Séminaire C2 du vendredi 19 mars 2021

Programme du séminaire

  • 10h00 -Mélissa Rossi, ANSSI, Assessing residual security of lattice-based cryptography
Abstract : This talk will present a framework for cryptanalysis of lattice-based schemes, when side information —in the form of «hints»— about the secret is available. This presentation outlines a joint work with Dana Dachman-Soled, Léo Ducas and Huijing Gong that was presented in CRYPTO 2020 (https://eprint.iacr.org/2020/292).
This framework generalizes the primal lattice reduction attack, and allows the progressive integration of hints before running a final lattice reduction step. The techniques for integrating hints include sparsifying the lattice, projecting onto and intersecting with hyperplanes, and/or altering the distribution of the secret vector. The main contribution is to propose a toolbox and a methodology to integrate such hints into lattice reduction attacks and to predict the performance of those lattice attacks with side information.
While initially designed for side-channel information, this framework can also be used in other cases. For example, one can simply exploit constraints imposed by certain schemes (LAC, Round5, NTRU). Besides, I will present a way to use this framework combined with decryption failures information using a joint work with Jan-Pieter D’Anvers and Fernando Virdia presented in EUROCRYPT 2020 (https://eprint.iacr.org/2019/1399).


  • 11h00André Schrottenloher, CWI Amsterdam, Quantum tools for symmetric cryptanalysis
Abstract : The security of (symmetric) cryptosystems is based on cryptanalysis: our confidence stems from an extensive and continuous public scrutiny. Meanwhile, some consequences of large-scale quantum computing devices in cryptography are well-known, for example, the quadratic speedup of Grover’s algorithm against exhaustive key search. But the post-quantum security of symmetric cryptography cannot be precisely asserted unless we try to design more specific attacks, as we do in the classical world.
 In this talk, I will give two examples of symmetric designs that can be attacked with specific quantum algorithms. I will first focus on the Even-Mansour construction, which turns a public permutation into a cipher. Even-Mansour has a strong algebraic structure, and, in fact, it is completely broken if the adversary is allowed « superposition queries ». Yet, this structure seems difficult to exploit for an adversary making standard, classical queries, and this has been an open problem for a while. We will see how to improve significantly the quantum key-recovery on Even-Mansour using the « offline Simon’s algorithm »
(joint work with X. Bonnetain, A. Hosoyamada, M. Naya-Plasencia and Y. Sasaki).
Our second target will be multiple-encryption. It is a cipher obtained by composing independent ciphers in sequence, with independent keys. I will shortly introduce quantum merging algorithms (joint work with M. Naya-Plasencia) and their application to the exhaustive key search in multiple-encryption. These algorithms are quite generic and have many other applications in cryptography.