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.
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.
Modern information and communication technology (ICT), including internet, smart phones, cloud computing, global positioning system, e-commerce, e-Health, global communications and internet of things (IoT), all rely fundamentally - for identification, authentication, confidentiality and confidence - on cryptography. However, there is a high chance that most modern cryptography protocols will be annihilated upon the arrival of quantum computers. This necessitates taking steps for making the current ICT systems secure against quantum computers. The task is a huge and time-consuming task and there is a serious probability that quantum computers will arrive before it is complete. Hence, it is of utmost importance to understand the risk and start planning for the solution now. \n \nAt this moment, there are two potential paths that lead to solution. One is the path of post-quantum cryptography: inventing classical cryptographic algorithms that are secure against quantum attacks. Although they are hoped to provide security against quantum attacks for most situations in practice, there is no mathematical proof to guarantee unconditional security (`unconditional security' is a technical term that means security is not dependent on a computational hardness assumption). This has driven many to choose the second path: quantum cryptography (QC). \n \nQuantum cryptography - utilizing the power of quantum mechanics - can guarantee unconditional security in theory. However, in practice, device behavior varies from the modeled behavior, leading to side-channels that can be exploited by an adversary to compromise security. Thus, practical QC systems need to be security evaluated - i.e., scrutinized and tested for possible vulnerabilities - before they are sold to customers or deployed in large scale. Unfortunately, this task has become more and more demanding as QC systems are being built in various style, variants and forms at different parts of the globe. Hence, standardization and certification of security evaluation methods are necessary. Also, a number of compatibility, connectivity and interoperability issues among the QC systems require standardization and certification which makes it an issue of highest priority. \n \n \nIn this thesis, several areas of practical quantum communication systems were scrutinized and tested for the purpose of standardization and certification. At the source side, the calibration mechanism of the outgoing mean photon number - a critical parameter for security - was investigated. As a prototype, the pulse-energy-monitoring system (PEMS) implemented in a commercial quantum key distribution (QKD) machine was chosen and the design validity was tested. It was found that the security of PEMS was based on flawed design logic and conservative assumptions on Eve's ability. Our results pointed out the limitations of closed security standards developed inside a company and highlighted the need for developing - for security - open standards and testing methodologies in collaboration between research and industry. \n \nAs my second project, I evaluated the security of the free space QKD receiver prototype designed for long-distance satellite communication. The existence of spatial-mode-efficiency-mismatch side-channel was experimentally verified and the attack feasibility was tested. The work identified a methodology for checking the spatial-mode-detector-efficiency mismatch in these types of receivers and showed a simple, implementable countermeasure to block this side-channel. \n \nNext, the feasibility of laser damage as a potential tool for eavesdropping was investigated. After testing on two different quantum communication systems, it was confirmed that laser damage has a high chance of compromising the security of a QC system. This work showed that a characterized and side-channel free system does not always mean secure; as side-channels can be created on demand. The result pointed out that the standardization and certification process must consider laser-damage related security critical issues and ensure that it is prevented. \n \nFinally, the security proof assumptions of the detector-device-independent QKD (ddiQKD) protocol - that restricted the ability of an eavesdropper - was scrutinized. By introducing several eavesdropping schemes, we showed that ddiQKD security cannot be based on post selected entanglement. Our results pointed out that testing the validity of assumptions are equally important as testing hardware for the standardization and certification process. \n \nSeveral other projects were undertaken including security evaluation of a QKD system against long wavelength Trojan-horse attack, certifying a countermeasure against a particular attack, analyzing the effects of finite-key-size and imperfect state preparation in a commercial QKD system, and experimental demonstration of quantum fingerprinting. All of these works are parts of an iterative process for standardization and certification that a new technology - in this case, quantum cryptography- must go through before being able to supersede the old technology - classical cryptography. I expect that after few more iterations like the ones outlined in this thesis, security of practical QC will advance to a state to be called unconditional and the technology will truly be able to win the trust to be deployed on large scale.
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.
