On the Spectral theory of Isogeny Graphs and Quantum Sampling of Secure Supersingular Elliptic curves

TL;DR

This paper introduces quantum algorithms for sampling secure supersingular elliptic curves, leveraging spectral graph theory to ensure security and efficiency, with implications for cryptography and quantum verification protocols.

Contribution

It presents the first provable quantum polynomial-time algorithms for sampling supersingular elliptic curves with high probability, based on spectral delocalization and eigenvector properties.

Findings

Quantum algorithms achieve $ ilde{O}( ext{log}^4 p)$ complexity under certain assumptions.

Spectral delocalization results prove the Quantum Unique Ergodicity conjecture for isogeny graphs.

Eigenvalue separation properties improve security assumptions in isogeny-based cryptography.

Abstract

In this paper, we study the problem of sampling random supersingular elliptic curves with unknown endomorphism rings. This problem has recently gained considerable attention as many isogeny-based cryptographic protocols require such ``secure'' curves for instantation, while existing methods achieve this only in a trusted-setup setting. We present the first provable quantum polynomial-time algorithms for sampling such curves with high probability, one of which is based on an algorithm of Booher et. al. One variant runs heuristically in $\tilde{O}(\log^{4} p)$ quantum gate complexity, and in $\tilde{O}(\log^{13} p)$ under the Generalized Riemann Hypothesis, and outputs a curve that is provably secure assuming average-case hardness of the endomorphism ring problem. Another variant samples uniform $\mathcal{O}$-oriented curves with unknown endomorphism rings, for any imaginary quadratic order $\mathcal O$, with security based on the hardness of Vectorization problem. When accompanied by an interactive quantum computation verification protocol our algorithms provide a secure instantiation of the CGL hash function and related primitives. Our analysis relies on a new spectral delocalization result for supersingular $\ell$-isogeny graphs: we prove the Quantum Unique Ergodicity conjecture and provide numerical evidence for complete eigenvector delocalization. We also prove a stronger $\varepsilon$-separation property for eigenvalues of isogeny graphs than that predicted in the quantum money protocol of Kane, Sharif, and Silverberg, thereby removing a key heuristic assumption in their construction.