Attacking and fixing Helios: An analysis of ballot secrecy

Véronique Cortier 1 Ben Smyth 1
1 CASSIS - Combination of approaches to the security of infinite states systems
FEMTO-ST - Franche-Comté Électronique Mécanique, Thermique et Optique - Sciences et Technologies, Inria Nancy - Grand Est, LORIA - FM - Department of Formal Methods
Abstract : Helios 2.0 is an open-source web-based end-to-end verifiable electronic voting system, suitable for use in low-coercion environments. In this article, we analyse ballot secrecy in Helios and discover a vulnerability which allows an adversary to compromise the privacy of voters. The vulnerability exploits the absence of ballot independence in Helios and works by replaying a voter's ballot or a variant of it, the replayed ballot magnifies the voter's contribution to the election outcome and this magnification can be used to violated privacy. We demonstrate the practicality of the attack by violating a voter's privacy in a mock election using the software implementation of Helios. Moreover, the feasibility of an attack is considered in the context of French legislative elections and, based upon our findings, we believe it constitutes a real threat to ballot secrecy. We present a fix and show that our solution satisfies a formal definition of ballot secrecy using the applied pi calculus. Furthermore, we present similar vulnerabilities in other electronic voting protocols -- namely, the schemes by Lee \emphet al., Sako \& Kilian, and Schoenmakers -- which do not assure ballot independence. Finally, we argue that independence and privacy properties are unrelated, and non-malleability is stronger than independence.
Type de document :
Article dans une revue
Journal of Computer Security, IOS Press, 2013, 21 (1), pp.89-148. 〈10.3233/JCS-2012-0458〉
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Contributeur : Véronique Cortier <>
Soumis le : lundi 17 septembre 2012 - 13:17:03
Dernière modification le : jeudi 11 janvier 2018 - 06:24:26



Véronique Cortier, Ben Smyth. Attacking and fixing Helios: An analysis of ballot secrecy. Journal of Computer Security, IOS Press, 2013, 21 (1), pp.89-148. 〈10.3233/JCS-2012-0458〉. 〈hal-00732899〉



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