Secure Voting Protocols with Perfect Ballot Secrecy
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Securing voters' privacy and ensuring the integrity of the voting process are major design goals in voting systems. We propose secure voting protocols for two families of voting rules -- score-based rules and order-based rules. This is the first study that considers the question of secure multiparty computation of election results that such voting rules determine. The protocols output the winning candidate(s) while preserving the privacy of the voters and the secrecy of the ballots. They offer perfect secrecy in the sense that apart from their desired output, all other information is kept secret, including the ballots, intermediate values, the final score received by each candidate, and the final ranking of candidates. This, in turn, decreases the opportunities for voters to vote strategically. Our protocols are designed to deal with both semi-honest and rational voters. Voters of both types follow the protocol's specifications, but at the same time they try to infer information on the input of other voters from the messages which they receive during the protocol's run. While semi-honest voters submit legal votes, rational voters may submit illegal inputs in attempt to help their candidate of choice. Our protocols involve D ≥ 1 independent talliers who perform the tallying procedure on encrypted and secret-shared ballots in order to prevent them access to the actual ballots. Our protocols are collision-secure, in the sense that no party, either a voter or a tallier, can get any access to the ballots or any other intermediate results, unless all D talliers collude with at least one of the voters. We analyze the protocols' privacy-preservation and communication and computational costs, and show that they are compliant with the common desiderata of secure e-voting systems.
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