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Optics Express

Free-space optical wiretap channel and experimental secret key agreement in 78 km terrestrial link

Mikio Fujiwara, Toshiyuki Ito, Mitsuo Kitamura, Hiroyuki Endo, Orie Tsuzuki, Morio Toyoshima, Hideki Takenaka, Yoshihisa Takayama, Ryōsuke Shimizu, Masahiro Takeoka, Ryutaroh Matsumoto, Masahide Sasaki

Published Jul 18, 2018• 27 citations
Publisher

Abstract

Secret key agreement using physical properties of a wireless channel is becoming a promising scheme to establish a secret key between two users, especially in short-distance radio frequency (RF) communications. In this scheme, the existence of codes or key distillation that can make the leaked information to an eavesdropper arbitrarily small can be derived in an information theoretical way, given a priori knowledge on the channel linking a sender (Alice), a legitimate receiver (Bob), and an eavesdropper (Eve), which is called the wiretap channel. In practice, however, it is often difficult for Alice and Bob to get sufficient knowledge on Eve. In this study, we implement a free-space optical wiretap channel in a 7.8 km-terrestrial link and study how to estimate Eve's tapping ability, demonstrating high speed secret key agreement in the optical domain under a certain restricted condition of line-of-sight.

DOI
10.1364/oe.26.019513

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Institution of Engineering and Technology eBooks•Jan 5, 2021•1 citation
Secret key agreement using physical properties of a wireless channel is becoming a promising scheme to establish a secret key between two users, especially in short-distance radio frequency (RF) communications. In this scheme, the existence of codes or key distillation that can make the leaked information to an eavesdropper arbitrarily small can be derived in an information theoretical way, given a priori knowledge on the channel linking a sender (Alice), a legitimate receiver (Bob), and an eavesdropper (Eve), which is called the wiretap channel. In practice, however, it is often difficult for Alice and Bob to get sufficient knowledge on Eve. In this study, we implement a free-space optical wiretap channel in a 7.8 km-terrestrial link and study how to estimate Eve's tapping ability, demonstrating high speed secret key agreement in the optical domain under a certain restricted condition of line-of-sight.
Free-space optical (FSO) communications are becoming promising schemes for high-capacity wireless links due to their plentiful characteristics originated from higher carrier frequency. These characteristics also yield a greater security advantage over radio frequency counterparts: the physical ability of a wiretapper is reasonably restricted due to the high directionality of communication beam and the line-of-sight configuration of the link. Secret key agreement over FSO links (FSO-SKA) employs this security advantage as well as the post-processing over an authenticated public channel to establish an information-theoretic secure key which cannot be broken even with unbounded computer resources. In the previous works, the authors demonstrated the full-field implementations of FSO-SKA with a 7.8-km FSO link testbed including a probing station to estimate the possible wiretapping risks from the sidelobe of the communication beam. In the demonstration, however, there is still room to improve the secret key rate by exploiting the optical fading which contains additional information about random states of the FSO links. We here propose a novel protocol for FSO-SKA employing such channel state information. In the protocol, the legitimate receiver decides whether to discard the received symbols or not according to the received optical power at the time. Based on the experimental data from the FSO link testbed, we demonstrate that the proposed protocol improves the secret key rate compared with our previous result. To our best knowledge, this is the first demonstration that exploits the effect of atmospheric turbulences to improve the security performance of communication systems. We anticipate that this idea will be applicable on the broader areas of FSO communications and opens a way toward practical wireless network spanned by FSO links.
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.
Abstract Free-space optical (FSO) links offer a practical approach to realize quantum key distribution (QKD) in a global scale. However, when one wants to further extend the distance from the geostationary orbit to the ground, currently known QKD schemes cannot realize practical key rates mainly due to the diffraction losses of a laser beam. If the facts that the FSO links are highly directional and must be used in the line-of-sight (LoS) condition are taken into account, one may impose some physical restrictions on an eavesdropping model to explore longer-distance QKD. In this paper, we propose a novel FSO secret key agreement scheme, line-of-sight QKD (LoS-QKD), based on a quantum wiretap channel. In our model, an eavesdropper can tap only a limited fraction of the FSO signal beam but perform any physically allowable operations on the tapped signals. Fading effects which are significant in the FSO links are fully taken into account. We provide a security proof for the differential phase shift (DPS) keying scheme in terms of the metric which meets the composability. We investigate numerically the performances of LoS-QKD with DPS keying, including finite-length analysis, showing that our proposed scheme can realize high-speed and long-distance secret key agreement with information-theoretic security.
The conventional omnipotent eavesdropper assumption in quantum cryptography study can be too strict for some realistic scenarios. In this paper, we study the secret key distillation over a satellite-to-satellite free space optics channel in which we assume that the eavesdropper (Eve) is restricted in her ability of power collection due to the limited size of her aperture but can change the position of her aperture to gain advantages over the communication parties (Alice and Bob) and we determine the achievable key rate lower and upper bounds with respect to different scenarios. We first study the case where Eve is behind Bob and we prove that the optimal eavesdropping strategy for her in long-distance transmission case is to place her aperture on the beam transmission axis and set Bob-to-Eve distance equal to Alice-to-Bob distance. We also show that the achievable key rate would be characterized by a Bessel function integral related to Eve's position in a short-distance transmission case. We then investigate the case where Eve is before Bob and show similar results with Eve's and Bob's roles exchanged.
Hybrid Free-Space Optical (FSO) and millimeter Wave (mmWave) systems have emerged as a promising candidate for high data rate wireless transmissions due to the unique complementary properties against the different channel and environment conditions. Consequently, in this study, we investigate the hybrid FSO-mmWave systems from a physical-layer security point of view in the presence of different types of eavesdroppers, where the communication between two legitimate peers takes place over both FSO and RF links simultaneously. In particular, exponential atmospheric turbulence and Weibull fading channels are considered for FSO and mmWave links, respectively. We examine practical scenarios to eavesdrop the legitimate communication, and discuss the effects of random radio power of mmWave link and optical irradiance of FSO link on the probability of achieving a secure transmission. The impact of the fundamental physical layer parameters on the secrecy performance of the hybrid system is analyzed by obtaining analytical derivations of the probability of strictly positive secrecy capacity (SPSC) for different types of eavesdroppers, namely RF-, FSO- and Hybrid-Eve. In the light of results, we show that the analytical expressions are in exact agreement with the Monte-Carlo simulations.
Free-space optical (FSO) communications offer high-capacity wireless transmission due to their plentiful properties originated from higher carrier frequency. These properties also yield a greater security advantage: the high directionality of the laser beam and the line-of-sight configuration can reasonably restrict an attack model launched by an eavesdropper (Eve). Secret key agreement over FSO links (FSO-SKA) employs this security advantage for key establishment between two distant parties, which is secure against Eve even with unbounded computer resources. In this chapter, we numerically evaluate the performance of FSO-SKA for satellite laser communications under the given power constraint. We also compare the performance of FSOSKA and quantum key distribution (QKD). Our result shows that FSO-SKA can generate a key even for the distance between geostationary orbit satellite and ground station. We anticipate that FSO-SKA has a potential to extend the secure satellite networks to global scale, which is hard only with QKD.