Eylon Yogev (Weizmann Institute):On Distributional Collision Resistant Hashing

Collision resistant hashing is a fundamental concept that is the basis for many of the important cryptographic primitives and protocols. Collision resistant hashing is a family of compressing functions such that no efficient adversary can find {\em any} collision given a random function in the family.

  

In this work we study a relaxation of collision resistance called \emph{distributional} collision resistance, introduced by Dubrov and Ishai (STOC '06). This relaxation of collision resistance only guarantees that no efficient adversary, given a random function in the family, can \emph{sample} a pair $(x,y)$ where $x$ is uniformly random and $y$ is uniformly random conditioned on colliding with $x$.

  

Our first result shows that distributional collision resistance can be based on the existence of \emph{multi}-collision resistance hash (with no additional assumptions). Multi-collision resistance is another relaxation of collision resistance which guarantees that an efficient adversary cannot find any tuple of $k>2$ inputs that collide relative to a random function in the family.  The construction is non-explicit, non-black-box, and yields an infinitely-often secure family. This partially resolves a question of Berman et al.\ (EUROCRYPT '18). We further observe that in a black-box model such an implication (from multi-collision resistance to distributional collision resistance) does not exist.

 

Our second result is a construction of a distributional collision resistant hash from the average-case hardness of SZK. Previously, this assumption was not known to imply any form of collision resistance (other than the ones implied by one-way functions).