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International Conference Laser Optics (ICLO)

Laser Damage Attack on a Simple Optical Attenuator Widely Used in Fiber-based QKD Systems

Kirill E. Bugai, Zyzykin A.P., Daniil S. Bulavkin, S.A. Bogdanov, Ivan S. Sushchev, D.A. Dvoretskiy

Published Jun 20, 2022• 5 citations
Publisher

Abstract

We present the results of an experimental study of laser damage attack (LDA) from the CW laser power up to 5.5 W at a wavelength of 1561 at a simple type of commercially available attenuator widely used in telecommunications and fiber-based QKD systems.
DOI
10.1109/iclo54117.2022.9839749

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The security of quantum key distribution (QKD) systems relies on the physical integrity of their components. While laser-damage attacks (LDAs) using high-power continuous-wave (cw) lasers have been well studied, the threat posed by pulsed lasers at alternative wavelengths remains underestimated. Here, we experimentally investigated the stability of four types of fiber-optic attenuators under exposure to sub-picosecond pulses at 1061 nm with average power reaching 1 W. Mechanical variable attenuators with blocking elements and fixed air-gap attenuators show resistance to this attack. MEMS-based variable attenuators exhibit increased attenuation or irreversible damage that causes a permanent reduction in attenuation of approximately 3.8 dB. For fixed attenuators with an absorption element, we demonstrate that initial pulsed irradiation significantly lowers the optical damage threshold of the components compared to direct cw attacks. The attenuation reduction achieved is up to 7 dB at a 1 W cw laser at 1550 nm. These results highlight the possibility of establishing a hidden side-channel for eavesdropping attacks and underscore the insufficiency of existing countermeasures against sophisticated LDA scenarios.
Quantum key distribution (QKD) systems provide quantum-safe key exchange. Therefore, complete security analysis of implementations of QKD protocols is in the focus of interest of a worldwide information-security community. For today, a number of QKD loopholes are closed by countermeasures, which are also considered in emerging QKD security evaluation and certification [1]–[3]. However, new threats to practical QKD implementations are still found, such as the laser-damage attack, which is a powerful hacking strategy. In investigations, CW laser radiation is most often used, but in contrast to it, the interaction of pulsed laser radiation with optical materials may lead to a wide range of effects, like nonlinear effects, dielectric breakdown, etc.
We report a new type of vulnerability based on a physical principle that has not been previously exploited in quantum hacking—optical pumping of a laser in practical implementations of quantum key distribution (QKD) systems. We show that it is possible to increase the pulse energy of a source laser diode not only by injection-locking it with external light near its emission wavelength of 1550 nm, but also by optically pumping it at a much shorter wavelength. We experimentally demonstrate a 10% increase in pulse energy when exposing the laser diode to continuous-wave (cw) laser light at 1310 nm with a power of 1.6 mW via its fiber pigtail. This effect may allow an eavesdropper to steal the secret key. A possible countermeasure is to install broadband optical filters and isolators at the source output and to characterize them during security certification.
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