Election Verifiability in Receipt-Free Voting Protocols

Electronic voting is a prominent example of conflicting requirements in security protocols, as the triad of privacy, verifiability and usability is essential for their deployment in practice. Receipt-freeness is a particularly strong notion of privacy, stating that it should be preserved even if voters cooperate with the adversary. While there are impossibility results showing we cannot have receipt-freeness and verifiability at the same time, there are several protocols that aim to achieve both, based on carefully devised trust assumptions. To evaluate their security, we propose a general symbolic definition of election verifiability, extending the state of the art to capture the more complex structure of receipt-free protocols. We apply this definition to analyse, using ProVerif, recent protocols with promising practical features: BeleniosRF and several variants of Selene. Against BeleniosRF, we find several attacks showing that verifiability in Belenios does indeed suffer from the attempt to introduce receipt-freeness. On the other hand, Selene satisfies a weaker notion of receipt-freeness, but we show that it satisfies verifiability in stronger corruption scenarios. We introduce a general frame-work to compare the verifiability of these protocols in various corruption scenarios and conclude with an analysis of SeleneRF, an attempt to get the best of both that we formalise in this paper. In addition to extending the symbolic model, our results point to foundational gaps in current cryptographic models for election verifiability, as they fail to uncover attacks that we do.