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Applied Optics

Experimental realization of optical key distribution under strong wiretap eavesdropping

Mateusz Kucharczyk, Marcin Jarzyna, Mikołaj Lasota, Piotr Leszek Kolenderski, Jaroslaw Piotr Turkiewicz, Patryk J. Urban, Konrad Banaszek

Published Aug 31, 2026• 0 citations
Publisher

Abstract

We report the experimental distribution of a cryptographic key based on binary intensity-modulation/direct-detection (IM/DD) transmission of an optical signal. The implemented secret key agreement protocol ensures security against passive wiretap eavesdropping, notably in the regime when the eavesdropper has an advantage in the intercepted signal strength over the legitimate recipient. Operating at a 1 GBaud symbol rate, the experimental fiber optic testbed yielded secret key rates between 43.5 Mbps at 61.1 km and 640 kbps at 81.6 km of the effective link length, with a 9% wiretap installed at the transmitter. The protocol performance was analyzed using a theoretical model incorporating transmitter noise and non-ideal key reconciliation efficiency. The presented results validate IM/DD optical key distribution as an efficient low-complexity solution for protecting optical communication systems against the predominant physical-layer threat of wiretap eavesdropping.

DOI
10.1364/ao.607192

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We report the experimental distribution of a cryptographic key based on binary intensity-modulation/direct-detection (IM/DD) transmission of an optical signal. The implemented secret key agreement protocol ensures security against passive wiretap eavesdropping, notably in the regime when the eavesdropper has an advantage in the intercepted signal strength over the legitimate recipient. Operating at a 1 GBaud symbol rate, the experimental fiber optic testbed yielded secret key rates between 43.5 Mbps at 61.1 km and 640 kbps at 81.6 km of the effective link length, with a 9% wiretap installed at the transmitter. The protocol performance was analyzed using a theoretical model incorporating transmitter noise and non-ideal key reconciliation efficiency. The presented results validate IM/DD optical key distribution as an efficient low-complexity solution for protecting optical communication systems against the predominant physical-layer threat of wiretap eavesdropping.
We analyze the theoretically optimal operation of an optical key distribution (OKD) link based on fine intensity modulation of an optical signal transmitted over an attenuating channel to a direct detection receiver. With suitable digital postprocessing, the users may generate a secret key that will be unknown to an unauthorized party collecting passively a fraction of the signal that escapes detection by the legitimate recipient. The security is ensured by the presence of the shot noise that inevitably accompanies an eavesdropper’s attempt to detect the collected signal. It is shown that the key amount depends on a ratio that compares the legitimate recipient’s and eavesdropper’s capabilities to detect the signal, including noise contributed by their respective detectors. A simple proportionality relation is derived in the strong eavesdropping regime, and closed expressions for the optimal depth of binary intensity modulation as well as the discrimination thresholds for hard-decoded direct detection are given. The presented results substantially simplify the design of practical OKD systems operating under changing external conditions, e.g. variable atmospheric absorption in the case of free-space optical links.
Optical key distribution (OKD) protects the physical layer of communication links by taking advantage of the inherent noise present in the photodetection process. It allows for efficient generation of a shared random key between two distant users that is secure against passive eavesdropping and can be subsequently used for cryptographic purposes. Moreover, it can be straightforwardly implemented over standard intensity modulation and direct detection links, making it an attractive alternative to quantum key distribution. Here we present a comprehensive security analysis against more powerful eavesdroppers possessing the ability to perform either coherent detection, or even quantum-optimal measurements on the intercepted transmission.
We propose and analyze numerically concurrent intensity modulation/direct detection (IM/DD) optical key distribution (OKD) combined with conventional data transmission over a LEO-to-ground optical link. The idea is validated by numerical simulations which indicate that in a realistic scenario it is possible to generate simultaneously up to 200 Mbits of a secret key and downlink transmit up to 140 Gbits of data during a single nearly-zenithal pass of a LEO satellite. Such lengths of the secret key are three orders of magnitude higher in comparison to typical quantum key distribution systems, although they are generated under different security assumptions. The simplicity and robustness of IM/DD OKD protocols could make them an attractive and cost-effective option to provide security in the physical layer of space communication systems.
In fiber-optic communication networks, research on data security at lower layers of the protocol stack and in particular at the physical layer by means of information-theoretic concepts is only in the beginning. Nevertheless, it has recently attracted quite some attention as it holds the promise of providing unconditional, perfect security without the need for secret key exchanges. In this paper, we analyze some important constraints that such concepts put on a potential implementation of physical-layer security. We review the fundamentals of physical-layer security on the basis of the commonly used AWGN wiretap channel model. For such channel model we summarize the security metrics which are typically used in information theory and in particular recall that, for secure communication over the AWGN channel, the legitimate receiver needs an SNR advantage over the eavesdropper. Next, we relate the information theoretic metrics to physically measurable quantities in optical communications engineering, namely optical signal-to-noise ratio (OSNR) and bit-error ratio (BER), and translate the information-theoretic wiretap scenario to a simple real-world point-to-point optical transmission link in which part of the light is wiretapped using a bend coupler. We investigate the achievable OSNR advantage under realistic assumptions for fiber loss, tap ratio, and noise budget and find that secure transmission is limited to a distance of a few tens of kilometers in this case. The maximum secure transmission distance decreases with an increasing tap ratio chosen by the eavesdropper. This can be only counteracted by monitoring the link loss towards the legitimate receiver which would force the eavesdropper to choose small tap ratios in order to remain undetected. In an outlook towards further research directions we identify information-theoretic approaches which could potentially allow to realize physical-layer security in more generalized scenarios or over longer distances.
In this study, we analyze the secret key capacity of intensity modulation/direct detection optical key distribution (IM/DD OKD) for a free-space optical (FSO) link between a low-Earth orbit satellite and an optical ground station. Focusing on downlink communication, we account for atmospheric turbulence, which causes random variations in the transmittance of the FSO channel. We implement an atmospheric channel model that accounts for absorption and scattering, geometric losses, pointing errors, and intensity fluctuations. The secret key capacity is quantified under different noise scenarios and reconciliation code efficiencies, assuming a hard decoding scheme. The performance of the IM/DD OKD protocol is compared under direct and reverse reconciliation regimes. Additionally, we examine the impact of weak and strong wind on the strength of atmospheric turbulence, leading to different results of the secret key capacity. Furthermore, we analyze the characteristics of error distributions that arise from protocol optimization. Our results provide insights into optimizing IM/DD OKD protocols for varying atmospheric conditions.
Free space optical (FSO) communications are enabling high-speed global wireless networks. Thanks to the highly directional nature of laser beam, they also yield a greater security advantage over radio frequency counterparts. When combined with a scheme of secret key agreement (SKA), FSO-SKA can establish at high speed a symmetric secret key which cannot be decrypted even by unbounded computer resources. Although there have been many theoretical studies on SKA, experimental investigations have been quite lacking, especially on quantifying eavesdropping risks and secret key rates in realistic environment. Here, we report the first full-field implementations of FSO-SKA in a 7.8-km terrestrial link with a probing station, enabling the estimation of eavesdropping risks. We attain the final key rates from 100 kbps to 7.77 Mbps under various atmospheric and beaming conditions even with total losses of 55dB or higher, in which known quantum key distribution schemes attain impractically low key rates.