Serpent is a block cipher designed by an international team of well-known researchers — Ross Anderson (UK), Eli Biham (Israel), and Lars Knudsen (Norway). It was designed for the AES competition and was one of the finalists in that competition, though not the winner. Like all AES candidates, it uses 128-bit blocks and supports key sizes of 128, 192 or 256 bits.
Serpent is an SP network with 32 rounds. It uses eight 4 by 4 S-boxes, but unlike other ciphers it does not use them all in each round. Instead each round uses eight copies of the same S-box, so that 32-bit computer instructions can do eight 4-bit operations in parallel. Each of the eight S-boxes is used in four different rounds.
The cipher is freely available for any use. It has a home page.