An Effective Lower Bound on the Number of Orientable Supersingular Elliptic Curves

International audienceIn this article, we prove a generic lower bound on the number of O-orientable supersingular curves over F p 2 , i.e curves that admit an embedding of the quadratic order O inside their endomorphism ring. Prior to this work, the only known effective lower-bound is restricted to small discriminants. Our main result targets the case of fundamental discriminants and we derive a generic bound using the expansion properties of the supersingular isogeny graphs. Our work is motivated by isogeny-based cryptography and the increasing number of protocols based on O-oriented curves. In particular, our lower bound provides a complexity estimate for the brute-force attack against the new O-uber isogeny problem introduced by De Feo, Delpech de Saint Guilhem, Fouotsa, Kutas, Leroux, Petit, Silva and Wesolowski in their recent article on the SETA encryption scheme