We present a symbolic definition of election verifiability for electronic voting protocols in the context of the applied pi calculus. Our definition is given in terms of boolean tests which can be performed on the data produced by an election. The definition distinguishes three aspects of verifiability, which we call individual verifiability, universal verifiability, and eligibility verifiability. It also allows us to determine precisely which aspects of the system's hardware and software must be trusted for the purpose of election verifiability. In contrast with earlier work our definition is compatible with a large class of electronic voting schemes, including those based on blind signatures, homomorphic encryption and mixnets. We demonstrate the applicability of our formalism by analysing two protocols which have been deployed; namely Helios 2.0, which is based on homomorphic encryption, and Civitas, which uses mixnets. In addition we consider the FOO protocol which is based on blind signatures.

Abstract-Helios 2.0 is an open-source web-based end-toend verifiable electronic voting system, suitable for use in lowcoercion environments. In this paper, we analyse ballot secrecy and discover a vulnerability which allows an adversary to compromise the privacy of voters. This vulnerability has been successfully exploited to break privacy in a mock election using the current Helios implementation. 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 in such settings. Finally, we present a fix and show that our solution satisfies a formal definition of ballot secrecy using the applied pi calculus.