Security proof of Counterfactual Quantum Cryptography against General Intercept-resend Attacks and Its Vulnerability

TL;DR

This paper provides a rigorous security proof for counterfactual quantum cryptography against intercept-resend attacks and reveals potential vulnerabilities in practical implementations due to device imperfections.

Contribution

It offers a tighter security proof for CQC against general intercept-resend attacks and identifies vulnerabilities arising from imperfect detectors in real-world setups.

Findings

CQC is secure against intercept-resend attacks under ideal conditions.

Practical CQC systems can be vulnerable to time-shift attacks due to detector inefficiencies.

Negative key rates are possible with imperfect apparatuses under certain attack strategies.

Abstract

Counterfactual quantum cryptography (CQC), recently proposed by Noh, is featured with no transmission of signal particles. This exhibits evident security advantage, such as its immunity to the well known PNS attack. In this paper, the theoretical security of CQC protocol against the general intercept-resend attacks is proved by bounding the information of an eavesdropper Eve more tightly than in Yin's proposal[Phys. Rev. A 82, 042335 (2010)]. It is also showed that practical CQC implementations may be vulnerable when equipped with imperfect apparatuses, by proving that a negative key rate can be achieved when Eve launches a time-shift attack based on imperfect detector efficiency.