidate AES algorithms 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

   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
   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.

   | Algorithm  |  Negotiable?  |  Default # of Rounds  |
   | MARS       |  Yes          |  32                   |
   | RC6        |  Yes          |  20                   |
   | Rijndael   |  Yes          |  10, 12, 14*          |
   | Serpent    |  Yes          |  32                   |
   | Twofish    |  Yes          |  16                   |

   *NOTE1: Rijndael's Default # of Rounds is dependent on key size. De-
   fault # of Rounds = keylen/32 + 6.

2.6  Cipher-specific Information


   MARS is IBM's submission to the AES competition. The inventors, who
   are from the US and Switzerland, are: Carolynn Burwick, Don Copper-
   smith, Edward D'Avignon, Rosario Gennaro, Shai Halevi, Charanjit Jut-
   la, Sstephen Matyas Jr., Luke O'Connor, Mohammad Peyravian, David
   Safford, and Nevenko Zunic, A patent application, IBM application
   CR99802, is pending.  However, the MARS homepage contains the follow-

Frankel,Glenn,Kelly                                             [Page 6]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   ing statement: "MARS is now available world-wide under a royalty-free
   license from Tivoli."  MARS is defined in [MARS-1] and [MARS-2]. A
   change to the key generation technique is described in [MARS-3].  The
   MARS homepage is:


   RC6 was invented by Ronald Rivest of MIT, and by Matthew Robshaw, Ray
   Sidney, and Yiqun Lisa Yin, all from RSA Laboratories. The name RC6
   is protected by a copyright. The algorithm is covered by USA patent
   number 5,724,428 (granted March 3, 1998); two other US patents are
   pending: application serial numbers 08/854,210 (filed April 21, 1997)
   and 09/094,649 (filed June 15, 1998). The RC6 family of algorithms is
   defined in [RC6].  The RC6 homepage is:


   Rijndael was invented by Joan Daemen from Banksys/PWI and Vincent Ri-
   jmen from ESAT-COSIC, both in Belgium.  It is not covered by any
   patents, and the Rijndael homepage contains the following statement:
   "Rijndael is available for free. You can use it for whatever purposes
   you want, irrespective of whether it is accepted as AES or not."  Ri-
   jndael's description can be found in [RIJNDAEL].  The Rijndael home-
   page is:


   Serpent was invented by Ross Anderson of Cambridge University, Eli
   Biham of the Technion, Israel and Lars Knudsen of the University of
   Bergen, Norway. Two UK patent applications are pending: 9722789.7
   (filed October 29, 1997) and 9722798.9 (filed October 30, 1997).
   However, the Serpent homepage contains the following statement: "Ser-
   pent is now completely in the public domain, and we impose no re-
   strictions on its use."  Serpent is defined in [SERPENT-1] and [SER-
   PENT-2].  The Serpent homepage is:


   Twofish was invented by Bruce Schneier, John Kelsey, Chris Hall and
   Niels Ferguson, all from Counterpane Systems, Doug Whiting of Hi/fn,
   and David Wagner from the University of California Berkeley.  It is
   not covered by any patents, and the Twofish homepage contains the
   following statement: "Twofish is unpatented, and the source code is
   uncopyrighted and license-free; it is free for all uses."  Twofish is
   defined in [TWOFISH-1] and [TWOFISH-2].  The Twofish homepage is:

2.7  Performance

   For a comparison table of the estimated speeds of these and other ci-
   pher algorithms, please see [PERF-1], [PERF-2], [PERF-3], or
   [PERF-4]. The AES homepage,, has pointers to

Frankel,Glenn,Kelly                                             [Page 7]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   other analyses. The individual cypher documents, [MARS-1], [MARS-2],
   [RC6], [RIJNDAEL], [SERPENT-1], [SERPENT-2], [TWOFISH-1] and
   [TWOFISH-2] also contain 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
   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 ECB 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
   these algorithms and other aspects of ESP, such as use of certain au-
   thentication 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.

3.3  IKE Interactions

   To facilitate the experimental use of the AES candidate ciphers, it
   would be useful to temporarily define standard IPsec ESP Transform
   Identifiers for each of the AES algorithms.  [DOI] reserves the val-

Frankel,Glenn,Kelly                                             [Page 8]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   ues 249-255 for "private use amongst cooperating systems."  The fol-
   lowing IPsec ESP Transform Identifiers are suggested for IKE interop-
   erability using the AES candidate ciphers:

   | Transform ID      |  Value  |
   | ESP_AES_MARS      |  249    |
   | ESP_AES_RC6       |  250    |
   | ESP_AES_RIJNDAEL  |  251    |
   | ESP_AES_SERPENT   |  252    |
   | ESP_AES_TWOFISH   |  253    |

   Since the AES candidate ciphers allow variable key lengths, the Key
   Length attribute MUST be specified in a Phase 2 exchange [DOI].  The
   Key Length attribute MAY be specified in a Phase 1 exchange [IKE]; if
   it is not specified, the default key length is 128 bits.

   If IKE is used to negotiate keys for the AES candidate ciphers, the
   recommended characteristics of the groups governing the Diffie-Hell-
   man exchange are as follows:

   | 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].

   Additional information about the relationship between the group gov-
   erning a Diffie-Hellman exchange and the symmetric keys derived from
   the exchange can be found in [KEYLEN-1].

