Exploiting re-voting in the Helios election system

TL;DR

This paper reveals a vulnerability in the Helios election system where an adversary can manipulate which ballots are tallied, undermining voter influence and election integrity.

Contribution

The paper identifies a flaw in Helios allowing adversaries to exploit re-voting, and proposes insights into how this can be prevented or mitigated.

Findings

Adversaries can cause non-last ballots to be tallied in Helios.

Manipulation of ballot contents can unduly influence election outcomes.

Helios's current design does not guarantee last-ballot-tallying integrity.

Abstract

Election systems must ensure that representatives are chosen by voters. Moreover, each voter should have equal influence. Traditionally, this has been achieved by permitting voters to cast at most one ballot. More recently, this has been achieved by tallying the last ballot cast by each voter. We show this is not achieved by the Helios election system, because an adversary can cause a ballot other than a voter's last to be tallied. Moreover, we show how the adversary can choose the contents of such a ballot, thus the adversary can unduly influence the selection of representatives.