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Internet Engineering Task Force                             R. Pereira
IP Security Working Group                         TimeStep Corporation
Internet Draft                                                R. Adams
Expires in six months                               Cisco Systems Inc.
                                                     September 4, 1998

                  The ESP CBC-Mode Cipher Algorithms

Status of this Memo

   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 editor.

   This document is an Internet-Draft.  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 draft documents are valid for a maximum of six
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   Distribution of this memo is unlimited.


   This document describes how to use CBC-mode cipher algorithms with
   the IPSec ESP (Encapsulating Security Payload) Protocol.  It not
   only clearly states how to use certain cipher algorithms, but also
   how to use all CBC-mode cipher algorithms.

R. Pereira, R. Adams                                          [Page 1]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

Table of Contents

   1. Introduction...................................................2
     1.1 Specification of Requirements...............................2
   2. Cipher Algorithms..............................................3
     2.1 Mode........................................................3
     2.2 Key Size....................................................3
     2.3 Weak Keys...................................................4
     2.4 Block Size and Padding......................................6
     2.5 Rounds......................................................6
     2.6 Backgrounds.................................................6
     2.7 Performance.................................................9
   3. ESP Payload....................................................9
     3.1 ESP Environmental Considerations............................9
     3.2 Keying Material............................................10
   4. Security Considerations.......................................10
   5. References....................................................10
   6. Acknowledgments...............................................12
   7. Editors' Addresses............................................12

1. Introduction

   The Encapsulating Security Payload (ESP) [Kent98] provides
   confidentiality for IP datagrams by encrypting the payload data to
   be protected.  This specification describes the ESP use of CBC-mode
   cipher algorithms.

   While this document does not describe the use of the default cipher
   algorithm DES, the reader should be familiar with that document.

   It is assumed that the reader is familiar with the terms and
   concepts described in the "Security Architecture for the Internet
   Protocol" [Atkinson95], "IP Security Document Roadmap" [Thayer97],
   and "IP Encapsulating Security Payload (ESP)" [Kent98] documents.

   Furthermore, this document is a companion to [Kent98] and MUST be
   read in its context.

1.1 Specification of Requirements

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
   NOT", and "MAY" that appear in this document are to be interpreted
   as described in [Bradner97].

R. Pereira, R. Adams                                          [Page 2]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

2. Cipher Algorithms

   All symmetric block cipher algorithms share common characteristics
   and variables.  These include mode, key size, weak keys, block
   size, and rounds.  All of which will be explained below.

   While this document illustrates certain cipher algorithms such as
   Blowfish [Schneier93], CAST-128 [Adams97], 3DES, IDEA [Lai] [MOV],
   and RC5 [Baldwin96], any other block cipher algorithm may be used
   with ESP if all of the variables described within this document are
   clearly defined.

2.1 Mode

   All symmetric block cipher algorithms described or insinuated
   within this document use Cipher Block Chaining (CBC) mode.  This
   mode requires an Initialization Vector (IV) that is the same size
   as the block size.  Use of a randomly generated IV prevents
   generation of identical ciphertext from packets which have
   identical data that spans the first block of the cipher algorithm's

   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 [Schneier95].

2.2 Key Size

   Some cipher algorithms allow for variable sized keys, while others
   only allow a specific key size.  The length of the key correlates
   with the strength of that algorithm, thus larger keys are always
   harder to break than shorter ones.

   This document stipulates that all key sizes MUST be a multiple of 8

   This document does specify the default key size for each cipher
   algorithm.  This size was chosen by consulting experts on the
   algorithm and by balancing strength of the algorithm with

R. Pereira, R. Adams                                          [Page 3]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

   | Algorithm    | Key Sizes (bits) | Popular Sizes   | Default  |
   | CAST-128 [1] | 40 to 128        | 40, 64, 80, 128 | 128      |
   | RC5          | 40 to 2040       | 40, 128, 160    | 128      |
   | IDEA         | 128              | 128             | 128      |
   | Blowfish     | 40 to 448        | 128             | 128      |
   | 3DES [2]     | 192              | 192             | 192      |


   [1] With CAST-128, keys less than 128 bits MUST be padded with
   zeros in the rightmost, or least significant, positions out to 128
   bits since the CAST-128 key schedule assumes an input key of 128
   bits.  Thus if you had a key with a size of 80 bits '3B5D831CFE',
   it would be padded to produce a key with a size of 128 bits

   [2] The first 3DES key is taken from the first 64 bits, the second
   from the next 64 bits, and the third from the last 64 bits.
   Implementations MUST take into consideration the parity bits when
   initially accepting a new set of keys.  Each of the three keys is
   really 56 bits in length with the extra 8 bits used for parity.

