18/06/2024 - Design of Secure Multi-Users Protocols: Applications to Bandits, Ticketing and File Transfer
Gaël MARCADET • Doctorant
Répétition de soutenance de thèseCryptographic protocols establish a series of interactions among numerous users to deliver specific functionality while ensuring various properties, a protocol being considered secure when it successfully ensures all intended properties. Accomplishing these properties requires the utilisation of cryptographic primitives, whose usage may entail computation overhead and hence limiting the scalability of the protocol. Throughout this manuscript, we focus on three problems dealing with multiple users.The first contribution focuses on the design of federated multi-armed bandits framework where a federation server acting as a learning agent repeatedly pull a bandit arm, the environment responding with a reward coming from an unknown distribution associated with the chosen arm. In this contribution, we introduce ProtoBandit, a secure federated Mbandits protocol fixing and extending our initial attempt Proto shown to be insecure. ProtoBandit is proved to prevent the federation to learn the reward distribution, the obtained rewards and the pulled bandit arm, at the cost of a large computation overhead due to the usage of expensive cryptographic primitives. In this second part of this contribution, we introduce ProtoBP a secure federated Mbandits protocol moving away from the blueprint of Proto and ProtoBandit, still preventing the federation server to learn sensitive data while achieving high-performance and hence to be scalable.The second contribution of this manuscript deals with a problem involving a large number of users, since it concerns the design of a ticketing system. Indeed, despite the high-demand, these systems provides very restricted guarantees. For instance, one may easily resell a ticket twice. Worse, majority of tickets are nominatives, revealing the identity of the ticket's owner. Using standard cryptographic primitives, we propose two scalable anonymous and transferable ticketing systems called ProtoTicket and ProtoAudit, ensuring users anonymity while featuring ticket purchasing, ticket refunding, ticket validation and ticket transferability. The difference between ProtoTicket and ProtoAudit lies in the ability to recover the identity of an attendee: In ProtoTicket, the anonymity of every user is guaranteed at any time, a property that still holds with ProtoAudit except for an additional third-party able to recover the identity of an attendee, at the cost of a slightly longer ticket validation.Our third and last contribution focuses on the broadcast file transfer problem consisting to share a file to a group of users. The trivial solution consisting to store files on a single publicly available server falls short for instance when the server is down or when the server handles a high number of requests, and is not suitable when the exchanged file contains sensitive data. In this contribution, we introduce an universally composable and efficient protocol allowing one to share a file only to a group of users while ensuring confidentiality and integrity of the file and sender authentication.
