Branching Heuristics in Differential Collision Search with Applications to SHA-512

Abstract

In this work, we present practical semi-free-start collisions for SHA-512 on up to 38 (out of 80) steps with complexity $2^{40.5}$. The best previously published result was on 24 steps. The attack is based on extending local collisions as proposed by Mendel et al. in their Eurocrypt 2013 attack on SHA-256. However, for SHA-512, the search space is too large for direct application of these techniques. We achieve our result by improving the branching heuristic of the guess-and-determine approach to find differential characteristics and conforming message pairs. Experiments show that for smaller problems like 27 steps of SHA-512, the heuristic can also speed up the collision search by a factor of $2^{20}$