   The reader should note that the minimum key size for all of the
   above cipher algorithms is 40 bits, and that the authors strongly
   advise that implementations do NOT use key sizes smaller than 40

2.3 Weak Keys

   Weak key checks SHOULD be performed.  If such a key is found, the
   key SHOULD be rejected and a new SA requested.  Some cipher
   algorithms have weak keys or keys that MUST not be used due to
   their weak nature.

   New weak keys might be discovered, so this document does not in any
   way contain all possible weak keys for these ciphers.  Please check
   with other sources of cryptography such as [MOV] and [Schneier] for
   further weak keys.

R. Pereira, R. Adams                                          [Page 4]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98


   No known weak keys.


   No known weak keys when used with 16 rounds.


   IDEA has been found to have weak keys.  Please check with [MOV] and
   [Schneier] for more information.


   Weak keys for Blowfish have been discovered.  Weak keys are keys
   that produce the identical entries in a given S-box.
   Unfortunately, there is no way to test for weak keys before the S-
   box values are generated.  However, the chances of randomly
   generating such a key are small.


   DES has 64 known weak keys, including so-called semi-weak keys and
   possibly-weak keys [Schneier95, pp 280-282].  The likelihood of
   picking one at random is negligible.

   For DES-EDE3, there is no known need to reject weak or
   complementation keys.  Any weakness is obviated by the use of
   multiple keys.

   However, if the first two or last two independent 64-bit keys are
   equal (k1 == k2 or k2 == k3), then the 3DES operation is simply the
   same as DES.  Implementers MUST reject keys that exhibit this

R. Pereira, R. Adams                                          [Page 5]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

2.4 Block Size and Padding

   All of the algorithms described in this document use a block size
   of eight octets (64 bits).

   Padding is used to align the payload type and pad length octets as
   specified in [Kent98].  Padding must be sufficient to align the
   data to be encrypted to an eight octet (64 bit) boundary.

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 Rounds       |
   | CAST-128           | No         | key<=80 bits, 12     |
   |                    |            | key>80 bits, 16      |
   | RC5                | No         | 16                   |
   | IDEA               | No         | 8                    |
   | Blowfish           | No         | 16                   |
   | 3DES               | No         | 48 (16x3)            |

2.6 Backgrounds


   The CAST design procedure was originally developed by Carlisle
   Adams and Stafford Tavares at Queen's University, Kingston,
   Ontario, Canada.  Subsequent enhancements have been made over the
   years by Carlisle Adams and Michael Wiener of Entrust Technologies.
   CAST-128 is the result of applying the CAST Design Procedure as
   outlined in [Adams97].

R. Pereira, R. Adams                                          [Page 6]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98


   The RC5 encryption algorithm was developed by Ron Rivest for RSA
   Data Security Inc. in order to address the need for a high-
   performance software and hardware ciphering alternative to DES. It
   is patented ( 5,724,428).  A description of RC5 may be found
   in [MOV] and [Schneier].


   Xuejia Lai and James Massey developed the IDEA (International Data
   Encryption Algorithm) algorithm.  The algorithm is described in
   detail in [Lai], [Schneier] and [MOV].

   The IDEA algorithm is patented in Europe and in the United States
   with patent application pending in Japan.  Licenses are required
   for commercial uses of IDEA.

   For patent and licensing information, contact:

         Ascom Systec AG, Dept. CMVV
         Gewerbepark, CH-5506
         Magenwil, Switzerland
         Phone: +41 64 56 59 83
         Fax: +41 64 56 59 90


   Bruce Schneier of Counterpane Systems developed the Blowfish block
   cipher algorithm.  The algorithm is described in detail in
   [Schneier93], [Schneier95] and [Schneier].


   This DES variant, colloquially known as "Triple DES" or as DES-
   EDE3, processes each block three times, each time with a different
   key.  This technique of using more than one DES operation was
   proposed in [Tuchman79].

