Rebound attacks on the reduced grøstl hash function

Grøstl is one of 14 second round candidates of the NIST SHA-3 competition. Cryptanalytic results on the wide-pipe compression function of Grøstl-256 have already been published. However, little is known about the hash function, arguably a much more interesting cryptanalytic setting. Also, Grøstl-512 has not been analyzed yet. In this paper, we show the first cryptanalytic attacks on reduced-round versions of the Grøstl hash functions. These results are obtained by several extensions of the rebound attack. We present a collision attack on 4/10 rounds of the Grøstl-256 hash function and 5/14 rounds of the Grøstl-512 hash functions. Additionally, we give the best collision attack for reduced-round (7/10 and 7/14) versions of the compression function of Grøstl-256 and Grøstl-512.