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Quantum Information and Computation

Zero-error attacks and detection statistics in the coherent one-way protocol for quantum cryptography

BranciardCyril, GisinNicolas, LütkenhausNorbert, ScaraniValerio

Published Sep 1, 2007• 0 citations
Publisher

Abstract

This is a study of the security of the Coherent One-Way (COW) protocol for quantumcryptography, proposed recently as a simple and fast experimental scheme. In thezero-error regime, the eavesdropper...
DOI
10.5555/2011734.2011740

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Abstract Coherent-one-way (COW) quantum key distribution (QKD) held the promise of distributing secret keys over long distances with a simple experimental setup. Indeed, this scheme is currently used in commercial applications. Surprisingly, however, it has been recently shown that its secret key rate scales at most quadratically with the system’s transmittance and, thus, it is not appropriate for long distance QKD transmission. Such pessimistic result was derived by employing a so-called zero-error attack, in which the eavesdropper does not introduce any error, but still the legitimate users of the system cannot distill a secure key. Here, we present an improved zero-error attack against COW-QKD that significantly outperforms previous approaches, and provides stronger restrictions on its maximum achievable distance in the absence of errors. This translates into an upper bound on its secret key rate that is more than an order of magnitude lower than previously known upper bounds.
To protect practical quantum key distribution (QKD) against photon-number-splitting attacks, one could measure the coherence of the received signals. One prominent example that follows this approach is coherent-one-way (COW) QKD, which is commercially available. Surprisingly, however, it has been shown very recently that its secret key rate scales quadratically with the channel transmittance and thus this scheme is unsuitable for long-distance transmission. This result was derived by using a zero-error attack, which prevents the distribution of a secure key without introducing any error. Here we study various countermeasures to foil zero-error attacks against COW QKD. They require either monitoring some additional available detection statistics or increasing the number of quantum states emitted. We obtain asymptotic upper security bounds on the secret key rate that scale close to linearly with the channel transmittance, thus suggesting the effectiveness of the countermeasures to boost the performance of this protocol.
Coherent One-Way (COW) is a simple and widely utilized quantum key distribution (QKD) protocol, favored in both experimental and commercial applications. In this paper, we compare the information that an eavesdropper, Eve, can acquire in both the 2-decoy and 1-decoy COW protocols. Our findings demonstrate that the 2-decoy protocol offers enhanced security compared to the original 1-decoy COW, all while preserving the simplicity of implementation. Furthermore, we have experimentally implemented the improved security COW-QKD (2-decoy protocol), achieving a secure key rate exceeding 0.5 kbps over a distance of 110 km (22 dB loss) in optical fiber, with an interference visibility greater than 80%. We also emphasize that monitoring the click rates of the data line detector for each state individually serves as a powerful method for detecting zero-error attacks.
Abstract Recent results have shown that the secret-key rate of coherent-one-way (COW) quantum key distribution (QKD) scales quadratically with the system’s transmittance, thus rendering this protocol unsuitable for long-distance transmission. This was proven by using a so-called zero-error attack, which relies on an unambiguous state discrimination (USD) measurement. This type of attack allows the eavesdropper to learn the whole secret key without introducing any error. Here, we investigate the feasibility and effectiveness of zero-error attacks against COW QKD with present-day technology. For this, we introduce two practical USD receivers that can be realized with linear passive optical elements, phase-space displacement operations and threshold single-photon detectors. The first receiver is optimal with respect to its success probability, while the second one can impose stronger restrictions on the protocol’s performance with faulty eavesdropping equipment. Our findings suggest that zero-error attacks could break the security of COW QKD even assuming realistic experimental conditions.
Coherent-one-way (COW) quantum key distribution (QKD) is a significant communication protocol that has been implemented experimentally and deployed in practical products due to its simple equipment requirements. However, existing security analyses of COW-QKD either provide a short transmission distance or lack immunity against coherent attacks in the finite-key regime. In this paper, we present a tight finite-key security analysis within the universally composable framework for a variant of COW-QKD, which has been proven to extend the secure transmission distance in the asymptotic case. We combine the quantum leftover hash lemma and entropic uncertainty relation to derive the key rate formula. When estimating statistical parameters, we use the recently proposed Kato's inequality to ensure security against coherent attacks and achieve a higher key rate. Our paper confirms the security and feasibility of COW-QKD for practical application and lays the foundation for further theoretical study and experimental implementation. Published by the American Physical Society 2024