R. Pereira, R. Adams                                          [Page 7]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

                        P1             P2             Pi
                         |              |              |
                  IV->->(X)    +>->->->(X)    +>->->->(X)
                         v     ^        v     ^        v
                      +-----+  ^     +-----+  ^     +-----+
                  k1->|  E  |  ^ k1->|  E  |  ^ k1->|  E  |
                      +-----+  ^     +-----+  ^     +-----+
                         |     ^        |     ^        |
                         v     ^        v     ^        v
                      +-----+  ^     +-----+  ^     +-----+
                  k2->|  D  |  ^ k2->|  D  |  ^ k2->|  D  |
                      +-----+  ^     +-----+  ^     +-----+
                         |     ^        |     ^        |
                         v     ^        v     ^        v
                      +-----+  ^     +-----+  ^     +-----+
                  k3->|  E  |  ^ k3->|  E  |  ^ k3->|  E  |
                      +-----+  ^     +-----+  ^     +-----+
                         |     ^        |     ^        |
                         +>->->+        +>->->+        +>->->
                         |              |              |
                         C1             C2             Ci

   The DES-EDE3-CBC algorithm is a simple variant of the DES-CBC
   algorithm [FIPS-46].  The "outer" chaining technique is used.

   In DES-EDE3-CBC, an Initialization Vector (IV) is XOR'd with the
   first 64-bit (8 byte) plaintext block (P1).  The keyed DES function
   is iterated three times, an encryption (Ek1) followed by a
   decryption (Dk2) followed by an encryption (Ek3), and generates the
   ciphertext (C1) for the block.  Each iteration uses an independent
   key: k1, k2 and k3.

   For successive blocks, the previous ciphertext block is XOR'd with
   the current plaintext (Pi).  The keyed DES-EDE3 encryption function
   generates the ciphertext (Ci) for that block.

   To decrypt, the order of the functions is reversed: decrypt with
   k3, encrypt with k2, decrypt with k1, and XOR the previous
   ciphertext block.

   Note that when all three keys (k1, k2 and k3) are the same, DES-
   EDE3-CBC is equivalent to DES-CBC.  This property allows the DES-
   EDE3 hardware implementations to operate in DES mode without

   For more explanation and implementation information for Triple DES,
   see [Schneier95].

R. Pereira, R. Adams                                          [Page 8]
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2.7 Performance

   For a comparison table of the estimated speed of any of these and
   other cipher algorithms, please see [Schneier97] or for an up-to-
   date performance comparison, please see [Bosseleaers].

3. ESP Payload

   The ESP payload is made up of the IV followed by raw cipher-text.
   Thus the payload field, as defined in [Kent98], is broken down
   according to the following diagram:

   |                                                               |
   +               Initialization Vector (8 octets)                +
   |                                                               |
   |                                                               |
   ~              Encrypted Payload (variable length)              ~
   |                                                               |
    1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

   The IV field MUST be same size as the block size of the cipher
   algorithm being used.  The IV MUST be chosen at random.  Common
   practice is to use random data for the first IV and the last block
   of encrypted data from an encryption process as the IV for the next
   encryption process.

   Including the IV in each datagram ensures that decryption of each
   received 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 Environmental Considerations

   Currently, there are no known issues regarding interactions between
   these algorithms and other aspects of ESP, such as use of certain
   authentication schemes.

R. Pereira, R. Adams                                          [Page 9]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

3.2 Keying Material

   The minimum number of bits sent from the key exchange protocol to
   this ESP algorithm must be greater 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. Security Considerations

   Implementations are encouraged to use the largest key sizes they
   can when taking into account performance considerations for their
   particular hardware and software configuration.  Note that
   encryption necessarily impacts both sides of a secure channel, so
   such consideration must take into account not only the client side,
   but the server as well.

   For information on the case for using random values please see

   For further security considerations, the reader is encouraged to
   read the documents that describe the actual cipher algorithms.

5. References

   [Adams97]   C. Adams, "The CAST-128 Encryption Algorithm",
               RFC2144, 1997.

   [Atkinson98]S. Kent, R. Atkinson, "Security Architecture for the
               Internet Protocol", draft-ietf-ipsec-arch-sec-04

   [Baldwin96] R.W. Baldwin, R. Rivest, "The RC5, RC5-CBC, RC5-CBC-
               Pad, and RC5-CTS Algorithms", RFC2040, October 1996

   [Bell97]    S. Bellovin, "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 (also
     {ps, pdf}).

