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 AES 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. 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 AES) for 128-bit keys, but somewhat faster for 256-bit keys.
As of 2004, there is no known attack on Twofish more efficient than brute force key search.
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