Nicolas BORDES: "Symmetric primitives, side-channel attacks and masking"

Directed by Jean-Guillaume Dumas (LJK). Co-supervised by Paolo Maistri (TIMA) and Pierre Karpman (LJK)
The first part of this thesis is concerned with the study of some properties of cryptographic per-mutations. It takes its source from a joint work with Joan Daemen, Daniël Kuijsters and GillesVan Assche published in the proceedings of CRYPTO 2021. These symmetric primitives can bedesigned using multiple approaches. One of them, popularized by the Advanced Encryption Stan-dard (AES), consists in grouping the bits, e.g. in bytes, and consistently processing them in thesegroups. This aligned approach leads to structures that make it possible to reason about the differ-ential and linear propagation properties using combinatorial arguments. In contrast, an unalignedapproach avoids any such grouping in its design, which however complexifies the analysis of thesame properties. In this thesis, we define formally what it means for a permutation to be alignedand study its impact on the differential and linear properties of four primitives adopting differentdesign strategies.

The second part of this thesis focuses on the secure implementation of symmetric primitives. Specifically, we study a class of attacks, side-channel attacks, where an attacker may be able toextract the secrets of a cryptographic algorithm by only measuring physical leakages from thecomponents computing it. One countermeasure against these attacks, called masking, leveragessecret-sharing schemes to split the sensitive data into random shares while allowing to securelycompute using this sharing. However, verifying that a masked implementation is indeed secureand the countermeasure itself are both costly. We improve the performance on this two aspects inan article published in the proceedings of EUROCRYPT 2021 with Pierre Karpman as a co-author. Following these results, we also propose a new masked version of the AES and we experimentallyverify its robustness against side-channel attacks.
Published on October 18, 2021