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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Author | Carlisle Adams | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 2144 (Informational) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
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) f = ((S1[Ia] ^ S2[Ib]) - S3[Ic]) + S4[Id] Type 2: I = ((Kmi ^ D) <<< Kri) f = ((S1[Ia] - S2[Ib]) + S3[Ic]) ^ S4[Id] Type 3: I = ((Kmi - D) <<< Kri) f = ((S1[Ia] + S2[Ib]) ^ S3[Ic]) - S4[Id] Rounds 1, 4, 7, 10, 13, and 16 use f function Type 1.Show full document text