Running Mixnet-Based Elections with Helios

The Helios voting system is an open-audit web-based voting system that has been used by various institutions in real-stake elections during the last few years. While targeting the simplicity of the election workflow, the homomorphic tallying process used in Helios limits its suitability for many elections (large number of candidates, specific ballot filling rules, . . . ). We present a variant of Helios that allows an efficient mixnet-based tallying procedure, and document the various choices we made in terms of election workflow and algorithm selection. In particular, we propose a modified version the TDH2 scheme of Shoup and Gennaro that we found particularly suitable for the encryption of the ballots. Our Helios variant has been tested in two multithousand voter elections. The lessons taken from the first of these elections motivated some changes into our procedure, which have been successfully experimented during the second election. Voter survey data are also presented.