The Supersingular Isogeny Problem in Genus 2 and Beyond

Abstract

Let (Formula Presented) be superspecial principally polarized abelian varieties of dimension (Formula Presented). For any prime (Formula Presented), we give an algorithm that finds a path (Formula Presented) in the (Formula Presented)-isogeny graph in (Formula Presented) group operations on a classical computer, and (Formula Presented) calls to the Grover oracle on a quantum computer. The idea is to find paths from A and A' to nodes that correspond to products of lower dimensional abelian varieties, and to recurse down in dimension until an elliptic path-finding algorithm (such as Delfs–Galbraith) can be invoked to connect the paths in dimension g=1. In the general case where A and A' are any two nodes in the graph, this algorithm presents an asymptotic improvement over all of the algorithms in the current literature. In the special case where A and A' are a known and relatively small number of steps away from each other (as is the case in higher dimensional analogues of SIDH), it gives an asymptotic improvement over the quantum claw finding algorithms and an asymptotic improvement over the classical van Oorschot–Wiener algorithm.