ABSTRACT Trojan horse attacks provide an effective means for the eavesdropper to access the sender which has a significant threat to the continuous variable quantum communication protocols. This study specifically focuses on the adverse effects of Trojan horse attacks on the fidelity of continuous‐variable quantum teleportation. A side‐channel attack scenario is considered in which an adversary injects ancillary photons into the transmission channel or measurement apparatus to probe or disrupt legitimate quantum signals. A comprehensive theoretical model is developed to describe how such injected photons interact with the teleportation channel, altering key parameters such as modulation variance and effective transmittance. Extensive numerical simulations quantify the explicit dependence of teleportation fidelity on the number of injected photons, demonstrating significant performance degradation under high injection rates. Furthermore, the study analyzes and evaluates the effectiveness of several practical countermeasures. This work not only establishes a rigorous theoretical framework for understanding Trojan horse vulnerabilities in quantum teleportation systems, but also provides actionable guidance for designing attack‐resistant quantum communication architectures.
ABSTRACT Trojan horse attacks provide an effective means for the eavesdropper to access the sender which has a significant threat to the continuous variable quantum communication protocols. This study specifically focuses on the adverse effects of Trojan horse attacks on the fidelity of continuous‐variable quantum teleportation. A side‐channel attack scenario is considered in which an adversary injects ancillary photons into the transmission channel or measurement apparatus to probe or disrupt legitimate quantum signals. A comprehensive theoretical model is developed to describe how such injected photons interact with the teleportation channel, altering key parameters such as modulation variance and effective transmittance. Extensive numerical simulations quantify the explicit dependence of teleportation fidelity on the number of injected photons, demonstrating significant performance degradation under high injection rates. Furthermore, the study analyzes and evaluates the effectiveness of several practical countermeasures. This work not only establishes a rigorous theoretical framework for understanding Trojan horse vulnerabilities in quantum teleportation systems, but also provides actionable guidance for designing attack‐resistant quantum communication architectures.
We present a Trojan-horse attack on the practical two-way continuous-variable quantum key distribution system. Our attack mainly focuses on the imperfection of the practical system that the modulator has a redundancy of modulation pulse-width, which leaves a loophole for the eavesdropper inserting a Trojan-horse pulse. Utilizing the unique characteristics of two-way continuous-variable quantum key distribution that Alice only takes modulation operation on the received mode without any measurement, this attack allows the eavesdropper to render all of the final keys shared between the legitimate parties insecure without being detected. After analyzing the feasibility of the attack, the corresponding countermeasures are put forward.
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.
Abstract Early security proofs of the keys in quantum cryptography systems were based on the assumption that the transmitting and receiving stations were completely isolated from the outside world—the eavesdropper. However, this condition cannot be implemented in practice, since quantum cryptography systems are open systems, in the sense that an eavesdropper can have indirect access, for example, through a fiber communication channel to critical equipment elements (phase modulators, intensity modulators, etc) using active sensing of the state of these elements—Trojan-horse attacks. A new Trojan-horse attack based on joint unambiguous measurements of reflected probing states from an intensity modulator and information states in a quantum communication channel is proposed. Estimates of attack parameters for a number of quantum key distribution protocols are given, when an attack leads to compromise of keys.
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.
Trojan horse attack is a common eavesdropping strategy which can attack various quantum secure communication systems. Its basic idea is to send auxiliary photons into a legitimate communicator's apparatuses and steal information by analyzing the reflected photons. In this paper, we consider a different kind of Trojan horse attack, the so-called counterfactual Trojan horse attack, which has not been studied in detail so far. In such an attack, the eavesdropper may steal the secret information by ``ghost'' photons, which can spontaneously avoid being detected, even if the detector is an ideal one. We present the details and requirements of such an attack. We also illustrate our results by considering two protocols, the ping-pong protocol and the counterfactual quantum cryptography. Furthermore, we discuss the nature of the counterfactual Trojan horse attack and the strategy to successfully deal with it. Our results indicate that additional resources may be required for the protection against such an attack.
Security analyses of quantum cryptographic protocols typically rely on certain conditions; one such condition is that the sender (Alice) and receiver (Bob) have isolated devices inaccessible to third parties. If an eavesdropper (Eve) has a side channel in one of the devices, then the key rate may be sensibly reduced. In this paper we consider an attack on a coherent-state protocol, where Eve not only taps the main communication channel but also hacks Alice's device. This is done by introducing a Trojan horse mode with low mean number of photons n, which is then modulated in a similar way to the signal state. First we show that this strategy can be reduced to an attack without side channels but with higher loss and noise in the main channel. Then we show how the key rate rapidly deteriorates for increasing photons n, being halved at long distances each time n+1 doubles. Our work suggests that Alice's device should also be equipped with sensing systems that are able to detect and estimate the total number of incoming and outgoing photons.