It has been established that the security of quantum key distribution protocols can be severely compromised were one to permit an eavesdropper to possess a very limited knowledge of the random sources used between the communicating parties. While such knowledge should always be expected in realistic experimental conditions, the result itself opened a new line of research to fully account for real-world weak randomness threats to quantum cryptography. Here we expand of this novel idea by describing a key distribution scheme that is provably secure against general attacks by a post-quantum adversary. We then discuss possible security consequences for such schemes under the assumption of weak randomness.
It has been established that the security of quantum key distribution protocols can be severely compromised were one to permit an eavesdropper to possess a very limited knowledge of the random sources used between the communicating parties. While such knowledge should always be expected in realistic experimental conditions, the result itself opened a new line of research to fully account for real-world weak randomness threats to quantum cryptography. Here we expand of this novel idea by describing a key distribution scheme that is provably secure against general attacks by a post-quantum adversary. We then discuss possible security consequences for such schemes under the assumption of weak randomness.
In usual security proofs of quantum protocols the adversary (Eve) is expected to have full control over any quantum communication between any communicating parties (Alice and Bob). Eve is also expected to have full access to an authenticated classical channel between Alice and Bob. Unconditional security against any attack by Eve can be proved even in the realistic setting of device and channel imperfection. In this paper we show that the security of quantum key distribution protocols is ruined if one allows Eve to possess a very limited access to the random sources used by Alice. Such knowledge should always be expected in realistic experimental conditions via different side channels.
Ideal quantum key distribution (QKD) protocols call for a source that emits single photon signals, but the sources used in typical practical realizations emit weak coherent states instead. A weak coherent state may contain more than one photon, which poses a potential security risk. QKD with weak coherent state signals has nevertheless been proven to be secure, but only under the assumption that the phase of each signal is random (and completely unknown to the adversary). Since this assumption need not be fully justified in practice, it is important to know whether phase randomization is really a requirement for security rather than a convenient technical assumption that makes the security proof easier. Here, we exhibit an explicit attack in which the eavesdropper exploits knowledge of the phase of the signals, and show that this attack allows the eavesdropper to learn every key bit in a parameter regime where a protocol using phase-randomized signals is provably secure. Thus we demonstrate that phase randomization really does enhance the security of QKD using weak coherent states. This result highlights the importance of a careful characterization of the source for proofs of the security of quantum key distribution.
In recent years, several hacking attacks have broken the security of quantum cryptography implementations by exploiting the presence of losses and the ability of the eavesdropper to tune detection efficiencies. We present a simple attack of this form that applies to any protocol in which the key is constructed from the results of untrusted measurements performed on particles coming from an insecure source or channel. Because of its generality, the attack applies to a large class of protocols, from standard prepare-and-measure to device-independent schemes. Our attack gives bounds on the critical detection efficiencies necessary for secure quantum key distribution, which show that the implementation of most partly device-independent solutions is, from the point of view of detection efficiency, almost as demanding as fully device-independent ones. We also show how our attack implies the existence of a form of bound randomness, namely nonlocal correlations in which a nonsignalling eavesdropper can find out a posteriori the result of any implemented measurement.
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.
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 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.