A number of questions associated with practical implementations of quantum cryptography systems having to do with unconditional secrecy, computational loads and effective secrecy rates in the presence of perfect and imperfect sources are discussed. The different types of unconditional secrecy, and their relationship to general communications security, are discussed in the context of quantum cryptography. In order to actually carry out a quantum cryptography protocol it is necessary that sufficient computational resources be available to perform the various processing steps, such as sifting, error correction, privacy amplification and authentication. We display the full computer machine instruction requirements needed to support a practical quantum cryptography implementation. We carry out a numerical comparison of system performance characteristics for implementations that make use of either weak coherent sources of light or perfect single photon sources, for eavesdroppers making individual attacks on the quantum channel characterized by different levels of technological capability. We find that, while in some circumstances it is best to employ perfect single photon sources, in other situations it is preferable to utilize weak coherent sources. In either case the secrecy level of the final shared cipher is identical, with the relevant distinguishing figure-of-merit being the effective throughput rate.
A number of questions associated with practical implementations of quantum cryptography systems having to do with unconditional secrecy, computational loads and effective secrecy rates in the presence of perfect and imperfect sources are discussed. The different types of unconditional secrecy, and their relationship to general communications security, are discussed in the context of quantum cryptography. In order to actually carry out a quantum cryptography protocol it is necessary that sufficient computational resources be available to perform the various processing steps, such as sifting, error correction, privacy amplification and authentication. We display the full computer machine instruction requirements needed to support a practical quantum cryptography implementation. We carry out a numerical comparison of system performance characteristics for implementations that make use of either weak coherent sources of light or perfect single photon sources, for eavesdroppers making individual attacks on the quantum channel characterized by different levels of technological capability. We find that, while in some circumstances it is best to employ perfect single photon sources, in other situations it is preferable to utilize weak coherent sources. In either case the secrecy level of the final shared cipher is identical, with the relevant distinguishing figure-of-merit being the effective throughput rate.
We perform a comprehensive analysis of practical quantum cryptography (QC) systems implemented in actual physical environments via either free-space or fiber-optic cable quantum channels for ground-ground, ground-satellite, air-satellite and satellite-satellite links. (1) We obtain the complete universal expressions for the effective secrecy capacity and rate for QC systems taking into account the most general attacks on individual quantum bits, including explicit closed-form expressions for the necessary and sufficient amount of privacy amplification. Our analysis also includes the explicit calculation in detail of the total cost in bits of continuous authentication, thereby obtaining new results for actual ciphers of finite length. (2) We perform for the first time a detailed, explicit analysis of all systems losses due to propagation, errors, noise, etc. as appropriate to both optical fiber cable- and satellite communications-based implementations of QC. (3) We calculate for the first time all system load costs associated to classical communication and computational constraints that are ancillary to, but essential for carrying out, the pure QC protocol itself. (4) We introduce an extended family of generalizations of the Bennett-Brassard (BB84) QC protocol that equally provide unconditional secrecy but allow for the possibility of optimizing throughput rates against specific cryptanalytic attacks. (5) We obtain universal predictions for maximal rates that can be achieved with practical system designs under realistic environmental conditions. (6) We propose a specific QC system design that includes the use of a novel method of high-speed photon detection that may be able to achieve very high throughput rates for actual implementations in realistic environments.
We perform a comprehensive analysis of practical quantum cryptography (QC) systems implemented in actual physical environments via either free-space or fiber-optic cable quantum channels for ground-ground, ground-satellite, air-satellite and satellite-satellite links. (1) We obtain universal expressions for the effective secrecy capacity and rate for QC systems taking into account three important attacks on individual quantum bits, including explicit closed-form expressions for the requisite amount of privacy amplification. Our analysis also includes the explicit calculation in detail of the total cost in bits of continuous authentication, thereby obtaining new results for actual ciphers of finite length. (2) We perform for the first time a detailed, explicit analysis of all systems losses due to propagation, errors, noise, etc. as appropriate to both optical fiber cable- and satellite communications-based implementations of QC. (3) We calculate for the first time all system load costs associated to classical communication and computational constraints that are ancillary to, but essential for carrying out, the pure QC protocol itself. (4) We introduce an extended family of generalizations of the Bennett-Brassard (BB84) QC protocol that equally provide unconditional secrecy but allow for the possibility of optimizing throughput rates against specific cryptanalytic attacks. (5) We obtain universal predictions for maximal rates that can be achieved with practical system designs under realistic environmental conditions. (6) We propose a specific QC system design that includes the use of a novel method of high-speed photon detection that may be able to achieve very high throughput rates for actual implementations in realistic environments.
