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arXiv · 1806.03656

A note on the security of CSIDH

Jean-François Biasse·Annamaria Iezzi·Michael J. Jacobson Jr
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Abstract

We propose an algorithm for computing an isogeny between two elliptic curves $E_1,E_2$ defined over a finite field such that there is an imaginary quadratic order $\mathcal{O}$ satisfying $\mathcal{O}\simeq \operatorname{End}(E_i)$ for $i = 1,2$. This concerns ordinary curves and supersingular curves defined over $\mathbb{F}_p$ (the latter used in the recent CSIDH proposal). Our algorithm has heuristic asymptotic run time $e^{O\left(\sqrt{\log(|\Delta|)}\right)}$ and requires polynomial quantum memory and $e^{O\left(\sqrt{\log(|\Delta|)}\right)}$ classical memory, where $\Delta$ is the discriminant of $\mathcal{O}$. This asymptotic complexity outperforms all other available method for computing isogenies. We also show that a variant of our method has asymptotic run time $e^{\tilde{O}\left(\sqrt{\log(|\Delta|)}\right)}$ while requesting only polynomial memory (both quantum and classical).

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Jean-François Biasse, Annamaria Iezzi, Michael J. Jacobson Jr. 2018-06-10. A note on the security of CSIDH. https://arxiv.org/abs/1806.03656

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