Adapting Helios for Provable Ballot Privacy

Abstract

Abstract. Recent results show that the current implementation of He-lios, a practical e-voting protocol, does not ensure independence of the cast votes, and demonstrate the impact of this lack of independence on vote privacy. Some simple fixes seem to be available and security of the revised scheme has been studied with respect to symbolic models. In this paper we study the security of Helios using computational models. Our first contribution is a model for the property known as ballot privacy that generalizes and extends several existing ones. Using this model, we investigate an abstract voting scheme (of which the revised Helios is an instantiation) built from an arbitrary encryp-tion scheme with certain functional properties. We prove, generically, that whenever this encryption scheme falls in the class of voting-friendly schemes that we define, the resulting voting scheme provably satisfies ballot privacy. We explain how our general result yields cryptographic security guaran-tees for the revised version of Helios (albeit from non-standard assump-tions). Furthermore, we show (by giving two distinct constructions) that it is possible to construct voting-friendly encryption, and therefore voting schemes, using only standard cryptographic tools. We detail an instan-tiation based on ElGamal encryption and Fiat-Shamir non-interactive zero-knowledge proofs that closely resembles Helios and which provably satisfies ballot privacy. 1