Digital signatures are widely used in electronic communications to secure important tasks such as financial transactions, software updates, and legal contracts. The signature schemes that are in use today are based on public-key cryptography and derive their security from computational assumptions. However, it is possible to construct unconditionally secure signature protocols. In particular, using quantum communication, it is possible to construct signature schemes with security based on fundamental principles of quantum mechanics. Several quantum signature protocols have been proposed, but none of them has been explicitly generalised to more than three participants, and their security goals have not been formally defined. Here, we first extend the security definitions of Swanson and Stinson [1] so that they can apply also to the quantum case, and introduce a formal definition of transferability based on different verification levels. We then prove several properties that multiparty signature protocols with informationtheoretic security – quantum or classical – must satisfy in order to achieve their security goals. We also express two existing quantum signature protocols with three parties in the security framework we have introduced. Finally, we generalize a quantum signature protocol given in [2] to the multiparty case, proving its security against forging, repudiation and non-transferability. Notably, this protocol can be implemented using any pointto-point quantum key distribution network and therefore is ready to be experimentally demonstrated.
Digital signatures are widely used in electronic communications to secure important tasks such as financial transactions, software updates, and legal contracts. The signature schemes that are in use today are based on public-key cryptography and derive their security from computational assumptions. However, it is possible to construct unconditionally secure signature protocols. In particular, using quantum communication, it is possible to construct signature schemes with security based on fundamental principles of quantum mechanics. Several quantum signature protocols have been proposed, but none of them has been explicitly generalised to more than three participants, and their security goals have not been formally defined. Here, we first extend the security definitions of Swanson and Stinson [1] so that they can apply also to the quantum case, and introduce a formal definition of transferability based on different verification levels. We then prove several properties that multiparty signature protocols with informationtheoretic security – quantum or classical – must satisfy in order to achieve their security goals. We also express two existing quantum signature protocols with three parties in the security framework we have introduced. Finally, we generalize a quantum signature protocol given in [2] to the multiparty case, proving its security against forging, repudiation and non-transferability. Notably, this protocol can be implemented using any pointto-point quantum key distribution network and therefore is ready to be experimentally demonstrated.
Digital signatures are widely used for providing security of communications. At the same time, the security of currently deployed digital signature protocols is based on unproven computational assumptions. An efficient way to ensure an unconditional (information-theoretic) security of communication is to use quantum key distribution (QKD), whose security is based on laws of quantum mechanics. In this work we develop an unconditionally secure signature scheme that guarantees authenticity and transferability of arbitrary length messages in a QKD network. In the proposed setup, the QKD network consists of two subnetworks: (i) an internal network that includes the signer and with limitation on the number of malicious nodes and (ii) an external network that has no assumptions on the number of malicious nodes. A consequence of the absence of the trust assumption in the external subnetwork is the necessity of assistance from internal subnetwork recipients for the verification of message-signature pairs by external subnetwork recipients. We provide a comprehensive security analysis of the developed scheme, perform an optimization of the scheme parameters with respect to the secret key consumption, and demonstrate that the developed scheme is compatible with the capabilities of currently available QKD devices.
