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
months and may be updated, replaced, or obsolete 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."
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
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]
