Developers from SFB Lab, together with the Moscow State University Quantum Technology Center, have devised a way to protect quantum keys from photodetector blinding attacks. The method was described in the scientific journal Physical Review Applied, and the development itself was secured with a Russian patent.

A quantum key is a secret code formed from photons, which are the smallest particles of light. These particles are registered by special devices — single-photon detectors, a kind of "traps" that record each incoming photon and convert it into a key bit.

Fraudsters tried to exploit this loophole. Attackers affect the detectors with optical radiation, which takes the device out of normal operation. The detector stops registering photons independently and enters a state where the registration results are controlled by the attacking party. This created risks that an outsider could gain access to the key, and the substitution could go unnoticed for a long time.

The authors of the study proposed a new protection mechanism and made the detector's sensitivity unpredictable. During operation, it constantly and randomly changes. To "blind" the detector, a hacker needs to precisely select the light intensity. But it changes in the detector every second, and it is impossible to guess it.

A failure in such cases signals an alarm, and the system understands that someone tried to interfere with the channel. By counting incidents, it calculates how many key bits the attacker could have obtained and discards only those bits. The entire key remains in place and continues to work. The method has already been tested on a standard serial detector.