Collision Attack on 5 Rounds of Grøstl.

In this article, we describe a novel collision attack for up to 5 rounds of the Grostl hash function. This significantly improves upon the best previously published results on 3 rounds. By using a new type of differential trail spanning over more than one message block we are able to construct collisions for Grostl-256 on 4 and 5 rounds with complexity of 2(67) and 2(120), respectively. Both attacks need 2(64) memory. Due to the generic nature of our attack we can even construct meaningful collisions in the chosen-prefix setting with the same attack complexity.