Electronic voting aims to guarantee the same security properties than traditional paper-based voting, namely vote privacy and verifiability. While the formal verification of security protocols is now a mature field with several powerful tools such as ProVerif or Tamarin, verifying voting protocols still pushes existing tools at their limits. We present in this work a proof framework for ProVerif that models an extensible election setting with an unbounded number of concurrent elections, voters and votes. We also develop a library of lemmas to aid the automatic verification. Crucially, the framework allows to reuse the same protocol model for both privacy and verifiability proofs. We apply the framework to several protocols of the literature and industry, showing the flexibility and applicability of our framework.

Ideal functionalities are used to study increasingly complex protocols within the Universal Composability framework. However, such functionalities are often complex themselves, making it difficult to assess whether they truly fulfill their promises. In this paper, we present four attacks on functionalities from various applications (e-voting, SMPC, anonymous lotteries, and smart metering), demonstrating that they do not capture the intuitively expected properties.We argue that ideal functionalities should not merely be justified secure at a high level but rigorously proven to be so. To this end, we propose a methodology that combines game-based proofs and computer-aided verification: ideal functionalities can in fact be treated as protocols, and one can use traditional game-based proofs to study them, where any game-based security property proven on the functionality does transfer to any protocol that realizes it. We also propose fixed versions of the ideal functionalities we studied, and formally define the security properties they should satisfy through a game. Finally, using Squirrel, a proof assistant for protocol security, we formally prove that the fixed functionalities verify the specified game-based security properties.

CHVote is one of the two main electronic voting systems developed in the context of political elections in Switzerland, where the regulation requires a specific setting and specific trust assumptions. We show that actually, CHVote fails to achieve vote secrecy and individual verifiability (here, recorded-as-intended), as soon as one of the online components is dishonest, contradicting the security claims of CHVote. In total, we found 9 attacks or variants against CHVote, 2 of them being based on a bug in the reference implementation. We confirmed our findings through a proof-of-concept implementation of our attacks.

Postal voting is a frequently used alternative to on-site voting.Traditionally, its security relies on organizational measures, andvoters have to trust many entities.In the recent years, several schemes have been proposed to addverifiability properties to postal voting, while preserving vote privacy. Postal voting comes with specific constraints. We conduct a systematic analysis of this setting and we identify a list of generic attacks, highlighting that some attacks seem unavoidable. This study is applied to existing systems of the literature. We then propose Vote&Check, a postal voting protocol which provides a high level of security, with a reduced number of authorities. Furthermore, it requires only basic cryptographic primitives, namely hash functions and signatures. The security properties are proven in a symbolic model, with the help of the ProVerif tool.

In this work we consider Belenios-CaI, a protocol offering a cast-as-intended mechanism and building upon Belenios, a voting system used in about 7000 elections to date. We modify the design of BeleniosCai from the user perspective without changing its core cryptographic mechanism. The goal is to increase its usability by letting the voter simply check whether two symbols are equal or different. We conducted a user-study among 165 participants in a research center to evaluate the usability of our implementation of Belenios-CaI. Since the cast-as-intended mechanism assumes that voters make some random choices, we also evaluate whether the choices made by voters are sufficiently "random" to provide verifiability and whether it could affect their privacy. The study shows that, for our population, Belenios-CaI is considered as usable with the random choices of the voters seeming sufficient for verifiability and privacy.

—Coercion-resistance is a security property of elec- tronic voting, often considered as a must-have for high-stake elections. The JCJ voting scheme, proposed in 2005 by Juels, Catalano and Jakobsson, is still the reference paradigm when designing a coercion-resistant protocol. We highlight a weakness in JCJ that is also present in all the systems following its general structure. This comes from the procedure that precedes the tally, where the trustees remove the ballots that should not be counted. This phase leaks more information than necessary, leading to potential threats for the coerced voters. Fixing this leads to the notion of cleansing-hiding , that we apply to form a variant of JCJ that we call CHide. One reason for the problem not being seen before is the fact that the associated formal definition of coercion-resistance was too weak. We therefore propose a definition that takes into account more behaviors such as revoting or the addition of fake ballots by authorities. We then prove that CHide is coercion-resistant for this definition.

The Swiss Chancellery has developed highly demanding requirements in terms of security for electronic voting. A few protocols have been proposed that meet the requirements. We propose a very simple protocol that satisfies the Swiss requirements while achieving an additional property: secrecy against a dishonest voting device, thanks to code voting. One key feature of our protocol is to use very short codes (typically one or two digits), so that it can be easily used by voters. Moreover, it only relies on standard primitives. Using the tool ProVerif, we formally prove vote privacy and end-to-end verifiability under the trust model of the Swiss Chancellery, assuming a dishonest voting device.

