Penetration testing of a quantum key distribution system as a black box

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.