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arXiv:2101.07192 · New Journal of Physics

Zero-error attack against coherent-one-way quantum key distribution

Róbert Trényi, Marcos Curty

Published Aug 17, 2021• 26 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1088/1367-2630/ac1e41

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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.
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.
The performance of quantum key distribution (QKD) is severely limited by multiphoton pulses emitted by laser sources due to the photon-number splitting attack. Coherent-one-way (COW) QKD has been introduced as a promising solution to overcome this limitation, and thus extend the achievable distance of practical QKD. Indeed, thanks to its experimental simplicity, the COW protocol is already used in commercial applications. Here, we derive simple upper security bounds on its secret key rate, which demonstrate that it scales at most quadratically with the system's transmittance, thus solving a long-standing problem. That is, in contrast to what has been claimed, this approach is inappropriate for long-distance QKD transmission. Remarkably, our findings imply that all implementations of the COW protocol performed so far are insecure.
Coherent-one-way quantum key distribution (COW-QKD), which requires a simple experimental setup and has the ability to withstand photon-number-splitting attacks, has been not only experimentally implemented but also commercially applied. However, recent studies have shown that the current COW-QKD system is insecure and can only distribute secret keys safely within 20 km of the optical fiber length. In this study, we propose a practical implementation of COW-QKD by adding a two-pulse vacuum state as a new decoy sequence. This proposal maintains the original experimental setup as well as the simplicity of its implementation. Utilizing detailed observations on the monitoring line to provide an analytical upper bound on the phase error rate, we provide a high-performance COW-QKD asymptotically secure against coherent attacks. This ensures the availability of COW-QKD within 100 km and establishes theoretical foundations for further applications.
Coherent one-way quantum key distribution (COW-QKD) has been widely investigated, and even been deployed in real-world quantum network. However, the proposal of the zero-error attack has critically undermined its security guarantees, and existing experimental implementations have not yet established security against coherent attacks. In this work, we propose and experimentally demonstrate an information-theoretically secure COW-QKD protocol that can resist source side-channel attacks, with secure transmission distances up to 100 km. Our system achieves a secure key rate on the order of kilobits per second over 50 km in the finite-size regime, sufficient for real-time secure voice communication across metropolitan networks. Furthermore, we demonstrate the encrypted transmission of a logo with information-theoretic security over 100 km of optical fiber. These results confirm that COW-QKD can simultaneously provide simplicity and security, establishing it as a strong candidate for deployment in small-scale quantum networks.
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.