Current communication networks are based on classical physics and classical information-processing. However, for nearly a century, we have known that at its most fundamental level, the universe is governed by the laws of quantum mechanics. With quantum communication, new possibilities arise in our capabilities to transmit and process information which, in many cases, lead to advantages compared to what is classically possible. The entire scope of tasks for which quantum communication can offer improvements has not yet been fully explored, but several quantum protocols are known that can either perform tasks which are impossible with classical resources or can outperform classical protocols. These quantum protocols are well understood from a theoretical point of view, but many of them have never been demonstrated in practice. Thus, in the context of quantum communication, there is a significant gap between theory and experiment that must be removed in order to harness the advantages provided by quantum mechanics in a practical setting. \n \nIn this thesis, we develop a series of tools for developing and testing practical quantum communication protocols. Our main technique is a theoretical reformulation of existing quantum communication protocols that converts them into a form in which they can be demonstrated with existing experimental techniques. More precisely, they can be implemented using only coherent states of light and linear optics circuits while still retaining the crucial properties of the original abstract protocols. We use this result to construct practical protocols for the Hidden Matching problem and quantum fingerprinting. \n \n \nIn the case of quantum fingerprinting, we make a thorough analysis of the role played by experimental errors and show that our practical protocol can still be implemented in the presence of these imperfections. In fact, we report a proof of concept experimental demonstration of a quantum fingerprinting system that is capable of transmitting less information than the best known classical protocol for this problem. Our implementation is based on a modified version of a commercial quantum key distribution system using off-the-shelf optical components over telecom wavelengths, and is practical for messages as large as 100 Mbits, even in the presence of experimental imperfections. \n \nSimilarly, in the context of cryptography, we propose a multiparty quantum signature protocol that can be implemented from any point-to-point quantum key distribution network, proving its security against forging, repudiation and non-transferability. Crucially, since quantum key distribution is already a practical technology, so is this protocol. However, unlike other tasks in quantum communication, there has not been significant theoretical work on establishing a security model for quantum signature schemes. Consequently, we also constructed a security framework for these schemes and proved several properties that these protocols must satisfy in order to achieve their security goals. \n \nFinally, in addition to proposing new practical protocols, we provide a reliable data analysis technique to verify an important property of many quantum communication protocols: the presence of entanglement. Our technique is based on entanglement witnesses and it does not require the specification of a prior distribution nor the assumption of independent measurements. The technique is suitable to be used with nonlinear entanglement witnesses, which we show can be constructed from any linear witness and evaluated from the same experimental data. We also develop numerical tools necessary to employ this approach in practice, rendering the procedure ready to be applied to current experiments. We demonstrate this by analyzing the data of a photonic experiment generating two-photon states whose entanglement is verified with the use of an accessible nonlinear witness.
Modern cryptography encompasses much more than encryption of secret messages. Signature schemes are widely used to guarantee that messages cannot be forged or tampered with, for example in e-mail, software updates and electronic commerce. Messages are also transferrable, which distinguishes digital signatures from message authentication. Transferability means that messages can be forwarded; in other words, that a sender is unlikely to be able to make one recipient accept a message which is subsequently rejected by another recipient if the message is forwarded. Similar to public-key encryption, the security of commonly used signature schemes relies on the assumed computational difficulty of problems such as finding discrete logarithms or factoring large primes. With quantum computers, such assumptions would no longer be valid. Partly for this reason, it is desirable to develop signature schemes with unconditional or information-theoretic security. Quantum signature schemes are one possible solution. Similar to quantum key distribution (QKD), their unconditional security relies only on the laws of quantum mechanics. Quantum signatures can be realized with the same system components as QKD, but are so far less investigated. This talk aims to provide an introduction to quantum signatures and to review theoretical and experimental progress so far.
Digital signatures guarantee the authenticity and transferability of messages and are widely used in modern communication. The security of currently used classical digital signature schemes, however, relies on computational assumptions. In contrast, quantum digital signature (QDS) schemes offer information-theoretic security guaranteed by the laws of quantum mechanics. We present two QDS protocols which have the same experimental requirements as quantum key distribution, which is already commercially available. We also give a security proof for the presented QDS schemes against coherent forging attacks.
With the quantum computational paradigm rapidly developing across the world, new vulnerabilities on currently deployed cryptography protocols are to be taken in consideration. A possible answer to said vulnerabilities is using quantum technologies on cryptography. For a quantum-assisted cryptography protocol, key pairs can be generated via quantum state measurements. This protocol is known as quantum key distribution, of extended use for providing security against any algorithm-based attack to the keys. Ensured the key protection against eavesdroppers, a QKD-assisted protocol can focus its security analysis on party behaviour. Depending on the protocol rules and steps to follow, users can attempt attacks on it with the purpose of causing disagreement or misunderstanding between parties. In this work, we present a simulation environment for a hybrid, quantum-assisted digital signature (QDS) protocol. The configurations available for the simulated network as well as the digital signature protocol make it possible to design specific communication scenarios, such as attacks on the message integrity, forging and repudiation attempts or noisy communication across the quantum channels.
Quantum digital signatures (QDSs), which are unforgeable, non-repudiable, and transferable, are widely used in quantum communication networks. Unfortunately, previous protocols have often relied on three-party models for security analysis and experimental verification. As the number of parties increases, the number of quantum links required increases exponentially, undoubtedly leading to high channel losses and cost for multi-party QDSs. This study proposes a multi-party ring QDS scheme that can effectively deal with the issue of redundant quantum links. Furthermore, compared with the previously proposed QDS protocols, our protocol can prevent repudiation attacks without symmetry operation or swap test. Hence, it also can reduce the possibility of forging attacks by eliminating the possibility of forgery using illegal strings.