Partial-Collision Attack on the Round-Reduced Compression Function of Skein-256

The hash function Skein is one of 5 finalists of the NIST SHA-3 competition. It is based on the block cipher Threefish which only uses three primitive operations: modular addition, rotation and bitwise XOR (ARX). This paper proposes a free-start partial-collision attack on round-reduced Skein-256 by combing the rebound attack with the modular differential techniques. The main idea of our attack is to connect two short differential paths into a long one with another differential characteristic that is complicated. Following our path, we give a free-start partial-collision attack on Skein-256 reduced to 32 rounds with Hamming distance 50 and complexity about 2 hash computations. In particular, we provide practical near-collision examples for Skein-256 reduced to 24 rounds and 28 rounds in the fixed tweaks and choosing tweaks setting separately. As far as we know, this is the first construction of a non-linear differential path for Skein which can lead to significantly improvement over previous analysis.