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arXiv:2508.15197

The practical issues of side-channel-secure quantum key distribution

Cong Jiang, Hu, Xiao-Long, Zong‐Wen Yu, Xu Hai, Xiang‐Bin Wang

Published Aug 21, 2025• 0 citations
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Abstract

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 $10^{-6}$, Eve can scarcely extract additional key information from the reflections. This work makes the SCS protocol more practical.

DOI
10.48550/arxiv.2508.15197

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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.
Quantum key distribution (QKD) theoretically provides unconditional security between remote parties. However, guaranteeing practical security through device characterization alone is challenging in real-world implementations due to the multidimensional spaces in which the devices may be operated. The side channel secure (SCS) QKD protocol, which only requires bounding the upper limits of the intensities for the two states, theoretically provides a rigorous solution to the challenge and achieves measurement-device-independent security in detection and security for whatever multidimensional side channel attack in the source. Here, we demonstrate a practical implementation of SCS-QKD, achieving a secure key rate of 6.67 kbps through a 50.5 km fiber and a maximum distribution distance of 101.1 km while accounting for finite-size effects. Our experiment also represents an approximately 40 times improvement over the previous experiment.
Quantum key distribution (QKD) is secure in principle, but practical security can be undermined by discrepancies between real devices and the idealized models assumed in security proofs. Source side channels, including those exploited by Trojan-horse attacks, are particularly detrimental: neglecting them compromises implementation security, whereas accounting for them reduces performance. Here we propose a QKD source that is intrinsically robust against side-channel attacks. Unlike existing passive and modulator-free schemes, it requires neither post-selection of the emitted pulses nor devices with a perfect extinction ratio to suppress side channels, and it does not introduce correlations between the intensity and the encoded bit or basis. Consequently, under idealized source assumptions commonly adopted in proposals for existing schemes, conventional decoy-state security analyses apply directly, yielding substantially higher key rates. Our proposal appears to be within reach of current technology and therefore provides a clear and practical path toward implementation-secure QKD.
The side-channel-secure (SCS) quantum key distribution (QKD), SCS QKD has attracted much attention due to its higher security compared to the measurement-device-independent protocols. It is not only measurement-device-independent secure, but also secure against source-state side channels provided that Eve has no access to Alice's or Bob's setups inside their labs. In the original protocol for SCS QKD, the side channels and the intensities of the coherent sources are allowed to be different in different time windows, but the vacuum sources of Alice and Bob have to be perfect, which is impossible in practice due to the finite extinction ratio of the intensity modulators. In this paper, we present a method that does not need the perfect vacuum source requested by the original protocol and assures the result as secure as the original protocol. With the upper bounds of the amplitudes of the nonvacuum part of the sources, the secure key rate of our SCS QKD protocol here with imperfect vacuum and unstable sources can be calculated. The numerical results show that, the secure distance of the SCS QKD protocol exceeds 150 km, provided that the intensity of the imperfect vacuum source is less than ${10}^{\ensuremath{-}8}$, which can be achieved in the experiment by a two-stage intensity modulator, where we also show that the active odd-parity pairing method and standard pairing method using two-way classical communication can be applied to the SCS QKD protocol to improve the key rate. Given the side-channel security based on the existing technological level, this work makes it possible to realize side-channel-secure QKD with real devices.
Semi-quantum key distribution (SQKD) protocols are used to distribute secret keys between a quantum party and a classical party. However, existing SQKD protocols rely on two-way communication, and may still be vulnerable to Trojan horse side-channel attacks where Eve sends her own photon into a receiver's apparatus and measures the reflected photon to estimate the key. In this paper, we propose a practical SQKD with one-way key. This requires that the single photons travelling through the one-way channel are used to encode bit information, and the returned photons are used to quantify Eve's information, thus reducing the security analysis of the Trojan horse attack in SQKD. Meanwhile, our protocol with one basis enjoys security advantage in practical SQKD systems when source flaws are taken into account. In particular, the present protocol is secure under practical conditions when weak coherent pulses (WCP) are used. Our simulation results show that the protocol using WCP can distribute secret keys over a distance of 110 km without decoy states.
Based on the idea that there is no side channel in the vacuum state, the side-channel-free quantum key distribution (SCFQKD) protocol was proposed, which is immune to all attacks in the source side-channel space and all attacks in the detectors. In the original SCFQKD protocol, an important assumption is that Alice and Bob can produce the perfect vacuum pulses. But due to the finite extinction ratio of the intensity modulators, the perfect vacuum pulse is impossible in practice. In this paper, we solve this problem and make the quantum key distribution side-channel secure with real source device which does not emit perfect vacuum pulses. Our conclusion only depends on the upper bounds of the intensities of the sources. No other assumptions such as stable sources and stable side channels are needed. The numerical results show that, comparing with the results of SCFQKD protocol with perfect vacuum sources, the key rates and secure distance are only slightly decreased if the upper bound of the intensity of the imperfect vacuum source is less than 10−810^{-8}10−8 which can be achieved in experiment by two-stage intensity modulator. We also show that the two-way classical communication can be used to the data post-processing of SCFQKD protocol to improve the key rate. Specially, the active odd-parity pairing method can improve the key rates in all distances by about two times and the secure distance by about 40 km. Give that the side channel security based on imperfect vacuum, this work makes it possible to realize side channel secure QKD with real devices.