Unkeyed cryptographic

Skein

A SHA-3 competition finalist designed by Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas, and Jesse Walker (2008). Built on the Threefish tweakable block cipher in a Matyas-Meyer-Oseas-style mode (UBI: Unique Block Iteration). NIST picked Keccak over Skein in 2012, but Skein remains a well-studied conservative alternative.

At a glance

Threefish + UBI

Threefish is a tweakable block cipher with 64-bit-word ARX rounds (add, rotate, XOR). The UBI mode chains Threefish encryptions with the tweak encoding both the chaining info and a domain separator. UBI handles fixed-length hashing, MACs, KDFs, XOFs, and tree hashing all under one umbrella , one of the design’s nicer properties.

Why Skein lost to Keccak

Both were excellent SHA-3 candidates. NIST’s stated reasons for picking Keccak were a more conservative cryptographic margin and a construction (sponge) structurally distinct from SHA-2 , deliberate diversification in case SHA-2 was broken later. Skein was the software-performance favorite and had stronger 64-bit CPU numbers than Keccak.

Where it shows up

• OpenPGP , supported hash algorithm.
• Some research libraries , reference implementations exist in C, Rust, Go.
• SHAvite-3 / Skein hash analyses , academic comparisons of SHA-3 finalists.

References

• Schneier , Skein hash function home (with full specification)
• Skein v1.3 specification (NIST submission)
• SHA3-256 · Grøstl