Automated verification of equivalence properties of cryptographic protocols

The original publication is available at www.springerlink.comInternational audienceIndistinguishability properties are essential in formal verification of cryptographic protocols. They are needed to model anonymity of cryptographic protocols. They are needed to model anonymity properties, strong versions of confidentiality and resistance to offline guessing attacks, and can be conveniently modeled using process equivalences. We present a novel procedure to verify equivalence properties for bounded number of sessions. Our procedure is able to verify trace equivalence for determinate cryptographic protocols. On determinate protocols, trace equivalence coincides with observational equivalence which can therefore be automatically verified for such processes. When protocols are not determinate our procedure can be used for both under- and over-approximations of trace equivalence, which proved successful on examples. The procedure can handle a large set of cryptographic primitives, namely those which can be modeled by an optimally reducing convergent rewrite system. Although, we were unable to prove its termination, it has been implemented in a prototype tool and has been effectively tested on examples, some of which were outside the scope of existing tools

Formal Analysis of E-Cash Protocols

International audienceElectronic cash (e-cash) aims at achieving client privacy at payment, similar to real cash. Several security protocols have been proposed to ensure privacy in e-cash, as well as the necessary unforgery properties. In this paper, we propose a formal framework to define, analyze, and verify security properties of e-cash systems. To this end, we model e-cash systems in the applied π-calculus, and we define two client privacy properties and three properties to prevent forgery. Finally, we apply our definitions to an e-cash protocol from the literature proposed by Chaum et al., which has two variants and a real implementation based on it. Using ProVerif, we demonstrate that our framework is suitable for an automated analysis of this protocol

A Formal Taxonomy of Privacy in Voting Protocols

International audiencePrivacy is one of the main issues in electronic voting. We propose a family of symbolic privacy notions that allows to assess the level of privacy ensured by a voting protocol. Our definitions are applicable to protocols featuring multiple votes per voter and special attack scenarios such as vote-copying or forced abstention. Finally we employ our definitions on several existing voting protocols to show that our model allows to compare different types of protocols based on different techniques, and is suitable for automated verification using existing tools

How to prove security of communication protocols? A discussion on the soundness of formal models w.r.t. computational ones.

Security protocols are short programs that aim at securing communication over a public network. Their design is known to be error-prone with flaws found years later. That is why they deserve a careful security analysis, with rigorous proofs. Two main lines of research have been (independently) developed to analyse the security of protocols. On the one hand, formal methods provide with symbolic models and often automatic proofs. On the other hand, cryptographic models propose a tighter modeling but proofs are more difficult to write and to check. An approach developed during the last decade consists in bridging the two approaches, showing that symbolic models are sound w.r.t. symbolic ones, yielding strong security guarantees using automatic tools. These results have been developed for several cryptographic primitives (e.g. symmetric and asymmetric encryption, signatures, hash) and security properties. While proving soundness of symbolic models is a very promising approach, several technical details are often not satisfactory. Focusing on symmetric encryption, we describe the difficulties and limitations of the available results

Formal Treatment of Distributed Trust in Electronic Voting

Electronic voting systems are among the most security critical distributed systems. Different trust concepts are implemented to mitigate the risk of conspiracies endangering security properties. These concepts render systems often very complex and end users no longer recognize whom they need to trust. Correspondingly, specific trust considerations are necessary to support users. Recently, resilience terms have been proposed in order to express, which entities can violate the addressed security properties in particular by illegal collaborations. However, previous works derived these resilience terms manually. Thus, successful attacks can be missed. Based on this approach, we propose a framework to formally and automatically derive these terms. Our framework comprises a knowledge calculus, which allows us to model knowledge and reason about knowledge of collaborating election entities. The introduced framework is applied to deduce previously manually derived resilience terms of three remote electronic voting systems, namely Polyas, Helios and the Estonian voting system. Thereby, we were able to discover mistakes in previous derivations

When Are Three Voters Enough for Privacy Properties?

International audienceProtocols for secure electronic voting are of increasing societal importance. Proving rigorously their security is more challenging than many other protocols, which aim at authentication or key exchange. One of the reasons is that they need to be secure for an arbitrary number of malicious voters. In this paper we identify a class of voting protocols for which only a small number of agents needs to be considered: if there is an attack on vote privacy then there is also an attack that involves at most 3 voters (2 honest voters and 1 dishonest voter). In the case where the protocol allows a voter to cast several votes and counts, e.g., only the last one, we also reduce the number of ballots required for an attack to 10, and under some additional hypotheses, 7 ballots. Our results are formalised and proven in a symbolic model based on the applied pi calculus. We illustrate the applicability of our results on several case studies, including different versions of Helios and Prêt-` a-Voter, as well as the JCJ protocol. For some of these protocols we can use the ProVerif tool to provide the first formal proofs of privacy for an unbounded number of voters