Xavier Bultel

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Contactless communications have become omnipresent in our daily lives, from simple access cards to electronic passports. Such systems are particularly vulnerable to <i>relay attacks</i>, in which an adversary relays the messages from a prover to a verifier. Distance-bounding protocols were introduced to counter such attacks. Lately, there has been a very(More)
Distance-bounding protocols have been introduced to thwart relay attacks against contactless authentication protocols. In this context, verifiers have to authenticate the credentials of untrusted provers. Unfortunately, these protocols are themselves subject to complex threats such as terrorist-fraud attacks, in which a malicious prover helps an accomplice(More)
The MapReduce programming paradigm allows to process big data sets in parallel on a large cluster of commodity machines. The MapReduce users often outsource their data and computations to a public cloud provider. We focus on the fundamental problem of matrix multiplication, and address the inherent security and privacy concerns that occur when outsourcing(More)
Akari, Takuzu, Kakuro and KenKen are logic games similar to Sudoku. In Akari, a labyrinth on a grid has to be lit by placing lanterns, respecting various constraints. In Takuzu a grid has to be filled with 0’s and 1’s, while respecting certain constraints. In Kakuro a grid has to be filled with numbers such that the sums per row and column match given(More)
Ring and group signatures allow their members to anonymously sign documents in the name of the group. In ring signatures, members manage the group themselves in an ad-hoc manner while in group signatures, a manager is required. Moreover, k-times traceable group and ring signatures [1] allow anyone to publicly trace two signatures from a same user if he(More)