On the structural stability of phase-coded quantum cryptography against detector-blinding attacks

Quantum key distribution (QKD) systems are open systems. Thus, an eavesdropper can actively influence the elements of the system through the communication line, altering their regular operation. One known attack is an attack with detector blinding. With such an attack, the eavesdropper can obtain the entire key and remain undetectable. The paper proposes a fundamentally new method of detecting and protecting quantum cryptography systems with phase coding against the attack with avalanche detector blinding. The proposed method is based on diverting a part of the input signal in the receiving terminal to bypass the interferometer and deliver it to the same avalanche detectors. Measuring the detector response in different time windows, both for the bypassed signal and the normal operation range, one can effectively identify the ongoing detector blinding attack. The method does not require radical changes in existing quantum cryptography systems and is quite universal, since it can be used for any phase-coded quantum key distribution protocol. The method provides a fundamental strategy of detecting and protecting against the attack at the level of the quantum key distribution protocol, and not at the level of technical ‘patches’.