Eligibility checks are often abstracted away or omitted in voting protocols, leading to situations where the voting server can easily stuff the ballot box. One reason for this is the difficulty of bootstraping the authentication material for voters without relying on trusting the voting server. In this paper, we propose a new protocol that solves this problem by building on OpenID, a widely deployed authentication protocol. Instead of using it as a standard authentication means, we turn it into a mechanism that delivers transferable proofs of eligibility. Using zk-SNARK proofs, we show that this can be done without revealing any compromising information, in particular, protecting everlasting privacy. Our approach remains efficient and can easily be integrated into existing protocols, as we have done for the Belenios voting protocol. We provide a full-fledged proof of concept along with benchmarks showing our protocol could be realistically used in large-scale elections.

We propose the BeleniosCaI protocol, a variant of Belenios which brings the cast-as-inended property, in addition to other existing security properties. Our approach is based on a 2-part checksum that the voting device commits to, before being challenged to reveal one of them chosen at random by the voter. It requires only one device on the voter’s side and does not rely on previously sent data like with return codes. Compared to the classical Benaloh audit-or-cast approach, we still have cast-as-intended with only some probability, but the voter’s journey is more linear, and the audited ballot is really the one that is cast. We formally prove the security of BeleniosCaI w.r.t. end-to-end verifiability and privacy in a symbolic model, using the ProVerif tool.

Electronic voting systems should guarantee (at least) vote privacy and verifiability. Formally proving these two properties is challenging. Indeed, vote privacy is typically expressed as an equivalence property, hard to analyze for automatic tools, while verifiability requires to count the number of votes, to guarantee that all honest votes are properly tallied. We provide a full characterization of E2E-verifiability in terms of two simple properties, that are shown to be both sufficient and necessary. In contrast, previous approaches proposed sufficient conditions only. These two properties can easily be expressed in a formal tool like ProVerif but remain hard to prove automatically. Therefore, we provide a generic election framework, together with a library of lemmas, for the (automatic) proof of E2E-verifiability. We successfully apply our framework to several protocols of the literature that include two complex, industrial-scale voting protocols, namely Swiss Post and CHVote, designed for the Swiss context.

We propose an on-site voting system Themis, that aims at improving security when local authorities are not fully trusted. Voters vote thanks to voting sheets as well as smart cards that produce encrypted ballots. Electronic ballots are systematically audited, without compromising privacy. Moreover, the system includes a precise dispute resolution procedure identifying misbehaving parties in most cases.

We propose an on-site voting system Themis, that aims at improving security when local authorities are not fully trusted. Voters vote thanks to voting sheets as well as smart cards that produce encrypted ballots. Electronic ballots are systematically audited, without compromising privacy. Moreover, the system includes a precise dispute resolution procedure identifying misbehaving parties in most cases. We conduct a full formal analysis of Themis using ProVerif, with a novel approach in order to cover the modular arithmetic needed in our protocol. In order to evaluate the usability of our system, we organized a voting experiment on a (small) group of voters.

Bounding the number of sessions is a long-standing problem in the context of security protocols. It is well known that even simple properties like secrecy are undecidable when an unbounded number of sessions is considered. Yet, attacks on existing protocols only require a few sessions. In this paper, we propose a sound algorithm that computes a sufficient set of scenarios that need to be considered to detect an attack. Our approach can be applied for both reachability and equivalence properties, for protocols with standard primitives that are type-compliant (unifiable messages have the same type). Moreover, when equivalence properties are considered, else branches are disallowed, and protocols are supposed to be simple (an attacker knows from which role and session a message comes from). Since this class remains undecidable, our algorithm may return an infinite set. However, our experiments show that on most basic protocols of the literature, our algorithm computes a small number of sessions (a dozen). As a consequence, tools for a bounded number of sessions like DeepSec can then be used to conclude that a protocol is secure for an unbounded number of sessions.

In most verifiable electronic voting schemes, one key step is the tally phase, where the election result is computed from the encrypted ballots. A generic technique consists in first applying (verifiable) mixnets to the ballots and then revealing all the votes in the clear. This however discloses much more information than the result of the election itself (that is, the winners, plus possibly some information required by law) and may offer the possibility to coerce voters. In this paper, we present a collection of building blocks for designing tally-hiding schemes based on multi-party computations. From these building blocks, we design a fully tally-hiding scheme for Condorcet elections. Our implementation shows that the approach is practical, at least for medium-size elections. Similarly, we provide the first tally-hiding schemes with no leakage for three important counting functions: D’Hondt, STV, and Majority Judgment. We prove that they can be used to design a private and verifiable voting scheme. We also unveil unknown flaws or leakage in some previously proposed tally-hiding schemes.