Mediated Semi-Quantum Key Distribution

TL;DR

This paper introduces a new semi-quantum key distribution protocol enabling classical users to securely establish a shared secret with an untrusted quantum server, demonstrating comparable security to fully quantum protocols.

Contribution

It presents the first secure semi-quantum protocol with an untrusted server, including a security proof and key rate analysis under worst-case adversarial conditions.

Findings

Protocol is unconditionally secure even with an all-powerful adversarial server.

Key rate lower bound depends on observed error rate.

Semi-quantum protocol achieves security comparable to fully quantum protocols.

Abstract

In this paper, we design a new quantum key distribution protocol, allowing two limited semi-quantum or "classical" users to establish a shared secret key with the help of a fully quantum server. A semi-quantum user can only prepare and measure qubits in the computational basis and so must rely on this quantum server to produce qubits in alternative bases and also to perform alternative measurements. However, we assume that the sever is untrusted and we prove the unconditional security of our protocol even in the worst case: when this quantum server is an all-powerful adversary. We also compute a lower bound of the key rate of our protocol, in the asymptotic scenario, as a function of the observed error rate in the channel allowing us to compute the maximally tolerated error of our protocol. Our results show that a semi-quantum protocol may hold similar security to a fully quantum one.