On Quantum Algorithms for Exact Polynomial System Solving
Semwell Ferrari
Abstract
A large-scale quantum computer, if constructed, would break the public-key cryptography in everyday use outright. It is therefore essential to gather evidence that current post-quantum schemes truly resist quantum attack. For multivariate schemes, whose security rests on the hardness of solving polynomial systems over a finite field (the PoSSo problem), this boils down to asking how far quantum algorithms can accelerate exact solving. The standard approach consists of linearisation methods such as XL and F4, which reduce solving to linear algebra on large Macaulay matrices; performing that linear algebra with a quantum linear-systems solver such as HHL, rather than classically, is the natural route to a speed-up. The ill-conditioning of those matrices, however, undermines the expected gain. We explore an alternative approach based on symbolic homotopy and Newton–Hensel lifting, a symbolic variant of Newton iteration. A numerical quantum version of it is due to Xue, Wu and Guo; we ask whether an exact one, over a finite field, can sidestep the construction of ill-conditioned matrices altogether. Our answer is that, given the current state of the art, it cannot. The thesis assembles the classical and quantum tools needed for a comparative complexity analysis and clarifies the obstacles that stand in the way of exact solving. The evidence gathered is that the quantum paradigms surveyed here do not threaten, at their current parameters, multivariate UOV-based schemes such as MAYO; a quantum attack on them, should one exist, would have to exploit their structure directly.