   [Bosselaers]A. Bosselaers, "Performance of Pentium

   [Bradner97] S. Bradner, "Key words for use in RFCs to indicate
               Requirement Levels", RFC2119, March 1997

R. Pereira, R. Adams                                         [Page 10]
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   [Crypto93]  J. Daemen, R. Govaerts, J. Vandewalle, "Weak Keys for
               IDEA", Advances in Cryptology, CRYPTO 93 Proceedings,
               Springer-Verlag, pp. 224-230.

   [FIPS-46]   US National Bureau of Standards, "Data Encryption
               Standard", Federal Information Processing Standard
               (FIPS) Publication 46, January 1977.

   [Kent98]    S. Kent, Atkinson, R., "IP Encapsulating Security
               Payload (ESP)", draft-ietf-ipsec-esp-v2-04

   [Lai]       X. Lai, "On the Design and Security of Block Ciphers",
               ETH Series in Information Processing, v. 1, Konstanz:
               Hartung-Gorre Verlag, 1992.

   [Madson98]  C. Madson, N. Dorswamy, "The ESP DES-CBC Cipher
               Algorithm With Explicit IV", draft-ietf-ipsec-ciph-

   [MOV]       A. Menezes, P. Van Oorschot, S. Vanstone, "Handbook of
               Applied Cryptography", CRC Press, 1997. ISBN 0-8493-

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

   [Schneier93]B. Schneier, "Description of a New Variable-Length
               Key, 64-Bit Block Cipher", from "Fast Software
               Encryption, Cambridge Security Workshop Proceedings",
               Springer-Verlag, 1994, pp. 191-204.

   [Schneier95]B. Schneier, "The Blowfish Encryption Algorithm - One
               Year Later", Dr. Dobb's Journal, September 1995,

   [Schneier97]B. Scheier, "Speed Comparisons of Block Ciphers on a
               Pentium." February 1997,

   [Thayer97]  R. Thayer, N. Doraswamy, R. Glenn, "IP Security
               Document Roadmap", draft-ietf-ipsec-doc-roadmap-02

   [Tuchman79] Tuchman, W, "Hellman Presents No Shortcut Solutions to
               DES", IEEE Spectrum, v. 16 n. 7, July 1979, pp. 40-41.

R. Pereira, R. Adams                                         [Page 11]
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6. Acknowledgments

   This document is a merger of most of the ESP cipher algorithm
   documents.  This merger was done to facilitate greater
   understanding of the commonality of all of the ESP algorithms and
   to further the development of these algorithm within ESP.

   The content of this document is based on suggestions originally
   from Stephen Kent and subsequent discussions from the IPSec mailing
   list as well as other IPSec drafts.

   Special thanks to Carlisle Adams and Paul Van Oorschot both of
   Entrust Technologies who provided input and review of CAST.

   Thanks to all of the editors of the previous ESP 3DES documents; W.
   Simpson, N. Doraswamy, P. Metzger, and P. Karn.

   Thanks to Brett Howard from TimeStep for his original work of ESP-

   Thanks to Markku-Juhani Saarinen, Helger Lipmaa and Bart Preneel
   for their input on IDEA and other ciphers.

7. Editors' Addresses

     Roy Pereira
     TimeStep Corporation
     +1 (613) 599-3610 x 4808

     Rob Adams
     Cisco Systems Inc.
     +1 (408) 457-5397


     Robert W. Baldwin
     <> or <>
     RSA Data Security, Inc.
     +1 (415) 595-8782

     Greg Carter
     Entrust Technologies
     +1 (613) 763-1358

R. Pereira, R. Adams                                         [Page 12]
Internet Draft    The ESP CBC-Mode Cipher Algorithms            Sep-98

     Rodney Thayer
     Sable Technology Corporation
     +1 (617) 332-7292

   The IPSec working group can be contacted via the IPSec working
   group's mailing list ( or through its chairs:

     Robert Moskowitz
     International Computer Security Association

     Theodore Y. Ts'o
     Massachusetts Institute of Technology

R. Pereira, R. Adams                                         [Page 13]