Multicollision Attacks on Some Generalized Sequential Hash Functions

A multicollision for a function is a set of inputs whose outputs are all identical. A. Joux showed multicollision attacks on the classical iterated hash function. He also showed how these multicollision attacks can be used to get a collision attack on a concatenated hash function. In this paper, we study multicollision attacks in a more general class of hash functions which we term "generalized sequential hash functions." We show that multicollision attacks exist for this class of hash functions provided that every message block is used at most twice in the computation of the message digest