SPIE Quantum Technologies

Forward-looking criteria for the certification of quantum key distribution (Conference Presentation)

Marco Lucamarini, J. F. Dynes, Zhiliang L. Yuan, Martin B. Ward, Andrew J. Shields

Published May 29, 2018• 0 citations

Abstract

Technological advances in quantum computers and number theory have the potential to compromise the security of existing cryptographic protocols. Quantum key distribution (QKD) offers the possibility of information theoretic security and is theoretically unbreakable. Therefore it is the natural candidate to face the above digital threat. However, in implementing QKD, it is important to check that the components employed do not deviate from their expected behaviour, to avoid opening the door to new security loopholes [1]. For this reason, it is necessary to characterise the real behaviour of the components, build reliable models and include them in the security analysis. Here we introduce a set of techniques and measurements to ease this characterisation process. We discuss explicit examples applied to the source [2], the boundaries [3] and the detection unit [4] of a QKD apparatus. These methods pave the way to the future certification of QKD systems. [1] K. Tamaki, M. Curty, and M. Lucamarini, “Decoy-state quantum key distribution with a leaky source,” New J. Phys 18, 65008 (2016). [2] J. F. Dynes et al., “Testing the photon-number statistics of a quantum key distribution light source,” arXiv:1711.00440 (2017). [3] M. Lucamarini et al., “Practical Security Bounds Against the Trojan-Horse Attack in Quantum Key Distribution,” Phys. Rev. X 5, 031030 (2015). [4] A. Koehler-Sidki et al., “Setting best practice criteria for self-differencing avalanche photodiodes in quantum key distribution,” SPIE Proc. 10442, Quant. Inf. Sci. Tech. III, 104420L (2017).

DOI: 10.1117/12.2317935

Related papers

Toward standardization of Quantum Key Distribution

Poompong Chaiwongkhot

Information security becomes an inseparable part of our everyday life. An encryption \nmethod widely used today is public-key encryption. The security of this method is based on \na hard to solve mathematical problems against an adversary with limited computational \npower. Such an assumption could be broken as our understanding of the mathematics \nbeing improved or new computation tools being developed. One such tool that poses a \nthreat to the public key encryption is a quantum computer. As a result, a new encryption \nmethod with a new security assumption is required. \nQuantum key distribution is a point-to-point symmetric key distribution method with \nsecurity based on the law of physics. In theory, the key generated by QKD is informationtheoretic secured. However, in practice, physical devices could have flaws or possess some \nbehaviors deviated from the theoretical model. These imperfections could open security \nloopholes for an adversary to exploit, compromising the security. Thus the security verification and system characterization of practical implementation of QKD are necessary. \nThe necessity of this verification is further emphasized as several QKD systems are being \ncommercialized and used in several discrete communication links today. \nTo extend this new encryption system’s practical implementation on a wider network \nscale requires a set of standards or common practices for developers and service providers \nto follow. This set of rules is set to ensure the compatibility of different device models in \nthe network and ensure the security of each component in the system, which would affect \nthe security of the system as a whole. \nTo fulfill standardization and certification criteria, a record of best practice on security \nanalysis, system design, device characterization, and security verification of QKD implementation is required. The research projects throughout my Ph.D. study contribute toward \nthis practice. These studies also address some issues and provide possible solutions to the \ndevelopment of a standard for QKD. This thesis is a collection of six experimental studies \non performance evaluation and security verification of different components of practical \nquantum key distribution systems. \nThe first study is a comparison between the performance of the QKD system with \nquantum dot (QD) as a single-photon source and the performance of QKD with weakcoherent pulsed (WCP) source. The result shows that the QKD with QD could generate \nthe key at higher channel loss than WCP QKD using the same laser source. This result \nshows the potential of QKD with a single-photon source as a candidate for secret key \ndistribution over high channel loss, such as up-link satellite-based QKD. \nThe second study is a theoretical study on the method to characterize the QKD system \nagainst the Trojan-horse attack being considered as a standard for the QKD system. The \nresult shows a possible loophole of this method against a more powerful adversary than \nassumed in the previously proposed model. An improved version of characterization against \na more general form of Trojan-horse attack has been proposed. \nThe third experiment is on the information leakage from a free-space QKD receiver due \nto detector backflash, a photon produced by the detector upon detection. The result shows \nthat the backflash photons carry the information of the ’clicked’ detector that could be \ntransmitted back to the channel and discriminated by Eve. An experimental demonstration \nof this attack has been performed. Countermeasure both in theory and practical setup has \nbeen proposed. \nThe next experiment is on the effect of atmospheric turbulence on Eve’s spatial-mode \ndetection efficiency mismatch attack on the free-space QKD system. We show that, by \nusing a phase-only spatial light modulator (SLM) and hologram created by Zernike polynomials, atmospheric turbulence with various strength covered from sea level to upper \natmosphere can be experimentally emulated in the lab environment. We then use that \nsetup to show the limit of the distance that Eve’s attack is successful. The theoretical \nlimit of the attack distance also shown. \nIn the fifth study, we use the SLM and Zernike polynomial holograms to characterize a \nfree-space QKD system against spatial mode attack. The result shows that, with higherorder spatial modes and finer control of wavefront intensity distribution, Eve could bypass \nthe countermeasure proposed in our previous study. We proposed a more robust version \nof countermeasure against spatial mode attack. The new countermeasure is verified by the \nSLM setup. \nThe last study is on the fake-state attack on the transition edge sensor (TES). The \nresult shows that TES’s voltage response can be deterministically controlled by Eve using \nbright laser through the input channel. It also shows that the photon number result from \nTES can be controlled by Eve. An attack model exploiting this imperfection has been \nshown. \nIn addition to the contribution to the standardization of the QKD system, I hope that \nthe result of this thesis would emphasize the necessity of security verification of the QKD \nsystem and the verification of countermeasure and characterization method against more \ngeneral attack model. Although the unconditional security, promised in theory, could not \nyet be achieved, this loop of hacking and patching should provide us information and insight \non which security level could be claimed from the practical QKD devices implementing \ntoday.

