Skein (hash function)

Skein is a cryptographic hash function that was one of five finalists in the NIST hash function competition. It was entered as a candidate for the SHA-3 standard and ultimately lost to Keccak. It was created by Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas and Jesse Walker. Skein is built on the Threefish tweakable block cipher, chained with a mode called Unique Block Iteration (UBI), a variant of the Matyas-Meyer-Oseas hash mode, and it natively supports arbitrary output lengths, so it can be treated as an extendable-output function. It supports internal state sizes of 256, 512 and 1024 bits, and its authors claim 6.1 cycles per byte on an Intel Core 2 Duo in 64-bit mode. Threefish gets its nonlinearity from additions and exclusive-ORs rather than from S-boxes. A 2010 attack combining rotational cryptanalysis with the rebound attack found rotational collisions in reduced-round Threefish, and the Skein team tweaked a key schedule constant for the third round of the competition in response. The algorithm and its reference implementation were given to the public domain.