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arXiv:2507.00159 · Physical Review Applied

Broadband optical time-domain reflectometry for security analysis of quantum key distribution

Klim D. Bondar, Ivan S. Sushchev, Daniil S. Bulavkin, Kirill E. Bugai, Anna S. Sidelnikova, Dmitri Melkonian, Veronika M. Vakhrusheva, Dmitriy A. Dvoretskiy

Published May 29, 2025• 2 citations
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Abstract

In this paper, we present a method for security analysis against the Trojan-horse attacks (THA) launched to a practical fiber-based quantum key distribution (QKD) system across a wide spectral range. To achieve this, we utilize optical time-domain reflectometry (OTDR) for the spectral reflectance analysis in the near-infrared range 1100 - 1800 nm with centimeter-level resolution and with a noise floor down to -80 dB. Finally, the total theoretical-security analysis against the THA side channel considering the spectral reflectance and transmittance data over this spectral range is conducted. To the best of our knowledge, our OTDR setup and the corresponding results are the first of their kind in a wide spectral range.

DOI
10.1103/368v-t34p

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In this paper, we present a method for security analysis against the Trojan-horse attacks (THA) launched to a practical fiber-based quantum key distribution (QKD) system across a wide spectral range. To achieve this, we utilize optical time-domain reflectometry (OTDR) for the spectral reflectance analysis in the near-infrared range 1100 - 1800 nm with centimeter-level resolution and with a noise floor down to -80 dB. Finally, the total theoretical-security analysis against the THA side channel considering the spectral reflectance and transmittance data over this spectral range is conducted. To the best of our knowledge, our OTDR setup and the corresponding results are the first of their kind in a wide spectral range.
Implementations of quantum key distribution (QKD) need vulnerability assessment against loopholes in their optical scheme. Most of the optical attacks involve injecting or receiving extraneous light via the communication channel. An eavesdropper can choose her attack wavelengths arbitrarily within the quantum channel passband to maximize the attack performance, exploiting spectral transparency windows of system components. Here we propose a wide-spectrum security evaluation methodology to achieve full optical spectrum safety for QKD systems. This technique requires transmittance characterization in a wide spectral band with a high sensitivity. We report a testbench that characterizes insertion loss of fiber-optic components in a wide spectral range of 400–2300 nm and up to 70 dB dynamic range. To illustrate practical application of the proposed methodology, we give a full Trojan-horse attack analysis for some typical QKD system configurations and discuss briefly induced-photorefraction and detector-backflash attacks. Our methodology can be used for certification of QKD systems.
The Trojan-horse attacks on quantum key distribution (QKD) systems are known to be a threat to secure key sharing between two parties (Alice and Bob). To protect a QKD system against such attacks a variety of passive optical and active optical devices embedded in Alice’s and Bob’s setup are widely used. However, a security analysis against the Trojan-horse attack is required to ensure the unconditional security of a QKD system. We present a security analysis against the Trojan-horse attacks on a practical fiber-based QKD phase-coding system in 400–1700 nm spectral range. The obtained results made it possible to calculate the higher limit of average photon number returned to Eve and the probabilities of Eve’s success of the attack on a particular QKD system.
This paper presents a theoretical and experimental demonstration of a security analysis of a Trojan-horse attack (THA) on a real-world quantum key distribution (QKD) system. We show that the upper bound on the information leakage depends solely on the fidelity between the states of the adversary. We find the lower bound for fidelity between THA states in both the polarization- and phase-coding BB84 protocols, considering both pure and mixed states. Our bounds depend only on the mean photon number per pulse available to an adversary. We also present an experimental analysis of a QKD system, including optical time-domain reflectometry measurements with centimeter resolution and spectral transmittance measurements for optical defense elements ranging from 1100 to 1800 nm with a noise floor lower than − 100 dB. Finally, by considering the optimal attack, we obtain the value of the mean photon number per pulse available to an adversary and calculate the key leakage that needs to be eliminated during the privacy amplification procedure. Published by the American Physical Society 2024
An eavesdropper Eve may probe a quantum key distribution (QKD) system by sending a bright pulse from the quantum channel into the system and analyzing the back-reflected pulses. Such Trojan-horse attacks can breach the security of the QKD system, if appropriate safeguards are not installed or if they can be fooled by the Eve. We present a risk analysis of such attacks based on extensive spectral measurements, such as transmittance, reflectivity, and detection sensitivity of some critical components used in a typical QKD systems. Our results indicate the existence of wavelength regimes, where the attacker gains considerable advantage as compared to launching an attack at 1550 nm. We also propose countermeasures to reduce the risk of such attacks.
Abstract The results of investigation of the spectral characteristics of some passive fiber-optic components, such as optical isolators, circulators, spectral filters and multiplexers, used in quantum key distribution (QKD) systems and contributing to the protection of QKD system from active optical probing attacks are presented. An assessment of the required degree of isolation is also given, which ensures reliable protection of the QKD system against Trojan-horse attacks under the most stringent conditions for legitimate users. Based on the formulated requirements and measurement results, the effectiveness of protection using the investigated passive insulating components is analyzed. Using the example of calculating the insulation for the QKD system scheme presented in the work, the achievement of the required level of isolation (150 dB) in the entire considered spectral range is shown.
Vulnerabilities of components used in quantum-key-distribution systems affect its implementation security and must be taken into consideration during system development and security analysis. In this paper, we investigate transmission of fiber optical elements, which are commonly used in quantum-key-distribution systems for designing countermeasures against Trojan-horse attacks, in the 150015001500--210021002100-nm\mathrm{nm}nm range. As a result, we find loopholes in their transmission spectra that open possibilities for eavesdropping. We also consider a simple passive countermeasure based on the violation of total internal reflection in a single-mode fiber, that provides additional insertion losses of at least 60 dB for double-pass Trojan-horse probe pulses for wavelengths longer than 1830 nm.