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Quantum cryptography and quantum cryptanalysis

Vadim Makarov

Published Jan 1, 2007• 15 citations
OpenAlex

Abstract

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].

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75•Jan 1, 2010•1 citation
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].
The main goals of cryptography are the encryption of messages to render them unintelligible to third parties and the authentication of messages to certify that they have not been modified. These goals can be accomplished if the sender ("Alice") and recipient ("Bob") both possess a secret random binary digit (bit) known as "key". It is essential that Alice and Bob acquire the key material with a high level of confidence that any third party ("Eve") does not have even partial information about the random bit sequence. If Alice and Bob communicate solely through classical messages (as opposed to Quantum cryptography), it is impossible for them to generate a certifiably secret key. QKD are the new generation of cryptographic systems which allow two remote parties (Alice and Bob) to generate a secret key with privacy guaranteed by quantum mechanics. They generate a random key securely over an optical fiber connection (also known as Quantum channel). This random key is then used for encryption and decryption of confidential messages, which then can be sent in encrypted form over any non-secure communication channel. In this thesis, we study two fiber-based QKD systems namely "oneway" and "two-way". Both systems have their unique advantage which distinguish them to one another. In one-way, the complexity of the electronic system may reduce. However more attention has to be made on the optical setup due to the requirement of active compensation. In the two-way, the requirement of optical setup may reduce but the attention moves to the electronic system which requires precise and short pulse especially for high speed in Alice system configuration. Our developed prototype is capable to support either one-way and two-way QKD system. We also solved some of the issues from the previous prototype Kumar [2008] which limited the system to be used in high speed. For instance: the synchronization system now uses a single synchronization signal per frame; the frame initialization time delay is reduced to 140ms per frame; pulse shaping distortion due to current consumption. We also introduced the security perspective for B92 protocol with uninformative states. This is done by utilizing the security analysis for BB84 such as entanglement distillation protocol (EDP) [...], smooth Ra'©nyi entropy [...] and composable security [...]. Numerous proposal on smooth Ra'©nyi entropy as general case [...] either for finite security analysis [...] or in asymptotic limit [...] assist us to deduce finite security perspective for B92 with uninformative states. [...] This thesis is organized as follow, initially starts with a general introduction to the cryptography and its relation with quantum cryptography. This is elaborated in Chapter One. In the Second Chapter we will go through the background of quantum mechanics and quantum information and introduce some parameters and theory mostly used in Quantum Key Distribution. These include quantum measurement, state behavior and the security analysis parameters. The second chapter will give the background concepts for the QKD in perspective of quantum information. The Third Chapter will explore more detailed information towards QKD. It starts with the basic architecture algorithm of the quantum cryptography system and details each components of the architecture. Later we focus on the security analysis specifically for the B92 protocol. Finally in the chapter, we will make some finite element analysis for the B92 protocol with uninformative states. In Chapter Four and onwards, we are discussing the experimental implementation and analysis. We begin with the heart of our system which is the field programmable gate array (FPGA) system. We explain the detailed architecture of our FPGA system and how the system works. The module that we develop in our FPGA system in order to work inside the QKD system is also explained in detail. We reserve the modification and advanced work of this system for the future by giving the original codes in the Appendix A. Chapter Five is one of the shortest chapters in this thesis. This chapter explains our electronic development and the opto-electronic device which are used in the QKD system. The main device that we develop for the QKD system is our Mezzanine board for FPGA. This mezzanine board supports some functions that are not available from the FPGA in order to make the system functional. Other developments include opto-electronic board and proportional-integral-derivative (PID) controller with the current driver. Chapter Six is the main experimental part. In this Chapter we start to give the introduction to our optical setup. We detail out our configuration of the setup and finally show our results taken from experimental work. Finally in Chapter Seven we conclude our work and purposed future work which actually need to done for the system.
Work presents the brief review of the contemporary state of quantum cryptography science. Main principles of the quantum cryptography protocol development are illustrated by giving the example of BB84 protocol. Examples of experimental setups for the quantum key distribution through the atmosphere and fiber quantum channel are described in this work. The work presents results of experiments conducted on the developed setup based on single photon polarization coding. Problem of single photon detection in the visible and near IR area is considered. Work describes developed fiber based quantum cryptography working with the telecom wavelength 1555 nm. With the use of this experimental setup using the 25 km single mode telecom fiber (1555 nm wavelength) between sender and receiver we achieved the 450 bit/s quantum key generation with the use of 5 MHz pulse repetition rate, ~0.2 photon/pulse. Given the achieved photodetector parameters an average quantum bit error rate is less than 3.7%.
In quantum cryptosystems, variations in detector efficiency can be exploited to stage a successful attack. This happens when the efficiencies of Bob's two detectors are different functions of a control parameter accessible to Eve (e.g., timing of the incoming pulses). It has previously been shown that the Bennett-Brassard 1984 (BB84) protocol is vulnerable to this attack. In this paper, we show that several other protocols and encodings may also be vulnerable. We consider a faked states attack in the case of a partial efficiency mismatch on the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol, and derive the quantum bit error rate as a function of detector efficiencies. Additionally, 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.
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.
We report several vulnerabilities found in Clavis2, the flagship quantum key distribution (QKD) system from ID Quantique. We show the hacking of a calibration sequence run by Clavis2 to synchronize the Alice and Bob devices before performing the secret key exchange. This hack induces a temporal detection efficiency mismatch in Bob that can allow Eve to break the security of the cryptosystem using faked states. We also experimentally investigate the superlinear behaviour in the single-photon detectors (SPDs) used by Bob. Due to this superlinearity, the SPDs feature an actual multi-photon detection probability which is generally higher than the theoretically-modelled value. We show how this increases the risk of detector control attacks on QKD systems (including Clavis2) employing such SPDs. Finally, we review the experimental feasibility of Trojan-horse attacks. In the case of Clavis2, the objective is to read Bob's phase modulator to acquire knowledge of his basis choice as this information suffices for constructing the raw key in the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol. We work in close collaboration with ID Quantique and for all these loopholes, we notified them in advance. Wherever possible, we or ID Quantique proposed countermeasures and they implemented suitable patches and upgrade their systems.
Cryptography has begun its journey into the field of quantum information theory. Classical cryptography has shown weaknesses, which may be exploited in the future, either by development in mathematics, or by quantum computers. Quantum key distribution (QKD) is a promising path for cryptography to enable secure communication in the future. Although the theory of QKD promises absolute security, the reality is that current quantum crypto systems have flaws in them, as perfect devices have proven impossible to build. However, this can be taken into account in security proofs to ensure security, even with flaws. Security loopholes in QKD systems are being discovered as development progresses. Nevertheless, the system being built at NTNU is intended to address them all, creating a totally secure system. During this thesis, work was continued assembling the interferometer which is the basis for encoding qubits. It was fully connected on an optical table, and interference was obtained. Concerning theoretical work, calculations for a photon source specific parameter was carried out. It consisted of expanding previous framework and applying the results in both an established security proof, and a recent generalization of this proof. Two source effects were in focus, the lasers random phase and its fluctuating pulse intensity. Where analytical derivation was no longer possible, Matlab was used for numerical calculations. Under the conditions of the framework and proofs this thesis lies on, randomized phase turned out to have a negligible improvement over the case of non-random phase. Fluctuating amplitude showed a larger effect, reducing system performance. The input parameters were extreme, thus in a realistic situation it should not affect system performance significantly. However, these fluctuations must be taken into account when proving system security.