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.

ProVerif is a popular tool for the fully automatic analysis of security protocols, offering very good support to detect flaws or prove security. One exception is the case of protocols with global states such as counters, tables, or more generally, memory cells. ProVerif fails to analyse such protocols, due to its internal abstraction. Our key idea is to devise a generic transformation of the security properties queried to ProVerif. We prove the soundness of our transformation and implement it into a front-end GSVerif. Our experiments show that our front-end (combined with ProVerif) outperforms the few existing tools, both in terms of efficiency and protocol coverage. We successfully apply our tool to a dozen of protocols of the literature, yielding the first fully automatic proof of a security API and a payment protocol of the literature.

Remote electronic voting is used in several countries for legally binding elections. Unlike academic voting protocols, these systems are not always documented and their security is rarely analysed rigorously. In this paper, we study a voting system that has been used for electing political representatives and in citizen-driven referenda in the Swiss canton of Neuchâtel. We design a detailed model of the protocol in ProVerif for both privacy and verifiability properties. Our analysis mostly confirms the security of the underlying protocol: we show that the Neuchâtel protocol guarantees ballot privacy, even against a corrupted server; it also ensures cast-as-intended and recorded-as-cast verifiability, even if the voter's device is compromised. To our knowledge, this is the first time a full-fledged automatic symbolic analysis of an e-voting system used for politicallybinding elections has been realized.

Many decision problems on security protocols can be reduced to solving so-called intruder constraints in Dolev Yao model. Most constraint solving procedures for protocol security rely on two properties of constraint systems called monotonicity and variable-origination. In this work we relax these restrictions by giving an NP decision procedure for solving general intruder constraints (that do not have these properties). Our result extends a first work by L. Mazaré in several directions: we allow non-atomic keys, and an associative, commutative and idempotent symbol (for modeling sets). We also give several new applications of the result.

Cryptographic protocols are crucial for securing electronic transactions. The confidence in these protocols can be increased by the formal analysis of their security properties. Although many works have been dedicated to standard protocols like Needham-Schroeder very few address the more challenging class of group protocols. We have introduced in previous work a synchronous model for group protocols, that generalizes standard protocol models by permitting unbounded lists inside messages. This approach also applies to analyzing Web services manipulating sequences of items. In this model we propose now a decision procedure for the sub-class of well-tagged protocols with autonomous keys.

We show that the insecurity problem for protocols with modular exponentiation and arbitrary products allowed in exponents is NP-complete. This result is based on a protocol and intruder model which is powerful enough to uncover known attacks on the Authenticated Group Diffie-Hellman (A-GDH.2) protocol suite. To prove our results, we develop a general framework in which the Dolev-Yao intruder is extended by generic intruder rules. This framework is also applied to obtain complexity results for protocols with commuting public key encryption.

We develop a general method for reasoning about contract-signing protocols using a specialized protocol logic. The method is applied to prove properties of the Asokan-Shoup-Waidner and the Garay-Jacobson-MacKenzie protocols. Our method offers certain advantages over previous analysis techniques. First, it is compositional: the security guarantees are proved by combining the independent proofs for the three sub-protocols of which each protocol is comprised. Second, the formal proofs are carried out in a "template" form, which gives us a reusable proof that may be instantiated for the ASW and GJM protocols, as well as for other protocols with the same arrangement of messages. Third, the proofs follow the design intuition. In particular, in proving game-theoretic properties like fairness, we demonstrate that the specific strategy that the protocol designer had in mind works, instead of showing that one exists. Finally, our results hold even when an unbounded number of sessions are executed in parallel.

We provide a method for deciding the insecurity of cryptographic protocols in presence of the standard Dolev-Yao intruder (with a finite number of sessions) extended with so-called oracle rules, i.e., deduction rules that satisfy certain conditions. As an instance of this general framework, we ascertain that protocol insecurity is in NP for an intruder that can exploit the properties of the XOR operator. This operator is frequently used in cryptographic protocols but cannot be handled in most protocol models. An immediate consequence of our proof is that checking whether a message can be derived by an intruder (using XOR) is in P. We also apply our framework to an intruder that exploits properties of certain encryption modes such as cipher block chaining (CBC).

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