## Section: New Results

### Elliptic curve and Abelian varieties cryptology

Participants : Jean-Marc Couveignes, Andreas Enge, Damien Robert.

In [27] J.-M. Couveignes and T. Ezome show how to efficiently evaluate functions, including Weil functions and canonical Theta functions, on Jacobian varieties and their quotients. They deduce a quasi-optimal algorithm to compute $(l,l)$ isogenies between Jacobians of genus two curves, using a compact representation and differential characterisation of isogenies in this context. This work has been submitted to the LMS Journal of Computation and Mathematics.

The paper [18] by J.-M. Couveignes and R. Lercier
describing the problem of parameterisations by radicals of low genus
algebraic curves has been accepted in *Advances in mathematics of
communications*.

In [31] D. Lubicz and D. Robert explain how to improve the arithmetic of Abelian and Kummer varieties. The speed of the arithmetic is a crucial factor in the performance of abelian varieties based cryptosystem. Depending on the cryptographic application, the speed record holder are elliptic curves (in the Edwards model) or the Kummer surface of an hyperelliptic curves of genus 2 (in the level 2 theta model). One drawback of the Kummer surface is that only scalar multiplications are available, which may be a drawback in certain cryptographic protocols. The previous known models to work on the Jacobian rather than the Kummer surface (Mumford coordinates or theta model of level 4) are too slow and not competitive with Elliptic Curves. This paper explains how to use geometric properties (like projective normality) to speed up the arithmetic. In particular it introduces a novel addition algorithm on Kummer varieties (compatible additions), and use it to enhance multi-exponentiations in Kummer varieties and to obtain new models of abelian surfaces where the scalar multiplication is as fast as on the Kummer surface.

In [32] (which has been accepted at LMS Journal of Computation and Mathematics), D. Lubicz and D. Robert explain how to compute isogenies between abelian varieties given algebraic equation of the kernel. The previous algorithms to compute isogenies between abelian varieties needed the coordinates of generators of the kernel. One drawback was that even if the kernel is rational, these generators may live in extension of large degree, especially for Abelian varieties defined over a number field rather than a finite field. This paper combines the use of formal coordinates together with a normalisation alongs linear subspaces of the kernel rather than the whole kernel to derive an algorithm which is quasi-optimal if the degree of the isogeny is ${\ell}^{g}$, for $\ell $ congruent to 1 modulo 4.

This article expands the article [17] by D. Cosset and D. Robert about the computation of $(\ell ,\ell )$-isogenies in dimension 2 which has been published in Mathematics of Computation.