Contributions to FHE security against CCA adversaries
Wednesday 10 December 2025 at 14h00
CEA Nano-Innov (Building 862, Amphi 33-34) and online
Abstract: Homomorphic encryption (FHE) is a cryptographic tool allowing the delegation of computations over sensitive data while preserving its confidentiality. Unfortunately, what makes the power of FHE, its malleability (the ability to perform computations over ciphertexts), is also one weak point. On their own, homomorphic schemes only provide a relatively low security guarantee (CPA security, which models passsive adversaries) in comparison to what is usually expected (CCA2 security, which models active adversaries) from more "classical" cryptosystems. This latter security level is known to be unachievable for FHE schemes, due to their malleability. The security of homomorphic cryptosystems has only been studied for a short time, and it is even more recent in the case of approximate homomorphic schemes. In the latter, there is a non-negligible probability of having an error in decryption (the obtained value differs slightly from the expected one). In this thesis, we introduce new security notions between CPA and CCA2, including vCCAD the new so far know strongest security notion achievable for approximate homomorphic schemes. We also provide constructions based on homomorphic schemes (approximate or not) achieving these new security notions.
The defense will be in English.
Jury:
- Dario Fiore, IMDEA Software Institute (Spain) - Reviewer
- Mark Manulis, Universität der Bundeswehr (München, Germany) - Reviewer
- Sébastien Canard, Télécom Paris / LTCI
- Mireille Fouquet, Université Paris Cité / IMJ
- David Pointcheval, CNRS ENS Paris / DIENS and Cosmian
- Adeline Roux-Langlois, CNRS Université de Caen / GREYC
- Caroline Fontaine, CNRS / LMF - Supervisor
- Renaud Sirdey, CEA / LIST - Supervisor
- Oana Stan, CEA / LIST - Supervisor