||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (September 2013)|
|Designers||Carlisle Adams, Stafford Tavares, Howard Heys, Michael Wiener|
|Key sizes||128, 160, 192, 224, or 256 bits|
|Block sizes||128 bits|
|Structure||generalised Feistel network|
In cryptography, CAST-256 (or CAST6) is a symmetric-key block cipher published in June 1998. It was submitted as a candidate for the Advanced Encryption Standard (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-boxes, 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 sizes are 128, 160, 192, 224 or 256 bits. CAST-256 is composed of 48 rounds, sometimes described as 12 "quad-rounds", arranged in a generalised Feistel network.
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 of CAST-256 in the standard single secret key setting that works for all keys is the zero-correlation cryptanalysis breaking 28 rounds with 2246.9 time and 298.8 data. 
- Bogdanov, Andrey; Leander, Gregor; Nyberg, Kaisa; Wang, Meiqin (2012). "Integral and multidimensional linear distinguishers with correlation zero.". Lecture Notes in Computer Science (Asiacrypt 2012) 7658: 244–261. doi:10.1007/978-3-642-34961-4.
- CAST-256 by John J. G. Savard
- 256bit Ciphers - CAST256 Reference implementation and derived code
- Standard Cryptographic Algorithm Naming: CAST-256
- RFC 2612
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