Vendredi 17 mars 2017 à 10h30 en salle C46
Pierrick Méaux (ENS)

Titre : Symmetric Encryption Scheme adapted to Fully Homomorphic Encryption Scheme: New Criteria for Boolean functions

Résumé :
Fully Homomorphic Encryption is a recent powerful cryptographic construction, which enables one to securely compute all functions on encrypted data, and decrypt the result of the function applied to the real data. This construction gives the possibility to securely delegate computation, which is a very important property with the increasing development of Cloud computing. Nevertheless, in current client-server frameworks, the client devices are too restricted to support pure FHE. In order to solve this problem, FHE has to be combined with primitives which incur small computation and communication cost: Symmetric Encryption schemes. In this talk, we will present a symmetric encryption scheme created for this context: the FLIP family of stream ciphers. This construction has an unusual design: at each clock cycle, the key register is updated by a different, publicly known, wire-cross permutation and then filtered by a Boolean function to produce one key-stream bit. Therefore, the security of the scheme crucially depends on this Boolean function, which should be robust relatively to standard cryptographic criteria and new ones. In this talk, we will talk about "low-cost" Boolean functions adapted to the FHE context; recurrent criteria on Boolean functions obtained by fixing some variables and Boolean criteria on restricted set of inputs.

Remarques : The presentation will be based on the following works: 1. Méaux, Journault, Standaert, Carlet. Towards stream ciphers for efficient FHE with low-noise ciphertexts. Eurocrypt 2016 2. Duval, Lallemand, Rotella. Cryptanalysis of the FLIP Family of Stream Ciphers. Crypto 2016 3. Carlet, Méaux, Rotella. Boolean functions with restricted input and their robustness; application to the FLIP cipher.