A New Distinguisher for CubeHash-8/b and CubeHash-15/b Compression Functions

CubeHash is one of the round 2 candidates of the public SHA-3 competition hosted by NIST. It was designed by Bernstein. In this paper we find a new distinguisher to distinguish CubeHash compression function from a random function. This distinguisher principle is based on rotational analysis that formally introduced by Khovratovich and Nikolic. In order to use this technique, we need to compute the probability that four swap functions in CubeHash round function preserve the rotational property for any input pair. We compute these probabilities and find a new distinguisher that distinguish CubeHash-8/b and CubeHash-15/b compression function from a random function with probability greater than 538.5 2 and 1009.7 2 , respectively. Until we know this is the first distinguisher for CubeHash compression function with more than 14 rounds.