  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.