Sep 3, 20200 citations

Finite-key security analysis for quantum key distribution with leaky sources

Weilong Wang, Kiyoshi Tamaki, Marcos Curty

Security proofs of quantum key distribution (QKD) typically assume that the devices of the legitimate users are perfectly shielded from the eavesdropper. This assumption is, however, very hard to meet in practice, and thus the security of current QKD implementations is not guaranteed. Here, we fill this gap by providing a finite-key security analysis for QKD which is valid against arbitrary information leakage from the state preparation process of the legitimate users. For this, we extend the techniques introduced by Tamaki et al (2016 New J. Phys. 18 065008) to the finite-key regime, and we evaluate the security of a leaky decoy-state BB84 protocol with biased basis choice, which is one of the most implemented QKD schemes today. Our simulation results demonstrate the practicability of QKD over long distances and within a reasonable time frame given that the legitimate users' devices are sufficiently isolated.

arXiv:1803.09508New Journal of PhysicsAug 6, 201850 citations

Best-Practice Criteria for Practical Security of Self-Differencing Avalanche Photodiode Detectors in Quantum Key Distribution

A. Koehler-Sidki, J. F. Dynes, Marco Lucamarini +4 more

Although quantum key distribution (QKD) promises information-theoretic security that can never be hacked, several studies have investigated how its security can be compromised by targeting the detectors in the system. Have they found truly fundamental problems with the physics of the scheme, or merely sloppy implementation? This study seeks to define best-practice criteria for these detectors, to distinguish between genuine loopholes and incorrect operation. The authors show that if these directions are followed, many of the attacks previously demonstrated simply do not work---bringing this technology a big step closer to everyday life.

