Review Your Choices: When Confirmation Pages Break Ballot Secrecy in Online Elections

Abstract Online voting systems typically display a confirmation screen allowing voters to confirm their selections before casting. This paper considers whether a network-based observer can extract information about voter selections from the length of the exchanged network data. We conducted a detailed analysis of the Simply Voting implementation, which had randomly varying lengths of exchanged data due to dynamic page content and gzip compression.

We demonstrated that we could correctly guess a voter’s selection with accuracy values ranging up to 100% in some instances. Even on more complex ballots, we generally could still rule out some combinations of candidates. We conducted a coordinated disclosure with the vendor and worked with them to roll out a mitigation.

To their credit, this discovery (and therefore its fix) was made possible by their willingness to provide a publicly accessible demo, which, as we will show, remains a rarity in the industry.