Detection of blinding attack in quantum key distribution

Security of an ideal system for quantum key distribution can be formally proved. However, technological imperfections of real systems can be misused by an eavesdropper to get information about the key without causing a detectable change in the error rate. An example of this is the blinding attack where the eavesdropper manipulates detectors of the receiving party. To avoid such an attack, technical modifications of the device are usually required. In this paper we show that an extended measurement, similar to the Bell test measurement, can reveal the blinding attack without any modification of the device.