Cryptology ePrint Archive: Report 2020/297

Random Self-reducibility of Ideal-SVP via Arakelov Random Walks

Koen de Boer and Léo Ducas and Alice Pellet-Mary and Benjamin Wesolowski

Abstract: Fixing a number field, the space of all ideal lattices, up to isometry, is naturally an Abelian group, called the *Arakelov class group*. This fact, well known to number theorists, has so far not been explicitly used in the literature on lattice-based cryptography. Remarkably, the Arakelov class group is a combination of two groups that have already led to significant cryptanalytic advances: the class group and the unit torus.

In the present article, we show that the Arakelov class group has more to offer. We start with the development of a new versatile tool: we prove that, subject to the Riemann Hypothesis for Hecke $L$-functions, certain random walks on the Arakelov class group have a rapid mixing property. We then exploit this result to relate the average-case and the worst-case of the Shortest Vector Problem in ideal lattices. Our reduction appears particularly sharp: for Hermite-SVP in ideal lattices of certain cyclotomic number fields, it loses no more than a $\tilde O(\sqrt n)$ factor on the Hermite approximation factor.

Furthermore, we suggest that this rapid-mixing theorem should find other applications in cryptography and in algorithmic number theory.

Category / Keywords: public-key cryptography / Ideal Lattices, Random Walk, Worst-Case Hardness, Arakelov, L-functions

Original Publication (with minor differences): IACR-CRYPTO-2020

Date: received 6 Mar 2020, last revised 7 Jul 2020

Contact author: K de Boer at cwi nl, ducas@cwi nl, alice pelletmary@kuleuven be, benjamin wesolowski@math u-bordeaux fr

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Version: 20200707:080743 (All versions of this report)

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