## Section: Application Domains

### Cryptology

The first application domain for our research is cryptology. This includes cryptography (constructive side) and cryptanalysis (breaking systems). For the cryptanalysis part, although it has practical implications, we do not expect any transfer in the classical sense of the term: it is more directed to governmental agencies and the end-users who build their trust, based on the cryptanalysis effort. It is noteworthy that analysis documents from governmental agencies (see e.g., [24] ) use cryptanalysis results as their key material.

#### Cryptography

Our cryptographic contributions are related to multiple facets of the large realm of curve-based cryptology. While it is quite clear that enough algorithms exist in order to provide cryptographers with elliptic curves having a suitably hard discrete logarithm (as found in cryptographic standards for instance), one must bear in mind that refinements of the requirements and extensions to curves of higher genus raise several interesting problems. Our work contributes to expanding the cryptographer's capabilities in these areas.

In the context of genus-2 curves, our work aims at two goals. First, improvements on the group law on selected curves yield better speed for the associated cryptosystems. The cryptographic primitives, and then the whole suite of cryptographic protocols built upon such curves would be accelerated. The second goal is the expansion of the set of curves that can be built given a set of desired properties. Using point counting algorithms for arbitrary curves, a curve offering a 128-bit security level, together with nice properties for fast arithmetic, has been computed by Caramel [5] . Another natural target for construction of curves for cryptography is also the suitability of curves for pairings. We expect to be able to compute such curves.

Important objects related to the structure of genus-2 curves are the isogenies between their Jacobians. Computing such isogenies is a key point in understanding important underlying objects such as the endomorphism ring, and can be useful in various situations, including for cryptographic or cryptanalytic applications. The team has produced important results in this context [7] , [3] .

#### Cryptanalysis

Our research on cryptanalysis is important for the cryptographic industry: by detecting weak instances, and setting new records we contribute to the definition of recommended families of systems together with their key sizes. The user's confidence in a cryptographic primitive is also related to how well the underlying problem is studied by researchers.

In particular, our involvement in computations with “NFS-like” algorithms encompasses of course the task of assessing the computational limits for integer factorization (as was done by the team by factoring RSA-768 [6] ) and discrete-logarithm computations (as was done by the team in 2013 for the field $\mathrm{GF}\left({2}^{809}\right)$ [25] ). The impact of the former is quite clear as it concerns the RSA algorithm; record-sized computations attract broad interest and determine updates on key-length recommendations. The latter are particularly important for pairing-based cryptography, since, in this context, one naturally encounters discrete-logarithm problems in extension fields of large degree. To this regard the breakthrough provided by the new quasi-polynomial discrete logarithm [26] is of course of utmost importance.