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IEEE Access

On the Atmospheric Turbulence-Based Cryptosystems With High Secret-Key Rate

Ivan B. Djordjević

Published Jan 1, 2025• 4 citations
Publisher

Abstract

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.

DOI
10.1109/access.2025.3625777

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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.
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.
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.
Secure data transmission is essential in today's world, and quantum key distribution (QKD) is widely recognized as the technology of choice to enable information-theoretical secure symmetric key generation. While optical fibers are typically used as a transmission channel, wireless transmission offers the flexibility to reach every part of the Earth; moreover, for Earth-Satellite QKD, wireless communication naturally becomes a necessity. In this study, we deployed a fiber-wireless optical system that transmitted a 64-QAM 400 Gbps classical signal for high-rate data exchange and a 1 MHz quantum signal for QKD. The impact of turbulence in the quantum system is evaluated by transmitting single-polarization quantum states through a turbulent free-space channel and by calculating the respective intrinsic quantum bit error rate (QBER). From this, we estimate the system's secret key rate (SKR) and its dependence on turbulence. The effects of atmospheric turbulence on the optical wireless signal were emulated and investigated by using a custom-made atmospheric chamber to induce different turbulent scenarios. The atmospheric chamber allowed us to vary the turbulence in a Rytov variance range from 0.06 to 2.52. The classical signal showed an overall average reliability of 82% for the mentioned turbulence regimes. The results of the quantum signal showed that for Rytov variances up to 1.3, and under the conditions tested, secret key generation is possible, reaching a maximum SKR of 8.7×10−4\mathbf {\times 10^{-4}}×10−4bits per pulse during the produced turbulence regimes.
Free-space optical (FSO) channels can be characterized by random power fluctuations due to atmospheric turbulence, which is known as scintillation. Weak coherent source based FSO quantum key distribution (QKD) systems suffer from the scintillation effect because during the deep channel fading the expected detection rate drops, which then gives an eavesdropper opportunity to get additional information about protocol by performing photon number splitting (PNS) attack and blocking single-photon pulses without changing QBER. To overcome this problem, in this paper, we study a large-alphabet QKD protocol, which is achieved by using pulse-position modulation (PPM)-like approach that utilizes the time-frequency uncertainty relation of the weak coherent photon state, called here TF-PPM-QKD protocol. We first complete finite size analysis for TF-PPM-QKD protocol to give practical bounds against non-negligible statistical fluctuation due to finite resources in practical implementations. The impact of scintillation under strong atmospheric turbulence regime is studied then. To overcome the secure key rate performance degradation of TF-PPM-QKD caused by scintillation, we propose an adaptation method for compensating the scintillation impact. By changing source intensity according to the channel state information (CSI), obtained by classical channel, the adaptation method improves the performance of QKD system with respect to the secret key rate. The CSI of a time-varying channel can be predicted using stochastic models, such as autoregressive (AR) models. Based on the channel state predictions, we change the source intensity to the optimal value to achieve a higher secret key rate. We demonstrate that the improvement of the adaptation method is dependent on the prediction accuracy.