ИСТИНА

For examining QKD systems for sustainability to the “backflash” attack, it is necessary to measure the probability of photon re-emission and calculate the maximum possible probability of backflash for the secret key to remain secure. In this paper, we present research on backflash probabilities dependencies on parameters of fiber-based QKD systems. The SPAD gate width for each QKD system and quantum communication protocol is individual, therefore, it is necessary to consider the dependence of the probability of photon re-emission on the SPAD gate width. Moreover, as a result of Laser Damage Attack, active elements, such as a variable attenuator or a pulsed laser, may not work quite stable, eventually, as far as the mean photon number per pulse (μ) varies within a small range, the maximum backflash probability can fluctuate. We demonstrate an experimental setup of correlation “backflash” measurements by optical reflectometry with very low dark count rate and parasitic noise, furthermore, the contribution of reflected photons is negligible in comparison with the classical optical reflectometry scheme. Therefore, the signal-to-noise ratio was increased by at least two orders of magnitude. The obtained experimental data demonstrate the variation of the backflash probability of the single-photon detector depending on the SPAD gate width and the mean photon number per pulse sent by Alice to Bob. Analysis of the calculated backflash probabilities enables to estimate the maximum possible information leakage, depending on the parameters of equipment. In the future, the obtained experimental results can be used to adjust the optimal parameters of QKD systems to guarantee communication protocol security.