In our recent paper, we proposed the secret-key generation (SKG) scheme leveraging both atmospheric turbulence and background radiation as sources of randomness, which provides high secret-key rates and is compatible with state-of-the-art optical communications. Unfortunately, its application is limited to the symmetric line-of-sight free-space optical (FSO) links. In this paper, we propose two SKG schemes suitable for asymmetric and non-line-of-sight (NLOS) communications over FSO links. The proposed schemes employ turbulence, solar background radiation, and post-quantum cryptography-based randomly generated Alice and Bob's PSK sequences. To reduce the system cost, an unmodulated carrier is transmitted alongside the modulated PSK signal. On the receiver side, the coherent detection is performed by beating the unmodulated carrier with the received modulated signal on the PIN-photodetector (or the avalanche photodiode), followed by the DC blocker to remove undesired DC components. The proposed SKG schemes are applicable to both physical-layer security (PLS) cryptosystems and protocols similar to the quantum key distribution (QKD). The proposed SKG schemes show excellent tolerance to the atmospheric turbulence effects compared to the traditional PLS schemes. Experimental results indicate that the secret-key capacity of the NLOS SKG scheme is significantly higher than the corresponding traditional PLS counterpart.
In our recent paper, we proposed the secret-key generation (SKG) scheme leveraging both atmospheric turbulence and background radiation as sources of randomness, which provides high secret-key rates and is compatible with state-of-the-art optical communications. Unfortunately, its application is limited to the symmetric line-of-sight free-space optical (FSO) links. In this paper, we propose two SKG schemes suitable for asymmetric and non-line-of-sight (NLOS) communications over FSO links. The proposed schemes employ turbulence, solar background radiation, and post-quantum cryptography-based randomly generated Alice and Bob's PSK sequences. To reduce the system cost, an unmodulated carrier is transmitted alongside the modulated PSK signal. On the receiver side, the coherent detection is performed by beating the unmodulated carrier with the received modulated signal on the PIN-photodetector (or the avalanche photodiode), followed by the DC blocker to remove undesired DC components. The proposed SKG schemes are applicable to both physical-layer security (PLS) cryptosystems and protocols similar to the quantum key distribution (QKD). The proposed SKG schemes show excellent tolerance to the atmospheric turbulence effects compared to the traditional PLS schemes. Experimental results indicate that the secret-key capacity of the NLOS SKG scheme is significantly higher than the corresponding traditional PLS counterpart.
In our recent paper, we proposed using the atmospheric turbulent channel as a source of randomness to simultaneously address various challenges in physical-layer security (PLS), quantum key distribution (QKD), and post-quantum cryptography (PQC). However, the long coherence time of atmospheric turbulence channels limits the achievable secret-key rates (SKRs). To address this issue, we initially leveraged the turbulence effects only during the system’s initialization phase. In this paper, we present a source-based secret key generation scheme that overcomes the limitations imposed by the long coherence time of the turbulent channel, without requiring initialization. The scheme utilizes both atmospheric turbulence and background radiation as sources of randomness. It offers security superior to PQC and is comparable to QKD, while enabling significantly higher SKRs, making it fully compatible with state-of-the-art optical communications.
To overcome various problems of QKD, postquantum cryptography (PQC), and physical-layer security (PLS) encryption protocols, in our recent paper we have proposed to employ the atmospheric turbulent channel as the source of randomness in a source type PLS cryptosystem. However, a long coherence time of atmospheric turbulence channels limits the secret-key rates (SKRs). In this invited paper, we describe an encryption scheme that overcomes the problem of the long coherence time of the turbulent channel. The proposed scheme has security better than PQC and is comparable to QKD but enables significantly higher SKRs.
