Cryptographic anonymity is a security property of cryptographic primitives or communication protocols eliminating or limiting user identification. It has been of predominant interest and has now evolved to become central to societal issues. In this thesis, we study electronic signatures with various anonymity properties, in order to develop mechanisms that will make it possible, to restrict the degree of identification to what is strictly necessary. We propose the study of the anonymity properties of linkable ring signatures. These signatures schemes ensure anonymity while maintaining a link between the signatures of the same signer, a property that is particularly relevant in the context of electronic voting, for example. We are also interested in signature delegation mechanisms. Furthermore, we propose a method for ensuring the anonymity of the delegate while limiting the number of signatures that can be issued. Exceeding the threshold implies revealing the identity of the signer, as well as the possibility of tracing all its issued signatures. In these two studies, we have adopted the paradigm of proven cryptography and will rely on so-called computational models. We also considering a more practical protocol - the EMV card payment protocol - we propose constructions allowing degrees of anonymity that comply with the current standard. We show that, despite the many normative constraints and laws imposing identification of parties, it is possible to design more privacy-friendly architectures. Our study is not limited to anonymity; we also look at other essential properties to moderate too strong anonymity and ensure that primitives can be easily integrated into concrete schemes.

Jury :

Pascal Lafourcade, LIMOS (directeur)
Xavier Bultel, INSA CVL, LIFO (encadrant)
Sébastien Canard, Télécom ParisTech (rapporteur)
Cristina Onete, XLIM, (rapporteuse)
Olivier Blazy, Ecole Polytechnique (examinateur)
Katharina Boudgoust, CNRS, LIRMM (examinatrice)
Dario Fiore, IMDEA (examinateur)
David Pointcheval, Cosmian (examinateur).