Paper 2025/1625

A Practical and Fully Distributed E-Voting Protocol for the Swiss Context

Abstract

Internet voting aims at guaranteeing both vote secrecy and verifiability. Advanced deployed systems, such as in Estonia or Switzerland, put a strong emphasis on cast-as-intended: even when a voter uses a corrupt device, they should be able to check that their intended vote is included in the tally. One approach relies on return codes, written on a voting card. This enables cast-as-intended even when the voter uses a single device that may be corrupted. While convenient for voters, this requires to generate the codes in a secure manner. This approach is deployed in Switzerland but the current system requires a single trusted setup authority: an offline component that is assumed to correctly generate the voting data. We propose a novel protocol that allows to distribute the trust during the setup. At the heart of our system lies an asymmetric protocol where several parties create the voting material, while allowing one of these parties to remain offline during the voting phase. Usability constraints are taken into account in our design, both in terms of computational complexity (linear setup and tally) and in terms of user experience (we ask the voter to type a high-entropy string only once). Our protocol is therefore a candidate for the next generation e-voting protocol in Switzerland. We provide an implementation of our protocol, demonstrating its practicability. Moreover, we formally prove vote privacy and verifiability. Our analysis takes into account the presence of short codes that an attacker can guess with non-negligible probability and reveals that making a significant change to the result remains exponentially hard.