To overcome the limitations of QKD, post-quantum cryptography, and computational security-based cryptography protocols in this paper, an atmospheric turbulence-controlled cryptosystem is proposed. The proposed encryption scheme employs the traditional scheme to utilize the atmospheric turbulence as the common source of randomness only in the initialization stage to determine the common parameters to be used in the proposed encryption scheme. To overcome low secret-key rates of traditional scheme, dictated by the long coherence time Tc of turbulence channel, the proposed encryption scheme updates the parameters of gamma-gamma distribution, used to generate irradiance samples for cumulative distribution function-based determination of the key, every Tc seconds and as such the final key is shaped by the atmospheric turbulence channel. We also describe a scheme that randomly selects one of several available paths in which the simultaneously measured irradiance samples, after interleaving, are used to generate the raw key. The secret-key rates of the proposed schemes are orders of magnitude higher compared to corresponding traditional QKD and source type physical-layer security schemes and are comparable with the state-of-the-art optical communication data rates.
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) communication provides a promising alternative solution to the traditional radio frequency (RF) communications. Compared to the RF technology, there are many advantages of FSO communications, such as ultra-high bandwidth, license-free spectrum, low inter-channel interference, and energy-efficient transmission. The potential applications for FSO communication include the last-one-mile access link from the fiber-based backbone network, low earth orbit (LEO) satellite communications, high-altitude base station platforms or vehicles, deep-space communications, and in-building automation connections. As the FSO technology becomes mature and increasingly involved, the security and reliability requirements of such applications also become crucial. Regarding the security issues, although the FSO communication is inherently more secure than the RF counterpart due to high directivity of its line-of-sight (LOS) link, it can still suffer from optical wire-tapping attack. There are many different application schemes that an eavesdropper can still probe the FSO link. Traditionally, to protect the communication, many works have been done on encryption layer and upper network layer, such as computationally intensive cryptographic algorithms and message exchanging protocols. In recent years, physical-layer security (PLS) has been getting more attentions as it provides an extra layer of security against eavesdropping. The PLS schemes, in general can be categorized into two main categories: classical PLS and quantum key distribution (QKD) protocols. The classical PLS is based on the information theoretic security analysis, while the QKD security is guaranteed by the quantum mechanics laws. Both techniques have been studied for FSO channels. In the first half of this dissertation, we study the PLS in a LOS FSO channel using orbital angular momentum (OAM) multiplexing and show that higher secrecy capacity can be achieved using OAM multiplexing technology in the presence of atmospheric turbulence effects. Then, we proposed an adaptation scheme for QKD system over FSO channel with different QKD protocols, i.e. BB84 and decoy state protocols. By adapting the source brightness based on the channel condition, we optimized the secret key rate (SKR) of the QKD system. Furthermore, we proposed a multiple spatial modes-based QKD system with backpropagation method to increase the total SKR through parallel channels. Regarding the reliability of an optical communication system as well as information reconciliation for both PLS and QKD schemes, the forward error correction (FEC) becomes an essential technique to enable high-speed transmission. The FEC with low-density parity-check (LDPC) codes has been studied for decades from LDPC block codes to the most recent spatially-coupled (SC) LDPC codes. For FSO communications, due to the time-varying nature of FSO channels, the capability of rate adaptation is important to achieve a consistent reliable transmission. In the second half of this dissertation, we proposed unified FPGA-based architecture to implement both quasi-cyclic (QC) LDPC codes and SC-LDPC codes with rate adaptation capability. The performance is verified through FPGA emulation to show good error floor properties. To further validate the rate adaptation capability, we have built a spatial light modulator (SLM)-based FSO channel emulator to test the adaptive coding scheme. Furthermore, we have conducted a fiber-based experiment with our rate adaptative coding scheme to show that the proposed adaptive coding scheme can be also used in dynamic fiber-based networks.
Security issues of terrestrial free-space optical (FSO) communications over strong atmospheric turbulence are investigated. Based on a subcarrier intensity modulated FSO system model, we first study the scintillation reciprocity of the system from a coherence time perspective. A private secret key-based cryptosystem with key management is introduced to enhance FSO security, and a key agreement approach is proposed based on statistics of the random atmospheric turbulence-induced fading channel measurements. Secret key rate of the key agreement scheme is studied for the strong turbulence channel characterized by the K-distribution. Practical key agreement protocol is designed based on channel identification.