Cryptography plays a key role in today's digital society. The most widely used encryption method is the public-key algorithm, but its security relies on unproven computational assumptions. In contrast, quantum key distribution (QKD) is an unbreakable encryption algorithm. QKD has been proven to provide information-theoretic security rigorously based on the laws of physics. Despite the beauty of QKD theory, in practice, QKD still suffers from practical side-channel attacks, due to the imperfections of real-life implementations. Such discrepancy between the theory and the practice is fatal to the security of a QKD system. This thesis addresses a number of the discrepancies in a standard QKD implementation, and it offers efficient countermeasures to resolve these discrepancies. A novel protocol -- measurement-device-independent (MDI) QKD - was proposed to remove all detector side-channel attacks. To apply MDI-QKD in real life, I have proposed practical protocols, conducted the first rigorous finite-key security proof, designed methodologies to optimize parameters, and demonstrated MDI-QKD experimentally. These results allow MDI-QKD mature enough to be implemented in real-life. Based on the framework of MDI-QKD, the only security issue left is the source. To solve the loophole due to the imperfect encoder, I have experimentally implemented a recent protocol, which enables QKD loss-tolerant to source flaws. This experiment for the first time shows secure QKD with imperfect state preparations at long distances. Another vulnerable part in the source is the ability to generate true randomness. I have developed a compact and cost-effective quantum random number generator (QRNG) prototype, which has a real-time generation rate of 1 Gbit/s with excellent stability. The simplicity and speed of my prototype prove the feasibility of a robust, low-cost, and fast QRNG. In summary, MDI-QKD enables new scientific developments in the field of quantum cryptography. This thesis solves the major practical issues in the implementation of MDI-QKD. The combination of MDI-QKD, imperfect sources, and QRNG consists of the foundation of a future practical side-channel-free QKD protocol. The success of this protocol can substitute current classical cryptosystems by offering people an unconditionally secure infrastructure, which will be the ultimate dream for researchers in secure communication.
As more aspects of modern society depend on digital communication, we increasingly rely on infrastructure that ensures the privacy and security of this communication. Classically, this has been provided by cryptographic protocols such as public-key encryption, in which secrets called keys are exchanged between different parties to enable secure communication. The rapid development of quantum algorithms which violate the assumptions of these protocols, however, poses a security challenge to modern cryptography. \n \nQuantum resources can also be used to strengthen cryptographic security, particularly the security of key exchange protocols. This approach, QKD, can be implemented by encoding in quantum systems such as single photons sent through free-space or a fiber. Fiber based QKD devices are already commercially available, but are fundamentally limited to distributing keys over a few hundred kilometers. To address this distance limitation, research QKD systems are being developed to exchange keys through free-space to satellites. This work considers practical challenges to building and testing both types of QKD devices. \n \nFirstly, we consider modeling and mission analysis for airborne demonstrations of QKD to stratospheric balloons and aircraft to simulate a satellite. Based on the mission parameters available for both platforms, we found aircraft platforms were more promising for testing prototype QKD satellite systems. We developed a mission planning tool to help design the flight geometries for testing the device. \n \nNext, we developed three new components for a QKD satellite prototype. The requirements for electro-optical devices in orbit are very different from lab environments, mandating new approaches to designing QKD devices. We developed a quad single photon detector package to meet the requirements for free-space links to low earth orbit. Moreover, we designed and built optical systems for analyzing the polarization of photons and an adaptive optics unit to increase the efficiency of collecting the encoded photons. All three devices were tested in conditions that simulated the time and loss of a satellite pass. \n \nFinally, we demonstrated a laser damage attack on a live commercial QKD system. Our attack injected additional optical power into the sender device to modify security-critical components. Specifically, our attack damaged the PIN diodes which monitor the encoded photon number, reducing their sensitivity or completely blinding them. Our damage could compromise the entire key, and was performed during system operation while raising no alarms. \n \nIn summary, this work shows the trade-offs of testing QKD payloads on different airborne platforms, develops components for a satellite QKD payload, and demonstrates a security vulnerability in a commercial QKD system that can fully compromise the key. These results help address practical challenges to building QKD devices, improving the security of modern cryptography.
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.
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).