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arXiv:1506.01989 · Physical Review X

Practical Security Bounds Against the Trojan-Horse Attack in Quantum Key Distribution

Marco Lucamarini, I. Choi, M. B. Ward, J. F. Dynes, Z. L. Yuan, A. J. Shields

Published Sep 9, 2015• 175 citations
arXivHTMLPDF

Abstract

In the quantum version of a Trojan-horse attack, photons are injected into the optical modules of a quantum key distribution system in an attempt to read information direct from the encoding devices.To stop the Trojan photons, the use of passive optical components has been suggested.However, to date, there is no quantitative bound that specifies such components in relation to the security of the system.Here, we turn the Trojan-horse attack into an information leakage problem.This allows us to quantify the system security and relate it to the specification of the optical elements.The analysis is supported by the experimental characterization, within the operation regime, of reflectivity and transmission of the optical components most relevant to security.

DOI
10.1103/physrevx.5.031030

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In the quantum version of a Trojan-horse attack, photons are injected into the optical modules of a quantum key distribution system in an attempt to read information direct from the encoding devices.To stop the Trojan photons, the use of passive optical components has been suggested.However, to date, there is no quantitative bound that specifies such components in relation to the security of the system.Here, we turn the Trojan-horse attack into an information leakage problem.This allows us to quantify the system security and relate it to the specification of the optical elements.The analysis is supported by the experimental characterization, within the operation regime, of reflectivity and transmission of the optical components most relevant to security.
Abstract Most security proofs of quantum key distribution (QKD) disregard the effect of information leakage from the users’ devices, and, thus, do not protect against Trojan-horse attacks (THAs). In a THA, the eavesdropper injects strong light into the QKD apparatuses, and then analyzes the back-reflected light to learn information about their internal setting choices. Only a few recent works consider this security threat, but predict a rather poor performance of QKD unless the devices are strongly isolated from the channel. Here, we derive finite-key security bounds for decoy-state-based QKD schemes in the presence of THAs, which significantly outperform previous analyses. Our results constitute an important step forward to closing the existing gap between theory and practice in QKD.
As the existence of non-zero reflection coefficients in the real component of continuous-variable quantum key distribution (CV-QKD) systems, Eve can probe the system by sending the bright light pulses into Alice’s set-up. With the analysis of back-reflections, Eve only takes a few back-reflected photons to intercept information and obtain the raw key bit. In this paper, the attack problems are converted into the information leakage problems. First, we analyzed the Trojan horse attacks with different wavelengths and confirmed its side effects, such as crosstalk and anti-Stokes Raman scattering, by a numerical simulation. Then, based on the wavelength-dependent property of beam splitter, we presented a practical way to estimate the deviation of shot noise and therefore correct the excess noise by inserting different wavelength pulses under joint attacks. Finally, we specified the security bounds of the system through quantifying the excess noise bounds caused by the Trojan horse attacks and provided a theoretical reference for the secret key transmission of system. As a consequence, the transmission errors within the security bounds can be negligible and the legitimate users will not perceive the presence of Eve.
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
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.
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.
Quantum secure direct communication (QSDC) enables the direct transmission of confidential information without pre-shared keys. However, device imperfections in practical systems can lead to information leakage. The Trojan-horse attack (THA), a stealthy side-channel threat, steals information by injecting photons into encoding devices and analyzing the reflected light. This paper focuses on the single-photon-based (DL04) protocol, establishing for the first time a comprehensive THA model against it. By combining weak coherent pulse (WCP) sources with the decoy-state method, we systematically analyze the security under THA during both the first and second rounds of transmission, derive analytical expressions for the secrecy message capacity, and quantitatively evaluate the impact of attack strength on system performance through numerical simulations. Our results demonstrate that the system’s security is highly sensitive to the average number of reflected Trojan photons, μout. For a first-round attack, the secrecy message capacity remains nearly identical to the attack-free case when μout≤10−8, but drops to zero when μout increases to 10−3. In contrast, the second round of attacks had a weaker impact. Even at the strongest attack intensity (μout=10−2), the system could still maintain a certain level of secrecy message capacity. When THA simultaneously targets both the first and second rounds, the overall security of the system is predominantly governed by the more sensitive first-round attack, whose security boundary remains largely comparable to that observed in a single first-round attack scenario. Furthermore, We perform parameter optimization to achieve the optimal secrecy message capacity for each channel attenuation. Our study provides the first quantitative security bound for THA on DL04 protocol, offering practical guidance for encoder isolation requirements in future implementations.