The CAST-128 Encryption Algorithm
RFC 2144
| Document | Type |
RFC - Informational
(May 1997; No errata)
Was draft-rfced-info-adams (individual)
|
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|---|---|---|---|
| Last updated | 2013-03-02 | ||
| Stream | Legacy | ||
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Network Working Group C. Adams
Request for Comments: 2144 Entrust Technologies
Category: Informational May 1997
The CAST-128 Encryption Algorithm
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
There is a need in the Internet community for an unencumbered
encryption algorithm with a range of key sizes that can provide
security for a variety of cryptographic applications and protocols.
This document describes an existing algorithm that can be used to
satisfy this requirement. Included are a description of the cipher
and the key scheduling algorithm (Section 2), the s-boxes (Appendix
A), and a set of test vectors (Appendix B).
TABLE OF CONTENTS
STATUS OF THIS MEMO.............................................1
ABSTRACT........................................................1
1. INTRODUCTION.................................................1
2. DESCRIPTION OF ALGORITHM.....................................2
3. INTELLECTUAL PROPERTY CONSIDERATIONS.........................8
4. SECURITY CONSIDERATIONS......................................8
5. REFERENCES...................................................8
6. AUTHOR'S ADDRESS.............................................8
APPENDICES
A. S-BOXES......................................................9
B. TEST VECTORS................................................15
1. Introduction
This document describes the CAST-128 encryption algorithm, a DES-like
Substitution-Permutation Network (SPN) cryptosystem which appears to
have good resistance to differential cryptanalysis, linear
cryptanalysis, and related-key cryptanalysis. This cipher also
possesses a number of other desirable cryptographic properties,
including avalanche, Strict Avalanche Criterion (SAC), Bit
Independence Criterion (BIC), no complementation property, and an
absence of weak and semi-weak keys. It thus appears to be a good
Adams Informational [Page 1]
RFC 2144 CAST-128 Encryption Algorithms May 1997
candidate for general-purpose use throughout the Internet community
wherever a cryptographically-strong, freely-available encryption
algorithm is required.
Adams [Adams] discusses the CAST design procedure in some detail;
analyses can also be obtained on-line (see, for example, [Web1] or
[Web2]).
2. Description of Algorithm
CAST-128 belongs to the class of encryption algorithms known as
Feistel ciphers; overall operation is thus similar to the Data
Encryption Standard (DES). The full encryption algorithm is given in
the following four steps.
INPUT: plaintext m1...m64; key K = k1...k128.
OUTPUT: ciphertext c1...c64.
1. (key schedule) Compute 16 pairs of subkeys {Kmi, Kri} from K
(see Sections 2.1 and 2.4).
2. (L0,R0) <-- (m1...m64). (Split the plaintext into left and
right 32-bit halves L0 = m1...m32 and R0 = m33...m64.)
3. (16 rounds) for i from 1 to 16, compute Li and Ri as follows:
Li = Ri-1;
Ri = Li-1 ^ f(Ri-1,Kmi,Kri), where f is defined in Section 2.2
(f is of Type 1, Type 2, or Type 3, depending on i).
4. c1...c64 <-- (R16,L16). (Exchange final blocks L16, R16 and
concatenate to form the ciphertext.)
Decryption is identical to the encryption algorithm given above,
except that the rounds (and therefore the subkey pairs) are used in
reverse order to compute (L0,R0) from (R16,L16).
See Appendix B for test vectors which can be used to verify
correctness of an implementation of this algorithm.
2.1. Pairs of Round Keys
CAST-128 uses a pair of subkeys per round: a 32-bit quantity Km is
used as a "masking" key and a 5-bit quantity Kr is used as a
"rotation" key.
Adams Informational [Page 2]
RFC 2144 CAST-128 Encryption Algorithms May 1997
2.2. Non-Identical Rounds
Three different round functions are used in CAST-128. The rounds are
as follows (where "D" is the data input to the f function and "Ia" -
"Id" are the most significant byte through least significant byte of
I, respectively). Note that "+" and "-" are addition and subtraction
modulo 2**32, "^" is bitwise XOR, and "<<<" is the circular left-
shift operation.
Type 1: I = ((Kmi + D) <<< Kri)
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