Improved cryptanalysis of skein

The hash function Skein is the submission of Ferguson et al. to the NIST Hash Competition, and is arguably a serious candidate for selection as SHA-3. This paper presents the rst third-party analysis of Skein, with an extensive study of its main component: the block cipher Three sh. We notably investigate near collisions, distinguishers, impossible di erentials, key recovery using related-key di erential and boomerang attacks. In particular, we present near collisions on up to 17 rounds, an impossible di erential on 21 rounds, a related-key boomerang distinguisher on 34 rounds, a known-related-key boomerang distinguisher on 35 rounds, and key recovery attacks on up to 32 rounds, out of 72 in total for Threefish-512. None of our attacks directly extends to the full Skein hash. However, the pseudorandomness of Threefish is required to validate the security proofs on Skein, and our results conclude that at least 36