Designed by Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, and Niels Ferguson; published in 1998. It uses a 256-bit key and 128-bit block and operates in XTS mode (see the section Modes of Operation). Twofish was one of the AES finalists. This cipher uses key- dependent S-boxes. Twofish may be viewed as a collection of 2128 different cryptosystems, where 128 bits derived from a 256-bit key control the selection of the cryptosystem . In , the Twofish team asserts that key-dependent S-boxes constitute a form of security margin against unknown attacks .