Weaknesses of the FORK-256 compression function

This report presents analysis of the compression function of a recently proposed hash function, FORK-256. We exhibit some unexpected differentials existing for the step transformation and show their possible uses in collision-finding attacks on different variants of FORK256. As a simple application of those observations we present a method of finding chosen IV collisions for a variant of FORK-256 reduced to two branches : either 1 and 2 or 3 and 4. Moreover, we present how those differentials can be used in the full FORK-256 to easily find messages with hashes differing by only a relatively small number of bits. We argue that this method allows for finding collisions in the full function with complexity not exceeding 2 hash evaluations, better than birthday attack and additionally requiring only a small amount of memory. Version: 2.1, last modified: November 29, 2006