arXiv:1712.06520Physical Review AppliedApr 18, 201820 citations

Quantum Cryptography: From Theory to Practice

Xiongfeng Ma

Quantum cryptography or quantum key distribution (QKD) applies fundamental laws of quantum physics to guarantee secure communication. The security of quantum cryptography was proven in the last decade. Many security analyses are based on the assumption that QKD system components are idealized. In practice, inevitable device imperfections may compromise security unless these imperfections are well investigated.\n\nA highly attenuated laser pulse which gives a weak coherent state is widely used in QKD experiments. A weak coherent state has multi-photon components, which opens up a security loophole to the sophisticated eavesdropper. With a small adjustment of the hardware, we will prove that the decoy state method can close this loophole and substantially improve the QKD performance. We also propose a few practical decoy state protocols, study statistical fluctuations and perform experimental demonstrations. Moreover, we will apply the methods from entanglement distillation protocols based on two-way classical communication to improve the decoy state QKD performance. Furthermore, we study the decoy state methods for other single photon sources, such as triggering parametric down-conversion (PDC) source. Note that our work, decoy state protocol, has attracted a lot of scientific and media interest. The decoy state QKD becomes a standard technique for prepare-and-measure QKD schemes.\n\nAside from single-photon-based QKD schemes, there is another type of scheme based on entangled photon sources. A PDC source is commonly used as an entangled photon source. We propose a model and post-processing scheme for the entanglement-based QKD with a PDC source. Although the model is proposed to study the entanglement-based QKD, we emphasize that our generic model may also be useful for other non-QKD experiments involving a PDC source. By simulating a real PDC experiment, we show that the entanglement-based QKD can achieve longer maximal secure distance than the single-photon-based QKD schemes.\n\nWe propose a time-shift attack that exploits the efficiency mismatch of two single photon detectors in a QKD system. This eavesdropping strategy can be realized by current technology. We will also discuss counter measures against the attack and study the security of a QKD system with efficiency mismatch detectors.

Feb 26, 200921 citations

Security of quantum key distribution with realistic sources

Xoel Sixto

Quantum Key Distribution (QKD) offers information-theoretic security grounded in the laws of quantum mechanics rather than computational assumptions. While its theoretical foundations are robust, practical implementations necessarily deviate from the idealized models assumed in security analyses, opening the door to side-channel attacks and implementation loopholes. In particular, most research to date has focused on mitigating vulnerabilities in the measurement devices, leading to the development of Measurement-Device-Independent QKD (MDI-QKD) and Twin-Field QKD (TF-QKD), which effectively close all receiver-side loopholes. By contrast, the security of the source has remained less explored, despite being critical for the overall integrity of QKD systems. The goal of this thesis is to develop security proofs that explicitly account for realistic imperfections in QKD sources. Special emphasis is placed on three main aspects: correlations between emitted pulses that violate independence assumptions, imperfect phase randomization arising from practical modulation schemes, and leakage of internal settings through passive or active side channels. By formulating rigorous methods to analyze these imperfections, the work aims to provide a framework that enables practitioners to quantify and mitigate implementation risks systematically. Additionally, this thesis seeks to clarify under which conditions the commonly used decoy-state method remains secure, and to what extent source assumptions can be relaxed without compromising performance. This thesis aims to advance the understanding of how real-world deviations impact QKD security and to contribute to the development of protocols and devices. Ultimately, this work is motivated by the ambition to help bring QKD from a theoretically promising idea to a mature technology deployable with confidence in practical scenarios.

Nov 7, 20250 citations

Numerical security analysis for practical quantum key distribution

Álvaro Navarrete, Guillermo Currás-Lorenzo, Margarida Pereira +1 more

Quantum key distribution (QKD) promises information-theoretic security based on quantum mechanics and idealized device models. Practical implementations, however, deviate from these models due to unavoidable device imperfections, and existing security proofs fall short of capturing the complexity of real-world systems. Here we introduce a versatile numerical finite-key security framework valid against general coherent attacks and applicable to a broad class of practical QKD setups. It accommodates most relevant imperfections at both transmitter and receiver, including non-independent-and-identically-distributed (non-IID) signals arising in high-speed QKD systems due to the limited bandwidth of optical modulators, while requiring only partial characterization of the apparatuses. We demonstrate the power of our framework by proving the security of a realistic decoy-state QKD implementation with laser sources, providing a practical route towards rigorous security certification of real-world QKD setups.

quant-pharXiv:2605.12984May 14, 20260 citations