Attacks exploiting deviation of mean photon number in quantum key distribution and coin tossing

TL;DR

This paper investigates how deviations in the mean photon number, due to calibration errors or attacks, impact the security of quantum communication protocols like QKD and coin tossing, highlighting vulnerabilities and implementation challenges.

Contribution

It models the effects of photon number deviations on protocol security, analyzes attack strategies, and examines security flaws in monitoring detectors for practical quantum systems.

Findings

Deviations in photon number can compromise protocol security.

Implementation flaws in monitoring detectors can be exploited by attackers.

Designing secure, loophole-free detectors remains a significant challenge.

Abstract

The security of quantum communication using a weak coherent source requires an accurate knowledge of the source's mean photon number. Finite calibration precision or an active manipulation by an attacker may cause the actual emitted photon number to deviate from the known value. We model effects of this deviation on the security of three quantum communication protocols: the Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol without decoy states, Scarani-Acin-Ribordy-Gisin 2004 (SARG04) QKD protocol, and a coin-tossing protocol. For QKD, we model both a strong attack using technology possible in principle, and a realistic attack bounded by today's technology. To maintain the mean photon number in two-way systems, such as plug-and-play and relativistic quantum cryptography schemes, bright pulse energy incoming from the communication channel must be monitored. Implementation of a monitoring detector has largely been ignored so far, except for ID Quantique's commercial QKD system Clavis2. We scrutinize this implementation for security problems, and show that designing a hack-proof pulse-energy-measuring detector is far from trivial. Indeed the first implementation has three serious flaws confirmed experimentally, each of which may be exploited in a cleverly constructed Trojan-horse attack. We discuss requirements for a loophole-free implementation of the monitoring detector.