Limits of the BRSIM/UC Soundness of Dolev-Yao Models with Hashes

Abstract

Automated tools such as model checkers and theorem provers for the analysis of security protocols typically abstract from cryptography by Dolev-Yao models, i.e., abstract term algebras replace the real cryptographic operations. Recently it was shown that in essence this approach is cryptographically sound for certain operations like signing and encryption. The strongest results show this in the sense of blackbox reactive simulatability (BRSIM)/UC with only small changes to both Dolev-Yao models and natural implementations. This notion essentially means the preservation of arbitrary security properties under active attacks in arbitrary protocol environments. We show that it is impossible to extend the strong BRSIM/UC results to usual Dolev-Yao models of hash functions in the general case. These models treat hash functions as free operators of the term algebra. This result does not depend on any restriction of the real hash function; even probabilistic hashing is covered. In contrast, we show that these models are sound in the same strict sense in the random oracle model of cryptography. For the standard model of cryptography, we also discuss several conceivable restrictions and extensions to the Dolev-Yao models and classify them into possible and impossible cases in the strong BRSIM/UC sense.