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Security Evaluation of Practical Quantum Communication Systems

Shihan Sajeed

Published Sep 29, 2017• 1 citation
OpenAlex

Abstract

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.

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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. 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ARPN Journal of Engineering and Applied Sciences•Jan 20, 2026•0 citations
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.
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.
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.
Quantum cryptography has emerged as a revolutionary technology for ensuring secure communication in the era of quantum computing. While existing research primarily focuses on theoretical frameworks and small-scale experimental setups, significant challenges remain in practical implementation, scalability, and security vulnerabilities. This study aims to bridge the gap between theory and real-world deployment by developing robust quantum cryptographic protocols that address key challenges such as noise management, side-channel attacks, and Trojan horse attacks. Additionally, we propose an optimized quantum key distribution (QKD) mechanism that ensures secure communication over long distances under realistic conditions. Our research integrates post-quantum cryptography with quantum cryptographic techniques to provide a hybrid security model that is resilient against both classical and quantum computing threats. By leveraging commercially available quantum hardware and advanced randomness extraction methods, this study contributes to the development of scalable, secure, and efficient quantum communication networks. The findings of this research will play a crucial role in advancing secure digital communication systems and fortifying data security in the post-quantum era.
Internet has become a global means of communication, turned our reality upside down. It has transformed communications, to the level that it is now our chosen standard of communication. Although there are many ways to protect information from undesired access, Quantum cryptography can be used to unconditionally secure data communications by applying the laws of quantum physics which overcomes the drawbacks of traditional classical cryptography, which depends on mathematical methods to limit attacks such as eavesdroppers. Quantum cryptography is considered to be a future replica of classical cryptography. In this survey paper, we will discuss about various protocols of cryptography. Also discussing about their vulnerabilities.
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.
Quantum computing poses a crucial threat against traditional cryptographic systems because it creates substantial weakness in existing encryption standards. This paper examines the quantum cryptography field by studying its three main frameworks, including Quantum Key Distribution (QKD) and post-quantum cryptographic (PQC) algorithms and hybrid security approaches. The research paper evaluates quantum- based protocols BB84 and E91 and PQC schemes using lattice and code methods in addition to machine learning-enhanced cryptography by analyzing existing works. The paper explores critical problems involving high execution expenses alongside scalability limitations and infrastructure compatibility problems, as well as side-channel attack susceptibility. Healthcare practitioners must focus on developing standardized deployment techniques alongside strong implementation strategies because these measures become essential for IoT and smart cities. This paper brings together insights from more than twenty-four fundamental studies to simplify the understanding of existing conditions and establish directions for secure quantum communication systems.