The cloudier side of cryptographic end-to-end verifiable voting: a security analysis of Helios

Helios is an open-audit internet voting system providing cryptographic protections to voter privacy, and election integrity. As part of these protections, Helios produces a cryptographic audit trail that can be used to verify ballots were correctly counted. Cryptographic end-to-end (E2E) election verification schemes of this kind are a promising step toward developing trustworthy electronic voting systems. In this paper we approach the discussion from the flip-side by exploring the practical potential for threats to be introduced by the presence of a cryptographic audit trail. We conducted a security analysis of the Helios implementation and discovered a range of vulnerabilities and implemented exploits that would: allow a malicious election official to produce arbitrary election results with accepting proofs of correctness; allow a malicious voter to cast a malformed ballot to prevent the tally from being computed; and, allow an attacker to surreptitiously cast a ballot on a voter's behalf. We also examine privacy issues including a random-number generation bias affecting the indistinguishably of encrypted ballots. We reported the issues and worked with the Helios designers to fix them.