70 research outputs found
Implementation vulnerabilities in general quantum cryptography
Quantum cryptography is information-theoretically secure owing to its solid
basis in quantum mechanics. However, generally, initial implementations with
practical imperfections might open loopholes, allowing an eavesdropper to
compromise the security of a quantum cryptographic system. This has been shown
to happen for quantum key distribution (QKD). Here we apply experience from
implementation security of QKD to several other quantum cryptographic
primitives. We survey quantum digital signatures, quantum secret sharing,
source-independent quantum random number generation, quantum secure direct
communication, and blind quantum computing. We propose how the eavesdropper
could in principle exploit the loopholes to violate assumptions in these
protocols, breaking their security properties. Applicable countermeasures are
also discussed. It is important to consider potential implementation security
issues early in protocol design, to shorten the path to future applications.Comment: 13 pages, 8 figure
Long-distance quantum key distribution with imperfect devices
Quantum key distribution (QKD) is one of the most promising techniques for the secure exchange of cryptographic keys between two users. Its unique property of relying on the laws of physics makes it an appealing tool for secure communications. While QKD has been implemented over distances on the order of a few hundreds of kilometers, the transmission rate of the key severely drops, when we go to further distances. An easy solution to this could rely on a network of trusted nodes. This solution, however, is far from ideal. In this thesis, we focus on obtaining long-distance secure communications by using trust-free intermediate nodes between two users. Quantum repeaters will then be at the core of our work and we analytically study different systems under realistic scenarios. We cover a range of repeater setups incorporating quantum memories (QMs), in terms of their short-term and long-term feasibility and in terms of ease of access for end users. We consider the main imperfections of the employed devices. In particular, we consider ensemble-based QMs, which offer a feasible route toward the implementation of probabilistic quantum repeaters. We study the effects of multiple excitations in such QMs and its effects on the key rate in a memory-assisted measurement device- independent QKD (MDI-QKD) system. We then analytically compare the performance of two probabilistic quantum repeater protocols by calculating their secure key rates. We identify under which regimes of operation one system outperforms the other. Source and memory imperfections are considered in our analysis. Finally, we combine a quantum repeater scheme with the MDI-QKD protocol and we derive the largest distances that is possible to reach under practical assumptions. Overall we obtain a realistic account of what can be done with existing technologies in order to improve the reach and the rate of QKD systems within a larger quantum network
Long-Distance Trust-Free Quantum Key Distribution
The feasibility of trust-free long-haul quantum key distribution (QKD) is addressed. We combine measurement-device-independent QKD (MDI-QKD), as an access technology, with a quantum repeater setup, at the core of future quantum communication networks. This will provide a quantum link none of whose intermediary nodes need to be trusted, or, in our terminology, a trust-free QKD link. As the main figure of merit, we calculate the secret key generation rate when a particular probabilistic quantum repeater protocol is in use. We assume the users are equipped with imperfect single photon sources, which can possibly emit two single photons, or laser sources to implement decoy-state techniques. We consider apparatus imperfection, such as quantum efficiency and dark count of photodetectors, path loss of the channel, and writing and reading efficiencies of quantum memories. By optimizing different system parameters, we estimate the maximum distance over which users can share secret keys when a finite number of memories are employed in the repeater setup
Quantum City: simulation of a practical near-term metropolitan quantum network
We present the architecture and analyze the applications of a
metropolitan-scale quantum network that requires only limited hardware
resources for end users. Using NetSquid, a quantum network simulation tool
based on discrete events, we assess the performance of several quantum network
protocols involving two or more users in various configurations in terms of
topology, hardware and trust choices. Our analysis takes losses and errors into
account and considers realistic parameters corresponding to present or
near-term technology. Our results show that practical quantum-enhanced network
functionalities are within reach today and can prepare the ground for further
applications when more advanced technology becomes available
Physical-Layer Security, Quantum Key Distribution and Post-quantum Cryptography
The growth of data-driven technologies, 5G, and the Internet place enormous pressure on underlying information infrastructure. There exist numerous proposals on how to deal with the possible capacity crunch. However, the security of both optical and wireless networks lags behind reliable and spectrally efficient transmission. Significant achievements have been made recently in the quantum computing arena. Because most conventional cryptography systems rely on computational security, which guarantees the security against an efficient eavesdropper for a limited time, with the advancement in quantum computing this security can be compromised. To solve these problems, various schemes providing perfect/unconditional security have been proposed including physical-layer security (PLS), quantum key distribution (QKD), and post-quantum cryptography. Unfortunately, it is still not clear how to integrate those different proposals with higher level cryptography schemes. So the purpose of the Special Issue entitled “Physical-Layer Security, Quantum Key Distribution and Post-quantum Cryptography” was to integrate these various approaches and enable the next generation of cryptography systems whose security cannot be broken by quantum computers. This book represents the reprint of the papers accepted for publication in the Special Issue
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Quantum key distribution beyond the repeaterless secret key capacity
Quantum communications promise to provide information theoretic security in the exchange of information. However, unlike their classical counterpart, they utilise dim optical pulses whose amplification is prohibited. Consequently, their transmission rate and range is confined below a theoretical limit known as repeaterless secret key capacity. Overcoming this limit with today’s technology was believed to be impossible until the recent proposal of Twin-field (TF) quantum key distribution (QKD), a scheme that uses phase-coherent optical signals and an auxiliary measuring station to distribute quantum information. Here, TF-QKD and its main variations are initially explored and compared in simulations, to assess their performance in different attributes. Such schemes are also practically implemented for the first time in two experiments. The first is a proof-of-principle implementation over significant channel losses, in excess of 90 dB. In the second, the setup is developed further and the protocol is implemented over real fibre channels exceeding 600 km in length, representing the first fibre-based secure quantum communication beyond the barriers of 600 km and 100 dB. In both cases, in the high loss/distance regime, the resulting secure key rates exceed the repeaterless secret key capacity, a result never achieved before.EPSRC, Toshiba Research Europ
Practical unconditionally secure signature schemes and related protocols
The security guarantees provided by digital signatures are vital to many modern applications such as online banking, software distribution, emails and many more. Their ubiquity across digital communications arguably makes digital signatures one of the most important inventions in cryptography. Worryingly, all commonly used schemes – RSA, DSA and ECDSA – provide only computational security, and are rendered completely insecure by quantum computers. Motivated by this threat, this thesis focuses on unconditionally secure signature (USS) schemes – an information theoretically secure analogue of digital signatures. We present and analyse two new USS schemes. The first is a quantum USS scheme that is both information-theoretically secure and realisable with current technology. The scheme represents an improvement over all previous quantum USS schemes, which were always either realisable or had a full security proof, but not both. The second is an entirely classical USS scheme that uses minimal resources and is vastly more efficient than all previous schemes, to such an extent that it could potentially find real-world application. With the discovery of such an efficient classical USS scheme using only minimal resources, it is difficult to see what advantage quantum USS schemes may provide. Lastly, we remain in the information-theoretic security setting and consider two quantum protocols closely related to USS schemes – oblivious transfer and quantum money. For oblivious transfer, we prove new lower bounds on the minimum achievable cheating probabilities in any 1-out-of-2 protocol. For quantum money, we present a scheme that is more efficient and error tolerant than all previous schemes. Additionally, we show that it can be implemented using a coherent source and lossy detectors, thereby allowing for the first experimental demonstration of quantum coin creation and verification
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