In
cryptography
Cryptography, or cryptology (from "hidden, secret"; and ''graphein'', "to write", or ''-logy, -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of Adversary (cryptography), ...
, CAST-256 (or CAST6) is a
symmetric-key block cipher
In cryptography, a block cipher is a deterministic algorithm that operates on fixed-length groups of bits, called ''blocks''. Block ciphers are the elementary building blocks of many cryptographic protocols. They are ubiquitous in the storage a ...
published in June 1998. It was submitted as a candidate for the
Advanced Encryption Standard
The Advanced Encryption Standard (AES), also known by its original name Rijndael (), is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001.
AES is a variant ...
(AES); however, it was not among the five
AES finalists. It is an extension of an earlier cipher,
CAST-128; both were designed according to the "CAST" design methodology invented by
Carlisle Adams and
Stafford Tavares.
Howard Heys and Michael Wiener also contributed to the design.
CAST-256 uses the same elements as CAST-128, including
S-box
In cryptography, an S-box (substitution-box) is a basic component of symmetric key algorithms which performs substitution. In block ciphers, they are typically used to obscure the relationship between the key and the ciphertext, thus ensuring Clau ...
es, but is adapted for a
block size of 128 bits – twice the size of its 64-bit predecessor. (A similar construction occurred in the evolution of
RC5 into
RC6). Acceptable
key size
In cryptography, key size or key length refers to the number of bits in a key used by a cryptographic algorithm (such as a cipher).
Key length defines the upper-bound on an algorithm's security (i.e. a logarithmic measure of the fastest known a ...
s are 128, 160, 192, 224 or 256 bits. CAST-256 is composed of 48 rounds, sometimes described as 12 "quad-rounds", arranged in a generalized
Feistel network
In cryptography, a Feistel cipher (also known as Luby–Rackoff block cipher) is a symmetric structure used in the construction of block ciphers, named after the German-born physicist and cryptographer Horst Feistel, who did pioneering researc ...
.
In
RFC 2612, the authors state that, "The CAST-256 cipher described in this document is available worldwide on a royalty-free and licence-free basis for commercial and non-commercial uses."
Currently, the best public
cryptanalysis
Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic se ...
of CAST-256 in the standard single secret key setting that works for all keys is the zero-correlation cryptanalysis breaking 28 rounds with 2
246.9 time and 2
98.8 data.
See also
*
AES process
References
External links
CAST-256by John J. G. Savard
*
ttp://www.users.zetnet.co.uk/hopwood/crypto/scan/cs.html Standard Cryptographic Algorithm Naming CAST-256
*
Block ciphers
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