Non-randomness in the Sarmal Compression Function

Sarmal is a hash function submitted to the NIST SHA-3 hash function competition. The design and structure of Sarmal is quite similar to that of ARIRANG, another SHA-3 candidate. We analyse the impact and applicability of recent attacks by Guo et al. on ARIRANG, with respect to Sarmal. Our results indicate that Sarmal is less vulnerable against this line of attack; in particular we were not able to obtain pseudo-collisions for Sarmal faster than using a generic attack. However, we have found that the compression function of Sarmal can be distinguished from a pseudorandom function with probability one, using only two compression function calls. This result is specific to the compression function, and does not seem extensible to the full hash function.