Message Recovery and Pseudo-preimage Attacks on the Compression Function of Hamsi-256

Hamsi is one of the second round candidates of the SHA-3 competition. In this study, we present non-random differential properties for the compression function of the hash function Hamsi-256. Based on these properties, we first demonstrate a distinguishing attack that requires a few evaluations of the compression function and extend the distinguisher to 5 rounds with complexity 2. Then, we present a message recovery attack with complexity of 2 compression function evaluations. Also, we present a pseudo-preimage attack for the compression function with complexity 2. The pseudo-preimage attack on the compression function is easily converted to a pseudo second preimage attack on Hamsi-256 hash function with the same complexity.