Paper 2006/281 Original: eprint.iacr.org/2006/281 ↗

Formalizing Human Ignorance: Collision-Resistant Hashing without the Keys

Abstract

There is a rarely mentioned foundational problem involving collision-resistant hash-functions: common constructions are keyless, but formal definitions are keyed. The discrepancy stems from the fact that a function H:{0,1}^* -> {0,1}^n always admits an efficient collision-finding algorithm, it's just that us human beings might be unable to write the program down. We explain a simple way to sidestep this difficulty that avoids having to key our hash functions. The idea is to state theorems in a way that prescribes an explicitly-given reduction, normally a black-box one. We illustrate this approach using well-known examples involving digital signatures, pseudorandom functions, and the Merkle-Damgard construction.