Serpent (cipher)

Serpent is a symmetric key block cipher that was a finalist in the Advanced Encryption Standard contest, where it ranked second to Rijndael. It was designed by Ross Anderson, Eli Biham and Lars Knudsen. Like the other contest submissions it has a block size of 128 bits and supports key sizes of 128, 192 or 256 bits. Internally it is a 32-round substitution-permutation network working on four 32-bit words, applying eight different 4-bit S-boxes in parallel and built so that every operation can run in 32-bit slices. The designers took a conservative approach, judging 16 rounds enough against known attacks but specifying 32 as insurance. Serpent is in the public domain and unpatented, so anyone may use it without license fees.