Quantum technologies are driving more and more interest both in academia and industry thanks to their promising performances in terms of security. In reality, practical systems suffer from imperfections compared to theoretical models which could be exploited by an eavesdropper if no protection is implemented. Bridging the gap between theory and practice is therefore one of the main challenges of this field today. During this thesis, I worked on different aspects of the practical security of quantum technologies. This ranges from the modeling of the entropy source of a quantum random number generator (QNRG) to the study of the vulnerabilities of quantum key distribution (QKD) implementations against hacking. In the first part, I focus on the modeling of a commercial QRNG chip from ID Quantique. More specifically, I present the model we developed for the quantum entropy source of the device. We estimate that this fully integrated device can provide a quantum min-entropy of 0.98 per bit. Importantly, this near-unity quantum entropy is achievable without post-processing reducing the power consumption of the chip, making it suitable for mobile devices. Moreover, we show that this high-quality entropy is robust against imperfections. On the side of QKD security, I begin by studying the behavior of negative-feedback avalanche diode (NFAD) detectors under a blinding attack. After showing their vulnerability to this attack and testing the resilience of a countermeasure based on the monitoring of the mean photocurrent, I present an improved countermeasure. This allows Bob to detect more advanced blinding strategies where Eve modulates her blinding power to reduce her impact on the mean photocurrent. In the last part of this thesis, I present a novel method to prevent the blinding attack based on statistical measurements with a multi-pixel detector. Through this approach, we can estimate an upper bound on Eve's information on the key exchanged. An analysis of the finite-key effects estimates that this countermeasure can be effective for distances up to 250km. The applicability of this countermeasure with current technology is shown with a 2-pixel superconducting detector.
Quantum technologies are driving more and more interest both in academia and industry thanks to their promising performances in terms of security. In reality, practical systems suffer from imperfections compared to theoretical models which could be exploited by an eavesdropper if no protection is implemented. Bridging the gap between theory and practice is therefore one of the main challenges of this field today. During this thesis, I worked on different aspects of the practical security of quantum technologies. This ranges from the modeling of the entropy source of a quantum random number generator (QNRG) to the study of the vulnerabilities of quantum key distribution (QKD) implementations against hacking. In the first part, I focus on the modeling of a commercial QRNG chip from ID Quantique. More specifically, I present the model we developed for the quantum entropy source of the device. We estimate that this fully integrated device can provide a quantum min-entropy of 0.98 per bit. Importantly, this near-unity quantum entropy is achievable without post-processing reducing the power consumption of the chip, making it suitable for mobile devices. Moreover, we show that this high-quality entropy is robust against imperfections. On the side of QKD security, I begin by studying the behavior of negative-feedback avalanche diode (NFAD) detectors under a blinding attack. After showing their vulnerability to this attack and testing the resilience of a countermeasure based on the monitoring of the mean photocurrent, I present an improved countermeasure. This allows Bob to detect more advanced blinding strategies where Eve modulates her blinding power to reduce her impact on the mean photocurrent. In the last part of this thesis, I present a novel method to prevent the blinding attack based on statistical measurements with a multi-pixel detector. Through this approach, we can estimate an upper bound on Eve's information on the key exchanged. An analysis of the finite-key effects estimates that this countermeasure can be effective for distances up to 250km. The applicability of this countermeasure with current technology is shown with a 2-pixel superconducting detector.
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.
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.
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.
Cryptography is essential for secure communication in the digital era. Today, public-key cryptography is widely employed, and has provided an efficient method for encrypting content and ensuring both confidentiality and authenticity of electronic communications. However, the security of these systems is based on assumptions of computational hardness within the constraints of current computing capability. Thus, as quantum computing becomes a reality, public-key algorithms will be genuinely vulnerable to attack. By contrast, quantum cryptography, which is based on quantum physics instead of mathematical assumptions, is able to achieve information-theoretic security. \n \nAdvances in practical quantum cryptographic systems have not kept pace with theory, where an eavesdropper can relatively easily exploit loopholes in practical implementations to compromise theory-proved security. Bridging the gap between perfect theory and imperfect practice has become a priority for the growing field of quantum key distribution (QKD), which has strived to strengthen the practical security of QKD systems. Among all the countermeasures against quantum hacking, the measurement-device-independent (MDI) QKD protocol is promising because it is immune to all side-channel attacks on measurement devices. However, the MDI QKD protocol has some limitations that critically restrict its practical usefulness. Technically, the MDI scheme is not compatible with existing QKD systems, and produces a low key rate. In addition, the theory underlying MDI QKD security is based on the use of trusted source stations. Thus, this protocol is not a universal solution. This thesis further investigates the practical security of quantum cryptography in and beyond MDI quantum cryptography. \n \nTo overcome the technical limitations of MDI QKD, we first evaluate two other countermeasures against imperfect detections. The first is an industrial patch based on random detection efficiency, recently implemented by ID Quantique in the commercial Clavis2 QKD system. While powerful, experimental testing shows that this countermeasure is not sufficient to defeat the detector blinding attack. The second countermeasure aims to achieve a higher key rate than MDI QKD while maintaining the same security properties. However, our research shows that detector-device-independent (DDI) QKD security is not equivalent to that of MDI QKD and, further, that DDI QKD is insecure against detector side-channel attacks. \n \nWhile this initial work points to the superior performance of MDI QKD systems, core challenges remain. The fundamental security assumption adopted for MDI QKD systems, regarding the exclusive use of trustable source stations, cannot always be satisfied in practice. Our study revealed several side channels of source devices. The first is disclosed from the implementation of a decoy-state protocol, which is widely used in QKD systems with weak coherent sources. The pump-current-modulated intensities result in a timing mismatch between the signal and decoy states, violating the key assumption in the decoy-state QKD protocol. Moreover, an active Eve can break the basic assumption about photon numbers in the QKD system. In this work, we experimentally demonstrate a laser seeding attack on the laser source, which shows that Eve can increase the emission power of the laser diode. Furthermore, by shining a high-power laser into an optical attenuator, Eve can decrease the attenuation values. The increase in laser emission power and the decrease in attenuation leads to an increase in mean photon numbers. \n \nIn summary, MDI QKD is a milestone in quantum cryptography. However, this thesis indicates the importance of continued investigations into the practical security of MDI QKD. The analysis of practical security should be extended to other countermeasures against side-channel attacks and the source stations in MDI QKD systems. Practical quantum hacking and security analysis promote the development of quantum cryptographic systems, which will eventually achieve the unconditional security claimed in theory.