The peculiar properties of quantum mechanics enable possibilities not allowed by classical physics. In particular, two parties can generate a random, secret key at a distance, even though an eavesdropper can do anything permitted by the laws of physics. Measuring the quantum properties of the signals generating the key, would ultimately change them, and thus reveal the eavesdropper’s presence. This exchange of a random, secret key is known as quantum cryptography.\n\nQuantum cryptography can be, and has been proven unconditionally secure using perfect devices. However, when quantum cryptography is implemented, one must use components available with current technology. These are usually imperfect. Although the security of quantum cryptography has been proven for components with certain imperfections, the question remains: can quantum cryptography be implemented in a provable, unconditionally secure way, using components available with current technology? This thesis contains both a theoretical, and an experimental contribution to the answer of this question. On the experimental side, components used in, and complete quantum cryptography systems have been carefully examined for security loopholes. In particular, it turned out that two commercial quantum cryptography systems contained loopholes, which would allow an eavesdropper to capture the full secret key, without exposing her presence. Furthermore, this detector control attack could be implemented with current technology. The attack is applicable against a variety of quantum cryptography implementations and protocols.\n\nThe theoretical contribution consists of security proofs for quantum cryptography in a very general setting. Precisely, the security is proven with arbitrary individual imperfections in the source and detectors. These proofs should make it possible to use a wide array of imperfect devices in implementations of quantum cryptography.\n\nFinally, a secure detection scheme is proposed, immune to the detector control attack and compatible with those security proofs. Therefore, if this scheme is implemented correctly, it offers provable security.
Quantum cryptography has attracted much recent attention due to its potential for providing secret communications that cannot be decrypted by any amount of computational effort. This is the first analysis of the secrecy of a practical implementation of the BB84 protocol that simultaneously takes into account and presents the full set of analytical expressions for effects due to the presence of pulses containing multiple photons in the attenuated output of the laser, the finite length of individual blocks of key material, losses due to error correction, privacy amplification, and authentication, errors in polarization detection, the efficiency of the detectors, and attenuation processes in the transmission medium. The analysis addresses eavesdropping attacks on individual photons rather than collective attacks in general. Of particular importance is the first derivation of the necessary and sufficient amount of privacy amplification compression to ensure secrecy against the loss of key material which occurs when an eavesdropper makes optimized individual attacks on pulses containing multiple photons. It is shown that only a fraction of the information in the multiple photon pulses is actually lost to the eavesdropper.
Communications in secrecy are often required in many commercial and military applications. Unfortunately, many cryptographic schemes in use today such as public-key cryptography based on the RSA algorithm would be broken with either unanticipated advances in hardware and algorithm or the advent of quantum computers. Quantum cryptography, on the other hand, has been proven secure even against the most general attack allowed by the laws of physics and is a promising technology poised for widespread adoption in realistic cryptographic applications. Quantum cryptography allows two parties to expand on a secret key that they have previously shared. Various quantum cryptographic protocols have been proposed to perform this task. In this paper, we survey some popular quantum cryptographic protocols (including the famous Bennett-Brassard 1984 protocol) and discuss their security. Specifically, we consider their security in two cases: the ideal case where a perfect single-photon source is used and the practical case where a realistic laser source is used. We compare the protocols and find that the efficient six-state protocol outperforms the others both in the tolerable quantum bit error rate and in the key generation rate when a realistic laser source is used.
Quantum cryptography, or quantum key distribution (QKD), provides a means of unconditionally secure communication. The security is in principle based on the fundamental laws of physics. Security proofs show that if quantum cryptography is appropriately implemented, even the most powerful eavesdropper cannot decrypt the message from a cipher.
The implementations of quantum crypto-systems in real life may not fully comply with the assumptions made in the security proofs. Such discrepancy between the experiment and the theory can be fatal to the security of a QKD system. In this thesis we address a number of these discrepancies.
A perfect single-photon source is often assumed in many security proofs. However, a weak coherent source is widely used in a real-life QKD implementation. Decoy state protocols have been proposed as a novel approach to dramatically improve the performance of a weak coherent source based QKD implementation without jeopardizing its security. Here, we present the first experimental demonstrations of decoy state protocols. Our experimental scheme was later adopted by most decoy state QKD implementations.
In the security proof of decoy state protocols as well as many other QKD protocols, it is widely assumed that a sender generates a phase-randomized coherent state. This assumption has been enforced in few implementations. We close this gap in two steps: First, we implement and verify the phase randomization experimentally; second, we prove the security of a QKD implementation without the coherent state assumption.
In many security proofs of QKD, it is assumed that all the detectors on the receiver's side have identical detection efficiencies. We show experimentally that this assumption may be violated in a commercial QKD implementation due to an eavesdropper's malicious manipulation. Moreover, we show that the eavesdropper can learn part of the final key shared by the legitimate users as a consequence of this violation of the assumptions.