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Journal of Experimental and Theoretical Physics Letters

Study of the Vulnerability of Neutral Optical Filters Used in Quantum Key Distribution Systems against Laser Damage Attack

S. V. Alferov, Kirill E. Bugai, Паргачёв Иван Андреевич

Published Jul 1, 2022• 8 citations
Publisher

Abstract

Quantum key distribution systems are accessible to an eavesdropper; i.e., the eavesdropper can attack not only quantum states but also the equipment. In the former case, vulnerabilities are removed by fundamental constraints (e.g., no-cloning theorem) or theoretical foundations for their removal exist. In the latter case, vulnerability is determined by the practical implementation of equipment. An attack with laser damage of optical components called laser damage attack can allow the eavesdropper to reduce the attenuation of optical elements and compromise distributed keys. In this work, a method based on neutral optical filters has been described for protection from laser damage attack against optical components. A scheme imitating actions of the eavesdropper in the process of laser damage attack has been presented. An approach has been proposed to calculate the parameters of optical elements after the attack. Processes occurring in samples exposed to intense laser radiation have been analyzed. The analysis of experimental data has indicated the stability and constraints of neutral optical filters used in quantum key distribution systems for protection from laser damage attack.

DOI
10.1134/s0021364022601117

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Quantum key distribution systems are accessible to an eavesdropper; i.e., the eavesdropper can attack not only quantum states but also the equipment. In the former case, vulnerabilities are removed by fundamental constraints (e.g., no-cloning theorem) or theoretical foundations for their removal exist. In the latter case, vulnerability is determined by the practical implementation of equipment. An attack with laser damage of optical components called laser damage attack can allow the eavesdropper to reduce the attenuation of optical elements and compromise distributed keys. In this work, a method based on neutral optical filters has been described for protection from laser damage attack against optical components. A scheme imitating actions of the eavesdropper in the process of laser damage attack has been presented. An approach has been proposed to calculate the parameters of optical elements after the attack. Processes occurring in samples exposed to intense laser radiation have been analyzed. The analysis of experimental data has indicated the stability and constraints of neutral optical filters used in quantum key distribution systems for protection from laser damage attack.
An attack on equipment with laser damage of optical components called " laser damage attack", can allow the eavesdropper to reduce the attenuation of optical elements and compromise distributed keys. A method of protection against this attack based on a device with a collapsing mirror has been considered. The conclusion based on experimental data about the effectiveness of the proposed method of protection has been made.. Keywords: quantum key distribution, laser damage attack, eavesdropper, symmetric cryptography.
Many quantum key distribution systems employ a laser followed by an optical attenuator to prepare weak coherent states in the source. Their mean photon number must be precalibrated to guarantee the security of key distribution. Here we experimentally show that this calibration can be broken with a high-power laser attack. We test four fiber-optic attenuator types used in quantum key distribution systems, and find that two of them exhibit a permanent decrease in attenuation after laser damage. This results in higher mean photon numbers in the prepared states and may allow an eavesdropper to compromise the key.
A well-protected and characterized source in a quantum key distribution system is needed for its security.Unfortunately, the source is vulnerable to light-injection attacks, such as Trojan-horse, laser-seeding, and laser-damage attacks, in which an eavesdropper actively injects bright light to hack the source unit.The hacking laser could be a high-power one that can modify properties of components via the laserdamage attack and also further help the Trojan-horse and other light-injection attacks.Here we propose a countermeasure against the light-injection attacks, consisting of an additional sacrificial component placed at the exit of the source.This component should either withstand high-power incoming light while attenuating it to a safe level that cannot modify the rest of the source, or get destroyed into a permanent high-attenuation state that breaks up the line.We demonstrate experimentally that off-the-shelf fiberoptic isolators and circulators have these desired properties, at least under attack by a continuous-wave high-power laser.
Quantum communication protocols are considered secure provided that all devices included in the system are fully characterized, and side channels are closed. However, as a result of laser radiation exposure, it is possible to change quantum communication systems components’ characteristics. This leads to vulnerabilities appearing in the quantum key distribution system. In this paper, we consider the effect of pulsed laser radiation on fiber-optic isolators used in quantum communication systems. Isolators protect the source of the system from attacking optical radiation coming from the “eavesdropping” side via the quantum channel. Lowering the isolation factor can bring the entire system out of a secure state. This gives an eavesdropper access to information about the secret key. The scenario of the most probable attack to the source of the quantum key distribution system via a pulsed laser was simulated. The experimental setup provided exposure of fiber isolators with pulsed laser radiation at a wavelength of 1064 nm (within the transparency window of the isolators) with a mean power up to 840 mW in four different pulse generation modes. The isolation factor and throughput of tested samples were monitored using a laser diode with a wavelength of 1550 nm and average power of 10.5 mW. Spectrally selective splitters were used to separate the lasers. It is shown that the isolation factor (isolator attenuation in the direction from the quantum channel to the system) at a wavelength of 1550 nm decreases from the initial value of 59.1 dB to 24.5 dB. The throughput (in the direction from the system to the quantum channel) at the same wavelength decreases from 0.6 dB to 1.2–12.3 dB or remains the same, depending on the acting pulsed laser radiation parameters. Temperature monitoring showed that the temperature of the isolator body changes insignificantly when exposed to pulsed radiation. The obtained effects of changing the isolation coefficient and throughput can be explained by the presence of nonlinear effects in the magneto-optical crystal of the isolator. The results of the work can be applicable in the practical evaluation of quantum communication systems security, in particular, the security evaluation of quantum key distribution systems. The results can be used to prepare the standards for certification procedures for assessing the security of quantum communication systems. The work gives recommendations for enhancing signal source unit security in quantum communication systems. Namely, as countermeasures to protect against the effects of pulsed laser radiation, it is proposed to set optical fuses with a limited threshold power, detectors for monitoring input optical radiation power, and narrow-band optical filters at the entrance of the quantum communication system.