Cryptanalysis of elliptic curve hidden number problem from PKC 2017

In PKC 2017, the elliptic curve hidden number problem (EC-HNP) was revisited in order to rigorously assess the bit security of the elliptic curve Diffie–Hellman key exchange protocol. In this paper, we solve EC-HNP by using the Coppersmith technique which combines the idea behind the second lattice method of Boneh, Halevi and Howgrave-Graham for solving the modular inversion hidden number problem. We show that the hidden point in EC-HNP can be recovered asymptotically if about half of the most significant bits of the x-coordinates of the corresponding points are given. A similar result is also obtained for the least significant bits. We provide better bounds than the one in the work of PKC 2017, which needs about 5/6 of the bits as a result of a rigorous algorithm. However, our solution is based on a heuristic assumption. We verify the validity of our heuristic algorithm by computer experiments.