Paper 2023/1289

Fully Tally-Hiding Verifiable E-Voting for Real-World Elections with Seat-Allocations

Abstract

Modern e-voting systems provide what is called verifiability, i.e., voters are able to check that their votes have actually been counted despite potentially malicious servers and voting authorities. Some of these systems, called tally-hiding systems, provide increased privacy by revealing only the actual election result, e.g., the winner of the election, but no further information that is supposed to be kept secret. However, due to these very strong privacy guarantees, supporting complex voting methods at a real-world scale has proven to be very challenging for tally-hiding systems. A widespread class of elections, and at the same time, one of the most involved ones is parliamentary election with party-based seat-allocation. These elections are performed for millions of voters, dozens of parties, and hundreds of individual candidates competing for seats; they also use very sophisticated multi-step algorithms to compute the final assignment of seats to candidates based on, e.g., party lists, hundreds of electoral constituencies, possibly additional votes for individual candidates, overhang seats, and special exceptions for minorities. So far, it has not been investigated whether and in how far such elections can be performed in a verifiable tally-hiding manner. In this work, we design and implement the first verifiable (fully) tally-hiding e-voting system for an election from this class, namely, for the German parliament (Bundestag). As part of this effort, we propose several new tally-hiding building blocks that are of independent interest. We perform benchmarks based on actual election data, which show, perhaps surprisingly, that our proposed system is practical even at a real-world scale. Our work thus serves as a foundational feasibility study for this class of elections.