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The Twofish algorithm


General  

Designers  Bruce Schneier 
First published  1998 
Derived from  Blowfish, SAFER, Square 
Related to  Threefish 
Certification  AES finalist 
Cipher detail  
Key sizes  128, 192 or 256 bits 
Block sizes  128 bits 
Structure  Feistel network 
Rounds  16 
Best public cryptanalysis  
Truncated differential cryptanalysis requiring roughly 2^{51} chosen plaintexts.^{[1]} Impossible differential attack that breaks 6 rounds out of 16 of the 256bit key version using 2^{256} steps.^{[2]} 
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 it was not selected for standardization. Twofish is related to the earlier block cipher Blowfish.
Twofish's distinctive features are the use of precomputed keydependent Sboxes, and a relatively complex key schedule. One half of an nbit key is used as the actual encryption key and the other half of the nbit key is used to modify the encryption algorithm (keydependent Sboxes). Twofish borrows some elements from other designs; for example, the pseudoHadamard transform^{[3]} (PHT) from the SAFER family of ciphers. Twofish has a Feistel structure like DES. Twofish also employs a Maximum Distance Separable matrix.
Back in 2000, on most software platforms Twofish was slightly slower than Rijndael (the chosen algorithm for Advanced Encryption Standard) for 128bit keys, but somewhat faster for 256bit keys. But after Rijndael was chosen as the Advanced Encryption Standard, Twofish has become much slower than Rijndael on the CPUs that support the AES instruction set.^{[4]}
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.
The Twofish cipher has not been patented and the reference implementation has been placed in the public domain. As a result, the Twofish algorithm is free for anyone to use without any restrictions whatsoever. It is one of a few ciphers included in the OpenPGP standard (RFC 4880). However, Twofish has seen less widespread usage than Blowfish, which has been available longer.
In 1999, Niels Ferguson published an impossible differential attack that breaks six rounds out of 16 of the 256bit key version using 2^{256} steps.^{[2]}
As of 2000^{[update]}, the best published cryptanalysis on the Twofish block cipher is a truncated differential cryptanalysis of the full 16round version. The paper claims that the probability of truncated differentials is 2^{−57.3} per block and that it will take roughly 2^{51} chosen plaintexts (32 petabytes 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: "But even from a theoretical perspective, Twofish isn't even remotely broken. There have been no extensions to these results since they were published in 2000."^{[5]}