# Threefish

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Threefish
General
Designers Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas, Jesse Walker
First published 2008
Related to Blowfish, Twofish
Cipher detail
Key sizes 256, 512 or 1024 bits
(key size is equal to block size)
Block sizes 256, 512 or 1024 bits
Rounds 72 (80 for 1024-bit block size)
Speed 6.1 cpb on Core 2.[1]

Threefish is a symmetric-key tweakable block cipher designed as part of the Skein hash function, an entry in the NIST hash function competition. Threefish uses no S-boxes or other table lookups in order to avoid cache timing attacks;[1] its nonlinearity comes from alternating additions with exclusive ORs. In that respect, it is similar to Salsa20, TEA, and the SHA-3 candidates CubeHash and BLAKE.

Threefish and the Skein hash function were designed by Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas, and Jesse Walker.

## Security

In October 2010, an attack that combines rotational cryptanalysis with the rebound attack was published. The attack mounts a known-key distinguisher against 53 of 72 rounds in Threefish-256, and 57 of 72 rounds in Threefish-512. It also affects the Skein hash function.[2] This is a follow-up to the earlier attack published in February, which breaks 39 and 42 rounds respectively.[3] In response to this attack, the Skein team tweaked the rotation constants used in Threefish and thereby the key schedule constants for round 3 of the NIST hash function competition.[1]

In 2009, a related key boomerang attack against a reduced round Threefish version was published. For the 32-round version, the time complexity is $2^{226}$ and the memory complexity is $2^{12}$; for the 33-round version, the time complexity is $2^{352.17}$ with a negligible memory usage. The attacks also work against the tweaked version of Threefish: for the 32-round version, the time complexity is $2^{222}$ and the memory complexity is $2^{12}$; for the 33-round version, the time complexity is $2^{355.5}$ with a negligible memory usage.[4]

## References

1. ^ a b c Ferguson et al. (2010-10-01). The Skein Hash Function Family. The paper in which Threefish was introduced.
2. ^ Dmitry Khovratovich, Ivica Nikolic, Christian Rechberger (2010-10-20). Rotational Rebound Attacks on Reduced Skein.
3. ^ Dmitry Khovratovich and Ivica Nikolić (2010). Rotational Cryptanalysis of ARX. University of Luxembourg.
4. ^ Jiazhe Chen; Keting Jia (2009-11-01). "Improved Related-key Boomerang Attacks on Round-Reduced Threeﬁsh-512".