Verifiability is a central property of modern e-voting systems. Intuitively, verifiability means that voters can check that their votes were actually counted and that the published result of the election is correct, even if the voting machine/authorities are (partially) untrusted. In this paper, we raise awareness of a simple attack, which we call a clash attack, on the verifiability of e-voting systems. The main idea behind this attack is that voting machines manage to provide different voters with the same receipt. As a result, the voting authorities can safely replace ballots by new ballots, and by this, manipulate the election without being detected. This attack does not seem to have attracted much attention in the literature. Even though the attack is quite simple, we show that, under reasonable trust assumptions, it applies to several e-voting systems that have been designed to provide verifiability. In particular, we show that it applies to the prominent ThreeBallot and VAV voting systems as well as to two e-voting systems that have been deployed in real elections: the Wombat Voting system and a variant of the Helios voting system. We discuss countermeasures for each of these systems and for (various variants of) Helios provide a formal analysis based on a rigorous definition of verifiability. More precisely, our analysis of Helios is with respect to the more general and in the area of e-voting often overlooked notion of accountability.

In this paper, we present new insights into central properties of voting systems, namely verifiability, privacy, and coercion-resistance. We demonstrate that the combination of the two forms of verifiability considered in the literature-individual and universal verifiability-are, unlike commonly believed, insufficient to guarantee overall verifiability. We also demonstrate that the relationship between coercion-resistance and privacy is more subtle than suggested in the literature.Our findings are partly based on a case study of prominent voting systems, ThreeBallot and VAV, for which, among others, we show that, unlike commonly believed, they do not provide any reasonable level of verifiability, even though they satisfy individual and universal verifiability. Also, we show that the original variants of ThreeBallot and VAV provide a better level of coercion-resistance than of privacy.

There have been intensive research efforts in the last two decades or so to design and deploy electronic voting (e-voting) protocols/systems which allow voters and/or external auditors to check that the votes were counted correctly. This security property, which not least was motivated by numerous problems in even national elections, is called verifiability. It is meant to defend against voting devices and servers that have programming errors or are outright malicious. In order to properly evaluate and analyze e-voting protocols w.r.t. verifiability, one fundamental challenge has been to formally capture the meaning of this security property. While the first formal definitions of verifiability were devised in the late 1980s already, new verifiability definitions are still being proposed. The definitions differ in various aspects, including the classes of protocols they capture and even their formulations of the very core of the meaning of verifiability. This is an unsatisfying state of affairs, leaving the research on the verifiability of e-voting protocols in a fuzzy state. In this paper, we review all formal definitions of verifiability proposed in the literature and cast them in a framework proposed by Küsters, Truderung, and Vogt (the KTV framework), yielding a uniform treatment of verifiability. This enables us to provide a detailed comparison of the various definitions of verifiability from the literature. We thoroughly discuss advantages and disadvantages, and point to limitations and problems. Finally, from these discussions and based on the KTV framework, we distill a general definition of verifiability, which can be instantiated in various ways, and provide precise guidelines for its instantiation. The concepts for verifiability we develop should be widely applicable also beyond the framework used here. Altogether, our work offers a well-founded reference point for future research on the verifiability of e-voting systems.

We consider the problem of establishing cryptographic guarantees-in particular, computational indistinguishability-for Java or Java-like programs that use cryptography. For this purpose, we propose a general framework that enables existing program analysis tools that can check (standard) non-interference properties of Java programs to establish cryptographic security guarantees, even if the tools a priori cannot deal with cryptography. The approach that we take is new and combines techniques from program analysis and simulation-based security. Our framework is stated and proved for a Java-like language that comprises a rich fragment of Java. The general idea of our approach should, however, be applicable also to other practical programming languages. As a proof of concept, we use an automatic program analysis tool for checking non-interference properties of Java programs, namely the tool Joana, in order to establish computational indistinguishability for a Java program that involves clients sending encrypted messages over a network, controlled by an active adversary, to a server.

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