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Versions: 00                                                            
Internet Engineering Task Force                             R. Pereira
IP Security Working Group                         TimeStep Corporation
Internet Draft                                               G. Carter
Expires in six months                             Entrust Technologies
                                                          July 2, 1997



                     The ESP CAST128-CBC Algorithm
              <draft-ietf-ipsec-ciph-cast128-cbc-00.txt>



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 (ipsec@tis.com) 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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   at any time. It is inappropriate to use Internet-Drafts as
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   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.

Abstract

   This document describes the CAST-128 block cipher algorithm as to
   be used with the IPSec Encapsulating Security Payload (ESP).











R. Pereira, G. Carter                                         [Page 1]


Internet Draft      The ESP CAST128-CBC Algorithm        July 2, 1997


Table of Contents

   1. Introduction...................................................2
     1.1 Specification of Requirements...............................2
   2. Cipher Algorithm...............................................2
     2.1 Rounds......................................................2
     2.2 Background on CAST-128......................................3
     2.3 Performance.................................................3
   3. Key Sizes......................................................3
     3.1 Weak Keys...................................................4
   4. ESP Payload....................................................4
     4.1 Block Size and Padding......................................4
     4.2 Interaction with Authentication Algorithms..................4
   5. Keying Material................................................5
   6. Security Considerations........................................5
   7. References.....................................................5
   8. Acknowledgments................................................5
   9. Editors' Addresses.............................................6

1. Introduction

   This document describes how the CAST-128 cipher algorithm may be
   used with the IPSec ESP protocol.

   It is assumed that the reader is familiar with the terms and
   concepts described in the "Security Architecture for the Internet
   Protocol" [Atkinson95] and "IP Encapsulating Security Payload
   (ESP)" [Kent97] documents.

   Furthermore, this document is a companion to [Kent97] 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].

2. Cipher Algorithm

   The symmetric block cipher algorithm used to secure ESP is CAST-128
   in CBC mode with a block size of 64 bits as described in [Adams97].

2.1 Rounds

   For key sizes up to and including 80 bits (i.e., 40, 48, 56, 64,
   72, and 80 bits), the algorithm is exactly as specified but MUST
   use 12 rounds.


R. Pereira, G. Carter                                         [Page 2]


Internet Draft      The ESP CAST128-CBC Algorithm        July 2, 1997



   For key sizes greater than 80 bits, the algorithm MUST use the full
   16 rounds.

2.2 Background on CAST-128

   The CAST design procedure was originally developed by Carlisle
   Adams and Stafford Travares 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].

2.3 Performance

   CAST-128 runs approximately 3 times faster than a highly optimized
   DES implementation and runs 5-6 times faster than the DES
   implementations found in typical applications.  This is based on a
   non optimized C++ implementation of CAST-128.  It can therefore be
   tuned to give even higher performance, if this is required.

   The following performance tests were run on a Pentium 90 MHz
   running the Windows NT operating system using 20 Kbyte buffers and
   do not include file I/O.  The DES-CBC implementation was not
   optimized for a 32 bit environment.

   CAST-128 64 bit key CBC encryption ........... 2,640,000 bytes/sec
   DES CBC encryption ............................. 504,000 bytes/sec

3. Key Sizes

   The CAST-128 encryption algorithm [Adams97] has been designed to
   allow a key size which can vary from 40 bits to 128 bits, in 8-bit
   increments (that is, the allowable key sizes are 40, 48, 56, 64,
   ..., 112, 120, and 128 bits. To facilitate interoperability, it is
   recommended that key sizes SHOULD be chosen from the set of 40, 64,
   80 and 128.

   For key sizes less than 128 bits, the key is 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
   `3B5D831CFE000000'.

   In order to avoid confusion, when variable key size operation is
   used, the name CAST-128 is to be considered synonymous with the
   name CAST5; this allows a keysize to be appended without ambiguity.


R. Pereira, G. Carter                                         [Page 3]


Internet Draft      The ESP CAST128-CBC Algorithm        July 2, 1997


   Thus, for example, CAST-128 with a 40 bit key is referred to as
   CAST5-40; where a 128 bit key is explicitly intended, the name
   CAST5-128 should be used.

3.1 Weak Keys

   CAST-128 no known weak keys.

4. ESP Payload

   CAST128-CBC requires an explicit Initialization Vector (IV) of 8
   octets (64 bits).  Thus the payload is made up of the 8 octet IV
   followed by raw cipher-text.  The IV SHOULD be chosen at random.
   Common practice is to use random data for the first IV and the last
   8 octets of encrypted data from an encryption process as the IV for
   the next encryption process.

   The payload field, as defined in [Kent97], is broken down according
   to the following diagram:

   +---------------+---------------+---------------+---------------+
   |                                                               |
   +                   Initialization Vector (IV)                  +
   |                                                               |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   ~              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


4.1 Block Size and Padding

   The ESP CAST-128 algorithm described in this document MUST use a
   block size of 8 octets (64 bits).

   When padding is required, it MUST be done according to the
   conventions specified in [Kent97].

4.2 Interaction with Authentication Algorithms

   This CAST-128 ESP document has no limitations on what
   authentication algorithm is used in ESP.






R. Pereira, G. Carter                                         [Page 4]


Internet Draft      The ESP CAST128-CBC Algorithm        July 2, 1997


5. 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 CAST-128 key is taken from the first <x> bits of the keying
   material, where <x> represents the required key size.

6. Security Considerations

   The ESP CAST-128 algorithm described in this document has the same
   security considerations as in [Adams97].

   Care should be taken when using small key sizes.  Smaller key sizes
   of 56 bits and below make brute force type attacks practical
   regardless of the cipher algorithm used.  It is therefore
   recommended that the ESP CAST-128 key size be at least 80 bits.
   Use of key sizes less than 80 bits is permitted, but careful
   considerations should be taken before its use.

7. References

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

   [Atkinson95] Atkinson, R., "Security Architecture for the Internet
   Protocol", draft-ietf-ipsec-arch-sec-01

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

   [Kent97] Kent, S., Atkinson, R., "IP Encapsulating Security Payload
   (ESP)", draft-ietf-ipsec-new-esp-01

8. Acknowledgments

   This document is based on suggestions from Stephen Kent and
   discussions from the IPSec mailing list as well as other IPSec
   drafts.

   Special thanks for Carlisle Adams and Paul Van Oorschot both of
   Entrust Technologies who provided input and review with respect to
   CAST-128.







R. Pereira, G. Carter                                         [Page 5]


Internet Draft      The ESP CAST128-CBC Algorithm        July 2, 1997


9. Editors' Addresses

     Roy Pereira
     <rpereira@timestep.com>
     TimeStep Corporation
     (613) 599-3610 x 4808

     Greg Carter
     <carterg@entrust.com>
     Entrust Technologies
     (613) 763-1358

   The IPSec working group can be contacted via the IPSec working
   group's mailing list (ipsec@tis.com) or through its chairs:

     Robert Moskowitz
     rgm@chrysler.com
     Chrysler Corporation

     Theodore Y. Ts'o
     tytso@MIT.EDU
     Massachusetts Institute of Technology




























R. Pereira, G. Carter                                         [Page 6]