Password-based authenticated key exchange (PAKE) are protocols which are designed to be secure even when the secret key used for authentication is a human-memorable password. In this paper, we consider PAKE protocols in the three-party scenario, in which the users trying to establish a common secret do not share a password between themselves but only with a trusted server. Towards our goal, we recall some of the existing security notions for PAKE protocols and introduce new ones that are more suitable to the case of generic constructions of three-party protocols. We then present a natural generic construction of a three-party PAKE protocol from any two-party PAKE protocol and prove its security. To the best of our knowledge, the new protocol is the first provably-secure PAKE protocol in the three-party setting.

International audienceThe aim of electronic voting schemes is to provide a set of protocols that allow voters to cast ballots while a group of authorities collect the votes and output the final tally. In this paper we describe a practical multi-candidate election scheme that guarantees privacy of voters, public verifiablity and robustness against a coalition of malicious authorities. Furthermore, we address the problem of receipt-freeness and incoercibility of voters. Our new scheme is based on the Paillier cryptosystem and on some related zero-knowledge proof techniques. The voting schemes are very practical and can be efficiently implemented in a real system

Abstract. In this paper, we study the problem of automatically verifying higherorder masking countermeasures. This problem is important in practice (weaknesses have been discovered in schemes that were thought secure), but is inherently exponential: for t-order masking, it involves proving that every subset of t intermediate variables is distributed independently of the secrets. Some type systems have been proposed to help cryptographers check their proofs, but many of these approaches are insufficient for higher-order implementations. We propose a new method, based on program verification techniques, to check the independence of sets of intermediate variables from some secrets. Our new language-based characterization of the problem also allows us to design and implement several algorithms that greatly reduce the number of sets of variables that need to be considered to prove this independence property on all valid adversary observations. The result of these algorithms is either a proof of security or a set of observations on which the independence property cannot be proved. We focus on AES implementations to check the validity of our algorithms. We also confirm the tool's ability to give useful information when proofs fail, by rediscovering existing attacks and discovering new ones.

Several public key cryptosystems with additional homomorphic properties have been proposed so far. They allow to perform c o mputation with e ncrypted data without the knowledge of any secret information. In many applications, the ability to perform decryption, i.e. the knowledge of the secret key, gives a huge power. A classical way to reduce the trust in such a secret owner, and consequently to increase the security, is to s hare the s ecret between many entities in such a w ay that cooperation between them is necessary to decrypt. In this paper, we propose a distributed version of the Paillier cryptosystem presented at Eurocrypt '99. This shared scheme can for example be used in an electronic voting scheme or in a lottery where a random number related to the winning ticket has to be jointly chosen by all participants.