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arXiv:2510.16868 · Physical Review A

Security against Trojan-horse attacks on a self-compensating all-fiber polarization modulator

Alberto De Toni, Aynur Cemre Aka, Costantino Agnesi, Davide G. Marangon, Giuseppe Vallone, Paolo Villoresi

Published Apr 22, 2026• 0 citations
arXivHTMLPDF

Abstract

Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Unlike classical cryptographic systems, the security of QKD is not reliant on computational assumptions but is instead rooted in the fundamental laws of physics. In a QKD protocol, any attempt by an eavesdropper to intercept the key is detectable: this provides an unprecedented level of security, making QKD an attractive solution for secure communication in an era increasingly threatened by the advent of quantum computers and their potential to break classical cryptographic systems. However, QKD also faces several practical challenges such as transmission loss and noise in quantum channels, finite key size effects, and implementation flaws in QKD devices. Addressing these issues is crucial for the large-scale deployment of QKD and the realization of a global quantum Internet. A whole body of research is dedicated to the hacking of the quantum state source, for example using (THAs), where the eavesdropper injects light into the system and analyzes the back-reflected signal. In this paper, we study the vulnerabilities against THAs of the iPOGNAC encoder, first introduced in [M. Avesani , ], to propose adapted countermeasures that can mitigate such attacks.

DOI
10.1103/bgdd-1vl9

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Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Unlike classical cryptographic systems, the security of QKD is not reliant on computational assumptions but is instead rooted in the fundamental laws of physics. In a QKD protocol, any attempt by an eavesdropper to intercept the key is detectable: this provides an unprecedented level of security, making QKD an attractive solution for secure communication in an era increasingly threatened by the advent of quantum computers and their potential to break classical cryptographic systems. However, QKD also faces several practical challenges such as transmission loss and noise in quantum channels, finite key size effects, and implementation flaws in QKD devices. Addressing these issues is crucial for the large-scale deployment of QKD and the realization of a global quantum Internet. A whole body of research is dedicated to the hacking of the quantum state source, for example using (THAs), where the eavesdropper injects light into the system and analyzes the back-reflected signal. In this paper, we study the vulnerabilities against THAs of the iPOGNAC encoder, first introduced in [M. Avesani , ], to propose adapted countermeasures that can mitigate such attacks.
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.
Quantum key distribution (QKD), which theoretically provides unconditional secure communication, has developed rapidly in the past decades. However, the practical QKD systems still have vulnerabilities due to device imperfections. Polarization-encoding systems are one of the most important branches in the field of QKD. To date, a variety of polarization-encoding schemes have been proposed to meet the requirements of high speed and robust modulation, but the security analysis of these schemes is often ignored. In this paper, we experimentally and theoretically evaluated the security of three typical polarization encoders against the Trojan horse attack (THA), which is recognized as one of the critical quantum attacks. We built a system to implement the THA and collected the reflection from QKD configuration with different encoders. According to the experimental results, we further analyzed the secure key rate and the resistance against THAs of the system while BB84 or measurement-device-independent QKD was applied. Our paper plays an important guiding role in the design of polarization encoders and the construction of secure QKD systems with realistic devices.
Quantum Key Distribution (QKD) leverages the principles of quantum mechanics to provide theoretically unconditional security for cryptographic key sharing. However, practical implementations remain vulnerable due to non-ideal devices and potential security loopholes at both the source and detection sides of QKD systems. The side-channel-secure (SCS) protocol addresses these challenges by encoding bits in vacuum and non-vacuum states and introducing a third-party measurement node, thereby repelling attacks targeting the detection side as well as external lab attacks on the source side. In this work, we consider the state-dependent correlated errors and Trojan-horse attack while preserving the SCS protocol's key advantage-specifically, requiring only upper bounds on intensities characterization without needing a full description of quantum states in infinite dimensions. Numerical results demonstrate that when the reflected light intensity from Trojan-horse attacks falls below 10−610^{-6}10−6, Eve can scarcely extract additional key information from the reflections. This work makes the SCS protocol more practical.
Quantum key distribution (QKD) leverages the principles of quantum mechanics to provide theoretically unconditional security for cryptographic key sharing. However, practical implementations remain vulnerable due to potential security loopholes at both the source and detection sides of QKD systems. The side-channel-secure (SCS) protocol addresses these challenges by encoding bits in vacuum and non-vacuum states and introducing a third-party measurement node, thereby repelling attacks targeting the detection side as well as external lab attacks on the source side. In this work, we consider the state-dependent correlated errors and Trojan-horse attack while preserving the SCS protocol’s key advantage—-specifically, requiring only upper bounds on intensity characterization without needing a full description of quantum states in infinite dimensions. Numerical results demonstrate that when the reflected light intensity from Trojan-horse attacks falls below 10 −6 , Eve can scarcely extract additional key information from the reflections. This work makes the SCS protocol more practical.
Measurement-device-independent quantum key distribution (MDI QKD) is a promising method for remote key sharing that can eliminate all detector side-channel attacks. However, current security proofs often overlook the potential information leakages from the legitimate users' devices. In a quantum version of Trojan-horse attack (THA), a malicious eavesdropper can inject bright light into the sources of a MDI QKD system and then analyze the back-reflected light to obtain their setting choices, thereby compromising the final security. Here, we derive the finite-key security bounds of decoy-state MDI QKD in the presence of THA, which significantly outperform previous analyses in terms of the secret-key rate and transmission distance. Specifically, we analyze a symmetric three-intensity decoy-state MDI QKD protocol and an efficient four-intensity decoy-state MDI QKD protocol. Our results represent a fundamental step in guaranteeing the implementation security of quantum communication systems.
The security proofs of the Quantum Key Distribution (QKD) protocols make certain assumptions about the operations of physical systems. Thus, appropriate modelling of devices to ensure that their operations are consistent with the models assumed in the security proof is imperative. In this paper, we explore the Trojan horse attack (THA) using Measurement Device Independent (MDI) QKD integrated photonic chips and how to avoid some of the security vulnerabilities using only on-chip components. We show that a monitor photodiode paired appropriately with enough optical isolation, given the sensitivity of the photodiode, can detect high power sniffing attacks. We also show that the placement of amplitude modulators with respect to back reflecting components and their switching time can be used to thwart a THA.