Twofish
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 The Twofish algorithm
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| Designer(s): | Bruce Schneier |
|---|---|
| First published: | 1998 |
| Derived from: | Blowfish, SAFER, Square |
| Certification: | AES finalist |
| Key size(s): | 128, 192 or 256 bits |
| Block size(s): | 128 bits |
| Structure: | Feistel network |
| Rounds: | 16 |
| Best public cryptanalysis: | |
| Truncated differential cryptanalysis requiring roughly 251 chosen plaintexts.[1] | |
In cryptography, Twofish is a symmetric key block cipher with a block size of 128 bits and key sizes up to 256 bits. It was one of the five finalists of the Advanced Encryption Standard contest, but was not selected for standardisation. Twofish is related to the earlier block cipher Blowfish.
Twofish's distinctive features are the use of pre-computed key-dependent S-boxes, and a relatively complex key schedule. One half of an n-bit key is used as the actual encryption key and the other half of the n-bit key is used to modify the encryption algorithm (key-dependent S-boxes). Twofish borrows some elements from other designs; for example, the pseudo-Hadamard transform (PHT) from the SAFER family of ciphers. Twofish uses the same Feistel structure as DES.
On most software platforms Twofish is slightly slower than Rijndael (the chosen algorithm for Advanced Encryption Standard) for 128-bit keys, but somewhat faster for 256-bit keys.[2]
Twofish was designed by Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, and Niels Ferguson; the "extended Twofish team" who met to perform further cryptanalysis of Twofish and other AES contest entrants included Stefan Lucks, Tadayoshi Kohno, and Mike Stay.
[edit] Cryptanalysis
As of 2007, the best published cryptanalysis on the Twofish block cipher is a truncated differential cryptanalysis of the full 16-round version. The paper claims that the probability of truncated differentials is 2-57.3 per block and that it will take roughly 251 chosen plaintexts (32 PiB worth of data) to find a good pair of truncated differentials.[1]
Bruce Schneier responds in a 2005 blog entry that this paper does not present a full cryptanalytic attack, but only some hypothesized differential characteristics. That is, from a practical perspective, Twofish isn't even remotely broken and there have been no extensions to these results since they were published in 2000.[3]
[edit] References
- ^ a b Shiho Moriai, Yiqun Lisa Yin (2000). "Cryptanalysis of Twofish (II)" (PDF). Retrieved on 2006-08-13.
- ^ Bruce Schneier, Doug Whiting (2000-04-07). "A Performance Comparison of the Five AES Finalists" (PDF/PostScript). Retrieved on 2006-08-13.
- ^ Schneier, Bruce (2005-11-23). Twofish Cryptanalysis Rumors. Schneier on Security blog. Retrieved on November 28, 2006.
- Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, Niels Ferguson (1998-06-15). "The Twofish Encryption Algorithm" (PDF/PostScript). Retrieved on 2007-03-04.
- Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, Niels Ferguson (1999-03-22). The Twofish Encryption Algorithm: A 128-Bit Block Cipher. New York City: John Wiley & Sons. ISBN 0-471-35381-7.
[edit] External links
- Twofish web page with full specifications, free source code, and other Twofish resources.
- David Wagner's sci.crypt post recommending AES over Twofish — Wagner was one of the designers of Twofish.
- SCAN's entry for Twofish
