The Complexity of Collision-Resistant Hashing

Item removed from eCommons on 2010-02-21 at the request of the author.A long-standing open problem on the intersection of Complexity Theory and Cryptography is whether the security of cryptographic primitives can be based on the worst-case hardness of NP. We show that, unless coNP $\subseteq$ AM, collision-resistant hash functions---one of the most central cryptographic primitives---cannot be based on the worst-case hardness of NP using any randomized Turing reduction; previously such separations were established only for restricted (e.g. non-adaptive) types of reductions. Under an average-case strengthening of the assumption that coNP $\not\subseteq$ AM, we furthermore rule out generic---but potentially non-black-box---constructions of collision-resistant hash functions from one-way functions (using Turing reductions); as far as we know, this yields the first non-black-box separation between cryptographic primitives