QUANTUM SECURE COMMUNICATION USING POLARIZATION HOPPING MULTI-STAGE PROTOCOLS

This dissertation presents a study of the security and performance of a quantum communication system using multi-stage multi-photon tolerant protocols. Multi-stage protocols are a generalization of the three-stage protocol proposed in 2006 by Subhash Kak. Multi-stage protocols use “Polarization Hopping,” which is the process of changing the polarization state at each stage of transmission. During the execution of a multi-stage protocol, the message transfer always starts by encoding a bit of information in a polarization state; for example, bit 0 is encoded using state |0⟩ and bit 1 is encoded using state|1⟩ whereas, on the channel, the state of polarization is given by α|├ 0⟩┤+β|├ 1⟩┤. In the following α and β are restricted to the real numbers i.e., the polarization stays on the equator of the Poincare sphere. A transformation applied by one communicating party at a given stage will result in new values of α and β. This dissertation analyzes the security of multi-stage, multi-photon tolerant protocols and proposes an upper bound on the average number of photons per pulse in the cases where Fock states and the cases where coherent states are used in the implementation of the three-stage protocol. The derived average number of photons is the maximum limit at which the three-stage protocol can operate at a quantum secure level while operating in a multi-photon domain. In addition, this dissertation studies the vulnerability of the multi-stage protocol to the Trojan horse attack, Photon Number splitting attack (PNS), Amplification attack, as well as the man-in-the middle attack. Moreover, this dissertation proposes a modified version of the multi-stage protocol. This modified version uses an initialization vector and implements a chaining mode between consecutive implementations of the protocol. The modified version is proposed in the case of the three-stage protocol and named a key/message expansion four variables three-stage protocol. The proposed nomenclature is based on the fact that an additional variable is added to secure the three-stage protocol. The introduction of this additional variable has the potential to secure the multi-stage protocol in the multi-photon regime. It results in the eavesdropper having a set of simultaneous equations where the number of variables exceeds the number of equations. The dissertation also addresses the performance of the multi-stage, multi-photon tolerant protocol. An average photon number of 1.5 photon/stage is used to calculate the maximum achievable distance and key transfer rates while using the single-stage protocol over fiber optic cables. We compute the increase in distance as well as data transfer rate while using the single-stage protocol. Channel losses as well as the detector losses are accounted for. Finally, an application of the multi-stage protocol in IEEE 802.11 is proposed. This application provides wireless networks with a quantum-level of security. It proposes the integration of multi-stage protocols into the four-way handshake of IEEE 802.11

Security Proofs for Quantum Key Distribution Protocols by Numerical Approaches

This thesis applies numerical methods to analyze the security of quantum key distribution (QKD) protocols. The main theoretical problem in QKD security proofs is to calculate the secret key generation rate. Under certain assumptions, this problem has been formulated as a convex optimization problem and numerical methods have been proposed to produce reliable lower bounds for discrete-variable QKD protocols. We investigate the applicability of these numerical approaches and apply the numerical methods to study a variety of protocols, including measurement-device-independent (MDI) protocols, variations of the BB84 protocol with a passive countermeasure against Trojan horse attacks, and the phase-encoding BB84 protocol using attenuated laser sources without continuous phase randomization

The Security of Practical Quantum Key Distribution

Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key between authorized partners connected by a quantum channel and a classical authenticated channel. The security of the key can in principle be guaranteed without putting any restriction on the eavesdropper's power. The first two sections provide a concise up-to-date review of QKD, biased toward the practical side. The rest of the paper presents the essential theoretical tools that have been developed to assess the security of the main experimental platforms (discrete variables, continuous variables and distributed-phase-reference protocols).Comment: Identical to the published version, up to cosmetic editorial change

Design and implementation of a high-speed free-space quantum key distribution system for urban scenarios

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid. Facultad de Ciencias, Departamento de Física de Materiales. Fecha de lectura: 21-06-201

Security Evaluation of Practical Quantum Communication Systems

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. At 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). Quantum 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. In 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. As 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. Next, 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. Finally, 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. Several 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

Practical free-space quantum key distribution

Within the last two decades, the world has seen an exponential increase in the quantity of data traffic exchanged electronically. Currently, the widespread use of classical encryption technology provides tolerable levels of security for data in day to day life. However, with one somewhat impractical exception these technologies are based on mathematical complexity and have never been proven to be secure. Significant advances in mathematics or new computer architectures could render these technologies obsolete in a very short timescale. By contrast, Quantum Key Distribution (or Quantum Cryptography as it is sometimes called) offers a theoretically secure method of cryptographic key generation and exchange which is guaranteed by physical laws. Moreover, the technique is capable of eavesdropper detection during the key exchange process. Much research and development work has been undertaken but most of this work has concentrated on the use of optical fibres as the transmission medium for the quantum channel. This thesis discusses the requirements, theoretical basis and practical development of a compact, free-space transmission quantum key distribution system from inception to system tests. Experiments conducted over several distances are outlined which verify the feasibility of quantum key distribution operating continuously over ranges from metres to intercity distances and finally to global reach via the use of satellites