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# Séminaire de Théorie Algorithmique des Nombres

Responsables : Razvan Barbulescu et Léo Poyeton

Page du séminaire

• Le 28 mars 2023 à 10:00
• Salle 2
Shane Gibbons (CWI, Netherlands)
Hull attacks on the Lattice Isomorphism Problem

The lattice isomorphism problem (LIP) asks one to find an isometry between two lattices. It has recently been proposed as a foundation for cryptography in independent works. This problem is the lattice variant of the code equivalence problem, on which the notion of the hull of a code can lead to devastating attacks. In this talk I will present the cryptanalytic role of an adaptation of the hull to the lattice setting, which we call the s-hull. Specifically, we show that the hull can be helpful for geometric attacks, for certain lattices the minimal distance of the hull is relatively smaller than that of the original lattice, and this can be exploited. The attack cost remains exponential, but the constant in the exponent is halved.
Our results suggests that one should be very considerate about the geometry of hulls when instantiating LIP for cryptography. They also point to unimodular lattices as attractive options, as they are equal to their own hulls. Remarkably, this is already the case in proposed instantiations, namely the trivial lattice $mathbb{Z}^n$ and the Barnes-Wall lattices.

• Le 4 avril 2023 à 10:00
• Salle 2
Jean Gillibert (Université de Toulouse 2)
Finite subgroups of $mathrm{PGL}_2(mathbb{Q})$ and number fields with large class groups

For each finite subgroup $G$ of $mathrm{PGL}_2(mathbb{Q})$, and for each integer $n$ coprime to $6$, we construct explicitly infinitely many Galois extensions of $mathbb{Q}$ with group $G$ and whose ideal class group has $n$-rank at least $#G-1$. This gives new $n$-rank records for class groups of number fields.

• Le 11 avril 2023 à 10:00
• Salle 2
Henry Bambury (ENS Ulm)
An inverse problem for isogeny volcanoes

Supersingular isogeny graphs are very complicated and intricate, and are used extensively by cryptographers. On the other side of things, the structure of ordinary isogeny graphs is well understood connected components look like volcanoes. Throughout this talk we will explore the ordinary $ell$-isogeny graph over $mathbb{F}_p$ for various prime numbers $ell$ and $p$, and answer the following question, given a volcano-shaped graph, can we always find an isogeny graph in which our volcano lives as a connected component?

• Le 25 avril 2023 à 11:00
• zoom 839 4179 5223 retransmited in room 2
Alessandro Languasco (University of Padova, Italy)
TBA

• Le 2 mai 2023 à 10:00
• Salle 2
Sorina Ionica (Université de Picardie)
TBA

• Le 9 mai 2023 à 10:00
• Salle 2
Sabrina Kunzweiler (IMB)
TBA

• Le 16 mai 2023 à 10:00
• Salle 2
Matthieu Lequesne (CWI, Netherlands)
TBA

• Le 23 mai 2023 à 10:00
• Salle 2
Boris Fuoutsa (EPFL, Switzerland)
TBA

• Le 30 mai 2023 à 10:00
• Salle 2
Sarah Arpin (University of Leinden, Netherlands)
Adding Level Structure to Supersingular Elliptic Curve Isogeny Graphs

The classical Deuring correspondence provides a roadmap between supersingular elliptic curves and the maximal orders which are isomorphic to their endomorphism rings. Building on this idea, we add the information of a cyclic subgroup of prime order N to supersingular elliptic curves, and prove a generalisation of the Deuring correspondence for these objects. We also study the resulting ell-isogeny graphs supersingular elliptic curve with level-N structure, and the corresponding graphs in the realm of quaternion algebras. The structure of the supersingular elliptic curve ell-isogeny graph underlies the security of a new zero-knowledge proof of isogeny knowledge [Basso-Codogni-Connolly-De Feo-Fouotsa-Lido-Morrison-Panny-Patranabis-Wesolowski 2022].

• Le 6 juin 2023 à 10:00
• Salle 2
Daan van Gent (University of Leinden, Netherlands)
TBA
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• Le 6 juin 2023 à 10:00
• Salle 2
Daan van Gent (University of Leinden, Netherlands)
TBA

• Le 13 juin 2023 à 10:00
• Salle 2
TBA
TBA

• Le 20 juin 2023 à 10:00
• Salle 2
TBA
TBA

• Le 27 juin 2023 à 10:00
• Salle 2
Agathe Houzelot (Labri)
TBA

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