Description
(Joint work with Thomas Peters, and David Pointcheval) We put forth a novel cryptographic primitive: encryption switching protocol (ESP), allowing to switch between two encryption schemes. Intuitively, this two-party protocol converts given ciphertexts from one scheme into ciphertexts of the same messages in the other scheme, for any polynomial number of switches, in any direction. Although ESP is a special kind of two-party computation protocol, it turns out that ESP implies general two-party computation under natural conditions. In particular, our new paradigm is tailored to the evaluation of functions over rings. Indeed, assuming the compatibility of two additively and multiplicatively homomorphic encryption schemes, switching ciphertexts makes it possible to efficiently reconcile the two internal laws. Since no such pair of schemes appeared in the literature, except for the non-interactive case of fully homomorphic encryption which still remains prohibitive in practice, we build the first ElGamal-like encryption scheme over (Zn;x) as a complement to the Paillier encryption scheme over (Zn;+), where n is a strong RSA modulus. Eventually, we also instantiate secure ESP between the two schemes, in front of malicious adversaries. Thanks to a pre-processing step, we manage to get an online communication in terms of group elements which neither depends on the security parameter nor on the modulus n. This makes use of a new technique called refreshable twin-ciphertext pool that is of independent interest.
Prochains exposés
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Cyclotomic Cosets: Hidden Subgroup and Quantum Sieving Algorithm for Prime-Power Moduli
Orateur : Mathias Boucher - Université de Rennes
The Dihedral Coset Problem (DCP), also known as the hidden shift problem, plays a fundamental role in post-quantum public-key cryptography, since the Learning-With-Error problem reduces to DCP. The “Simon meets Kuperberg” quantum algorithm by Bonnetain and Naya-Plasencia solves the DCP in quasi-polynomial time when the modulus is a power of two. We provide a generalization of this algorithm for[…] -
Adelic reduction of module lattices
Orateur : Henry Bambury - DGA-MI et Inria Rennes
We give a strict generalisation of the LLL algorithm over number fields, based on the reduction theory of $GL(n)$ over the adele ring of a number field. Our algorithm is free of heuristics, with rigorous bounds on output quality and complexity. -- based on joint work with Seungki Kim, Changmin Lee and Phong Nguyen ---
Cryptography
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European Cyber Week: atelier cryptographie post-quantique
Dans la continuité des éditions 2021, 2022 et 2024, la DGA — en partenariat avec CREACH LABS et avec le soutien de l'ANSSI, de l'IRISA, de l'IRMAR et du Pôle d'Excellence Cyber — organise la 4e édition de l'atelier consacré à la cryptographie post-quantique dans le cadre de l'European Cyber Week 2026. Attention, il faut s'inscrire (gratuitement) au préalable — s'inscrire à la conférence Les[…] -
Post-quantum day of the cryptography seminar
A scientific day devoted to post-quantum cryptography, held in the wake of the European Cyber Week, with talks more technical than those presented at the ECW. The talks will take place at Amphi E in building 02A. To register for lunch, please do so before oct 30th using this link. -
Chiffrement homomorphe à l'aide des groupes après Nuida
Orateur : Pierre Guillot - Ravel Technologies/Université de strasbourg
Nuida et d'autres ont montré comment on peut obtenir un protocole de chiffrement pleinement homomorphe si l'on sait chiffrer les éléments d'un groupe G convenable, tout en sachant faire l'opération de G "en homomorphe". Les exemples étaient jusqu'à présent limités au chiffrement d'un bit à la fois, avec beaucoup d'opérations à effectuer pour "simuler" les portes logiques usuelles. Je vais[…]-
Cryptography
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Post-Quantum Signatures from Symmetric-Key-Friendly Techniques: From MPC-in-the-Head to VOLE-in-the-Head
Orateur : Dung Bui - Northwestern University
The MPC-in-the-Head (MPCitH) paradigm turns a secure multiparty computation protocol into a zero-knowledge proof and, via Fiat–Shamir, into a signature scheme whose security relies only on symmetric-key primitives. The framework of Katz, Kolesnikov and Wang (KKW, CCS18), which moves most of the MPC work into a committed preprocessing phase that the verifier checks by opening all-but-one of the[…] -
Nouveaux doctorants
Exposés courts des nouveaux doctorants (Elouan, Quentin, Eliot)