The present thesis aimed to evaluate the efficiency and resilience of quantum cryptographic systems against malicious attacks, with emphasis on Quantum Key Distribution (QKD).Initially, fundamental concepts of quantum computing and quantum inform ation were examined, such as qubits, superposition, entanglement, decoherence and quantum gates, in order to establish the necessary theoretical background for the reader.Subsequently, key QKD protocols, such as BB84, E91, B92, BBM92 and Decoy-State techniques, were analyzed and compared in terms of security, practical applicability, and sensitivity to different categories of attacks.The methodology of the thesis was based on bibliographic and comparative analysis, using evaluation metrics such as Quantum Bit Error Rate (QBER), secret key generation rate, throughput, transmission distance, and channel losses.The analysis showed that simple theoretical attacks, such as intercept-resend, are usually more easily detectable due to an increase in QBER, whereas practical implementation attacks, such as detector blinding, time-shift, trojan-horse and photon-number-splitting attacks, can be more dangerous because they exploit device imperfections and do not always cause an obvious increase in errors.In conclusion, the theoretical security of QKD protocols is strong; however, their actual efficiency depends on the quality of implementation, technological limitations and the balance between protection and the final rate of secure key generation.
This doctoral thesis summarizes research in quantum cryptography done at the Department of Electronics and Telecommunications at the Norwegian University of Science and Technology (NTNU) from 1998 through 2007.\n\nThe opening parts contain a brief introduction into quantum cryptography as well as an overview of all existing single photon detection techniques for visible and near infrared light. Then, our implementation of a fiber optic quantum key distribution (QKD) system is described. We employ a one-way phase coding scheme with a 1310 nm attenuated laser source and a polarization-maintaining Mach-Zehnder interferometer. A feature of our scheme is that it tracks phase drift in the interferometer at the single photon level instead of employing hardware phase control measures. An optimal phase tracking algorithm has been developed, implemented and tested. Phase tracking accuracy of +-10 degrees is achieved when approximately 200 photon counts are collected in each cycle of adjustment. Another feature of our QKD system is that it uses a single photon detector based on a germanium avalanche photodiode gated at 20 MHz. To make possible this relatively high gating rate, we have developed, implemented and tested an afterpulse blocking technique, when a number of gating pulses is blocked after each registered avalanche. This technique allows to increase the key generation rate nearly proportionally to the increase of the gating rate. QKD has been demonstrated in the laboratory setting with only a very limited success: by the time of the thesis completion we had malfunctioning components in the setup, and the quantum bit error rate remained unstable with its lowest registered value of about 4%.\n\nMore than half of the thesis is devoted to various security aspects of QKD. We have studied several attacks that exploit component imperfections and loopholes in optical schemes. In a large pulse attack, settings of modulators inside Alice's and Bob's setups are read out by external interrogating light pulses, without interacting with quantum states and without raising security alarms. An external measurement of phase shift at Alice's phase modulator in our setup has been demonstrated experimentally. In a faked states attack, Eve intercepts Alice's qubits and then utilizes various optical imperfections in Bob's scheme to construct and resend light pulses in such a way that Bob does not distinguish his detection results from normal, whereas they give Bob the basis and bit value chosen at Eve's discretion. Construction of such faked states using several different imperfections is discussed. Also, we sketch a practical workflow of breaking into a running quantum cryptolink for the two abovementioned classes of attacks. A special attention is paid to a common imperfection when sensitivity of Bob's two detectors relative to one another can be controlled by Eve via an external parameter, for example via the timing of the incoming pulse. This imperfection is illustrated by measurements on two different single photon detectors. Quantitative results for a faked states attack on the Bennett-Brassard 1984 (BB84) and the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocols using this imperfection are obtained. It is shown how faked states can in principle be constructed for quantum cryptosystems that use a phase-time encoding, the differential phase shift keying (DPSK) and the Ekert protocols. Furthermore we have attempted to integrate this imperfection of detectors into the general security proof for the BB84 protocol. For all attacks, their applicability to and implications for various known QKD schemes are considered, and countermeasures against the attacks are proposed.\n\nThe thesis incorporates published papers [J. Mod. Opt. 48, 2023 (2001)], [Appl. Opt. 43, 4385 (2004)], [J. Mod. Opt. 52, 691 (2005)], [Phys. Rev. A 74, 022313 (2006)], and [quant-ph/0702262].