Anqi Huang

Quantum key distribution (QKD) establishes a shared secret between remote parties and is proven unbreakable in theory. Unfortunately, practical implementations of QKD exhibit device imperfections that lead to security vulnerabilities. Most of these vulnerabilities have been verified in a white-box testing scenario, when one has access to the system hardware for analysis. Here we implement automated penetration testing of a QKD system in a black-box setting, using only its public communication lines and a limited operator’s manual. Our implementation parses information transmitted over the classical communication line and toggles an optical delay in the quantum communication line. This enables it to tamper with the timing settings of detector gates in the QKD system during its calibration procedure and to passively eavesdrop on 98.97% of the sifted key. The entire testing process is fully automated and takes only minutes to initiate eavesdropping. Our work paves the way for automated penetration testing of QKD installations as a method of security verification.

In the transmitter of a quantum key distribution (QKD) system, a dense wavelength-division multiplexer (DWDM) is typically used to combine quantum and synchronization signals and is directly connected to the quantum channel. As a result, it becomes the first optical component exposed to laser-injection attacks. Therefore, understanding the behavior of DWDMs under such attacks is essential for assessing the practical security of QKD systems. In this work, we systematically investigate the characteristics of DWDMs under high-power laser illumination. Our experimental results show that certain DWDM samples exhibit pronounced changes in their spectral features once the injected laser power surpasses a specific threshold. Taking the Trojan-horse attack as an illustrative example, we further perform a theoretical analysis of the resulting spectral side channel and show that it can reduce the maximum secure transmission distance to below 66.9% of its original value. By combining experimental observations with theoretical modeling, this study advances the understanding of the influence of DWDMs on the practical security of QKD systems.

Quantum key distribution systems offer cryptographic security, provided that all their components are thoroughly characterized. However, certain components might be vulnerable to laser-damage attacks, particularly when subjected to untested laser parameters. Here, we show that exposing 1550-nm fiber-optic isolators to 1061-nm sub-nanosecond pulsed illumination with 1.16 W average power permanently degrades their isolation performance at 1550 nm, while their forward transparency is less affected. One experimental sample was exposed to 17 mW average power picosecond attacking pulses that temporarily reduced its isolation below the specified guaranteed minimum value. This indicates a potential security threat under these attacking laser regimes that need to be addressed by improving security analysis for various light-injection attacks.

The intensity correlations due to imperfect modulation during the quantum-state preparation in a measurement-device-independent quantum key distribution (MDI QKD) system compromise its security performance. Therefore, it is crucial to assess the impact of intensity correlations on the practical security of MDI QKD systems. In this work, we propose a theoretical model that quantitatively analyzes the secure key rate of MDI QKD systems under intensity correlations. Furthermore, we apply the theoretical model to a practical MDI QKD system with measured intensity correlations, which shows that the system struggles to generate keys efficiently under this model. We also explore the boundary conditions of intensity correlations to generate secret keys. This study extends the security analysis of intensity correlations to MDI QKD protocols, providing a methodology to evaluate the practical security of MDI QKD systems.