   For symmetric key lengths that exceed the output of the hash used to
   generate the key, the Diffie-Hellman shared secret MUST be hashed

Frankel,Glenn,Kelly                                             [Page 9]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   twice, and the resulting values combined to form the keying material
   [KEYLEN-2], as follows:

        P1 = Hash(0|shared_secret)
        P2 = Hash(1|shared_secret)

        keying_material = (P1 << shift_bits XOR P2)

   The first hash output, P1, is shifted left a variable number of bits,
   depending upon the hash and the key length, prior to XOR'ing it with
   the second hash output, P2.

   | Key Size  |  Hash   |  Dual DH?  |  # of Shift Bits  |
   | 128       |  MD5    |  N         |  -                |
   | 128       |  SHA-1  |  N         |  -                |
   | 192       |  MD5    |  Y         |  64               |
   | 192       |  SHA-1  |  Y         |  32               |
   | 256       |  MD5    |  Y         |  128              |
   | 256       |  SHA-1  |  Y         |  96               |

   If additional keying material is required for an authentication key,
   IKE's iterative key-boosting algorithm MUST be used [IKE, Section

4.   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.

   Because these candidate AES algorithms are relatively new and have
   only undergone limited cryptographic analysis, their use in IPsec im-
   plementations should be considered experimental.  Once NIST has pub-
   lished the AES FIPS, and at the recommendation of cryptographic ex-
   perts, 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
   the documents that describe the actual cipher algorithms.

Frankel,Glenn,Kelly                                            [Page 10]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

5.   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.

6.   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.

7.   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.{ps, pdf}).

   [CRYPTO-M]  A. Menezes, P. Van Oorschot, S. Vanstone, "Handbook of
               Applied Cryptography", CRC Press, 1997, ISBN

Frankel,Glenn,Kelly                                            [Page 11]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

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

   [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.

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

   [IKE]       Harkins, D. and D. Carrel, "The Internet Key Exchange
               (IKE)", draft-ietf-ipsec-ike-01.txt, May 1999.

   [IKE-ECC]   Panjwani, P. and Y. Poeluev, "Additional ECC Groups For
               IKE," draft-ietf-ipsec-ike-ecc-groups-01.txt,
               Septermber 1999.

   [ISAKMP]    Maughan, D., M. Schertler, M. Schneider, and J. Turner,
               "The Internet Security Association and Key Management
               Protocol (ISAKMP),"

   [KEYLEN-1]  Orman, H. and P. Hoffman, "Determining Strengths For
               Public Keys Used For Exchanging Symmetric Keys," draft-
               orman-public-key-lengths-00.txt, February 2000.

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

   [MARS-1]    Burwick, C., D. Coppersmith, E. D'Avignon, R. Gennaro,
               S. Halevi, C. Jutla, S. Matyas Jr., L. O'Connor, M.
               Peyravian, D. Safford, and N. Zunic, "MARS - a
               candidate cipher for AES," NIST AES Proposal, Jun 1998.

   [MARS-2]    Burwick, C., D. Coppersmith, E. D'Avignon, R. Gennaro,
               S. Halevi, C. Jutla, S. Matyas Jr., L. O'Connor, M.
               Peyravian, D. Safford, and N. Zunic, "The MARS
               Encryption Algorithm," NIST AES Proposal, Jun 1998.

   [MARS-3]    Zunic, N., "Suggested 'tweaks' for the MARS cipher,"
               NIST AES Proposal, May 1999.

   [PERF-1]    Bassham, L. III, "Efficiency Testing of ANSI C
               Implementations of Round1 Candidate Algorithms for the
               Advanced Encryption Standard".

Frankel,Glenn,Kelly                                            [Page 12]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

   [PERF-2]    Lipmaa, Helger, "Efficiency Testing Table."

   [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

   [PERF-4]    Schneier, B., J. Kelsey, D. Whiting, D. Wagner, C.
               Hall, and N. Ferguson, "Performance Comparison of the
               AES Submissions."

   [RC6]       Rivest, R., M. Robshaw, R. Sidney, and Y. Yin, "The
               RC6[TM] Block Cipher," NIST AES Proposal, Jun 1998.

   [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.

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

   [SERPENT-1] Anderson, R., E. Biham, and L. Knudsen, "Serpent: A
               Proposal for the Advanced Encryption Standard," NIST
               AES Proposal, Jun 1998.

   [SERPENT-2] Biham, E., R. Anderson, L. Knudsen, "Serpent: A New
               Block Cipher Proposal," Fast Software Encryption -
               FSE98, Springer LNCS, vol. 1372, pp. 222-238.

   [TWOFISH-1] Schneier, B., J. Kelsey, D. Whiting, D. Wagner, C.
               Hall, and N. Ferguson, "Twofish: A 128-Bit Block
               Cipher," NIST AES Proposal, Jun 1998.

   [TWOFISH-2] Schneier, B., J. Kelsey, D. Whiting, D. Wagner, C.
               Hall, and N. Ferguson, "The Twofish Encryption
               Algorithm: A 128-Bit Block Cipher," John Wiley & Sons,

Frankel,Glenn,Kelly                                            [Page 13]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000

8.   Authors' Addresses

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

        Rob Glenn
        820 West Diamond Ave.
        Room 455
        Gaithersburg, MD 20899
        Phone: +1 (301) 975-3667

        Scott Kelly
        RedCreek Communications
        3900 Newpark Mall Road
        Newark, CA 94560
        Phone: +1 (510) 745-3969

   The IPsec working group can be contacted through the chair:

        Ted T'so
        Massachusetts Institute of Technology

9.   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
   copyright notice or references to the Internet Society or other In-
   ternet organizations, except as needed for the purpose of developing
   Internet standards in which case the procedures for copyrights de-
   fined in the Internet Standards process must be followed, or as re-
   quired to translate it into languages other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

Frankel,Glenn,Kelly                                            [Page 14]

INTERNET DRAFT   <draft-ietf-ipsec-ciph-aes-cbc-00.txt>       March 2000


Frankel,Glenn,Kelly                                            [Page 15]