Twofish is a symmetric key block cipher. It operates on 128-bits block size and uses key sizes up to 256 bits. It uses 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 the key is used to modify the encryption algorithm. It borrows some elements from the pseudo-Hadamard transform (PHT) from the SAFER family of ciphers.
