A family of tweakable symmetric block-cipher algorithms with 256, 512 and 1024 bit block and key sizes.
Threefish is a family of tweakable symmetric block-cipher algorithms with 256, 512 and 1024 bit block and key sizes.
The Threefish block-cipher was designed as part of the skein hash function, which was a finalist in the NIST sha-3-competition.
See the wikipedia page for more information.