key: 2012-further-ballot-secrecy-attacks-in-Helios
type: techreport
title: Replay attacks that violate ballot secrecy in Helios
authors:
  - Ben Smyth
year: 2012
month: 1
number: 2012/185
institution: Cryptology ePrint Archive
description: >-
  This paper presents a replay attack against the Helios electronic voting protocol that violates
  ballot secrecy.
keywords: >-
  helios, electronic voting, privacy, secrecy, ballot secrecy, attack, vulnerability, exploit,
  ballot independence, homomorphic encryption, malleability, end-to-end verifiable
abstract: >-
  Helios 2.0 is a web-based end-to-end verifiable electronic voting system, suitable for use in
  low-coercion environments. In this paper we identify a vulnerability in Helios which allows an
  adversary to compromise the privacy of voters whom cast abstention votes. The vulnerability can be
  attributed to the absence of ballot independence and the use of homomorphic ElGamal encryption, in
  particular, these properties can be exploited by an adversary to construct a ballot related to an
  abstention vote cast by an honest voter and this ballot can be submitted by a corrupt voter to
  influence the election outcome, thereby introducing information that can be used to violate
  privacy. We demonstrate the attack by breaking privacy in a mock election using the current Helios
  implementation. It is unlikely that the vulnerability will be exploited in a real-world election
  and therefore our results are largely theoretical. Nonetheless, we cannot expect any computational
  proofs of ballot secrecy without fixing this vulnerability and, moreover, the attack methodology
  may be of interest – in particular, it could represent a viable threat to existing protocols in
  the literature – thus providing motivation to report these results.
links:
  - file: Smyth12a-attacking-Helios.pdf
    text: Technical Report
    alt:
      url: http://eprint.iacr.org/2012/185
      text: Cryptology ePrint Archive version