Reversing, Breaking, and Fixing the French Legislative Election E-Voting Protocol

We conduct a security analysis of the e-voting protocol used for the largest political election using e-voting in the world, the 2022 French legislative election for the citizens overseas. Due to a lack of system and threat model specifications, we built and contributed such specifications by studying the French legal framework and by reverse-engineering the code base accessible to the voters. Our analysis reveals that this protocol is affected by two design-level and implementation-level vulnerabilities. We show how those allow a standard voting server attacker and even more so a channel attacker to defeat the election integrity and ballot privacy due to 5 attack variants. We propose and discuss 5 fixes to prevent those attacks. Our specifications, the attacks, and the fixes were acknowledged by the relevant stakeholders during our responsible disclosure. They implemented our fixes to prevent our attacks for future elections. Beyond this protocol, we draw general lessons, recommendations, and open questions from this instructive experience where an e-voting protocol meets the real-world constraints of a large-scale, political election.