Functional Graph Revisited: Updates on (Second) Preimage Attacks on Hash Combiners.

This paper studies functional-graph-based (second) preimage attacks against hash combiners. By exploiting more properties of functional graph, we find an improved preimage attack against the XOR combiner with a complexity of 2(5n/8), while the previous best-known complexity is 2(2n/3). Moreover, we find the first generic second-preimage attack on Zipper hash with an optimal complexity of 2(3n/5).