A new framework for secure communication in physical layer is proposed. A network of users equipped with coupled dynamical systems is considered. The aim is to securely exchange messages between network nodes in the presence of an eavesdropper, referred to as Eve. Unlike a traditional wireless system, the messages to be conveyed are not sent directly through the medium. Instead, they are mapped to initial conditions of the dynamical system. Once the system converges to a steady state, conditions of the local system at each node is measured to recover the sent messages. A fundamental property of the proposed system which makes it secure is that Eve is not part of the dynamical system and hence, she does not observe the initial conditions of the nodes for steady state comparison measurement. In particular, a situation with two users is considered. The proposed system is then modeled as a two-way wiretap channel and the secrecy capacity region is derived under various conditions. A consequence of our result is that regardless of Eve's physical location and how strong her receiver is the achievable rates are positive, i.e., secure communication is possible. Furthermore, a radio frequency (RF) system is proposed to realize this model in a wireless setting by means of local coupled oscillators. In particular, a unidirectional master-slave coupling architecture is considered where a power-constrained slave node synchronizes its frequency with a high-power master node. It is shown how the coupling mechanism, realized by transmitting and receiving power between the RF front-ends of the master and the slave node, can be used by the slave node to securely send messages or to share secret keys with the master node. To the best of our knowledge, this is the first architecture providing physical layer secret key generation fully designed in the RF front-end. The proposed RF system is simulated using Advanced Design System (ADS). The simulation results are shown for a 10m channel link and confirm the security condition. The secret key generation rate is 2 bits per synchronization time-frame which is dominated by the wave propagation delay between the two nodes.
A new framework for secure communication in physical layer is proposed. A network of users equipped with coupled dynamical systems is considered. The aim is to securely exchange messages between network nodes in the presence of an eavesdropper, referred to as Eve. Unlike a traditional wireless system, the messages to be conveyed are not sent directly through the medium. Instead, they are mapped to initial conditions of the dynamical system. Once the system converges to a steady state, conditions of the local system at each node is measured to recover the sent messages. A fundamental property of the proposed system which makes it secure is that Eve is not part of the dynamical system and hence, she does not observe the initial conditions of the nodes for steady state comparison measurement. In particular, a situation with two users is considered. The proposed system is then modeled as a two-way wiretap channel and the secrecy capacity region is derived under various conditions. A consequence of our result is that regardless of Eve's physical location and how strong her receiver is the achievable rates are positive, i.e., secure communication is possible. Furthermore, a radio frequency (RF) system is proposed to realize this model in a wireless setting by means of local coupled oscillators. In particular, a unidirectional master-slave coupling architecture is considered where a power-constrained slave node synchronizes its frequency with a high-power master node. It is shown how the coupling mechanism, realized by transmitting and receiving power between the RF front-ends of the master and the slave node, can be used by the slave node to securely send messages or to share secret keys with the master node. To the best of our knowledge, this is the first architecture providing physical layer secret key generation fully designed in the RF front-end. The proposed RF system is simulated using Advanced Design System (ADS). The simulation results are shown for a 10m channel link and confirm the security condition. The secret key generation rate is 2 bits per synchronization time-frame which is dominated by the wave propagation delay between the two nodes.
In this work we present a novel technique for physical layer security in the Internet-of-Things (IoT) networks. In the proposed architecture, each IoT node generates a phase-modulated random key/data and transmits it to a master node in the presence of an eavesdropper, referred to as Eve. The master node, simultaneously, broadcasts a high power signal using an omni-directional antenna, which is received as interference by Eve. This interference masks the generated key by the IoT node and will result in a higher bit-error rate in the data received by Eve. The two legitimate intended nodes communicate in a full-duplex manner and, consequently, subtract their transmitted signals, as a known reference, from the received signal (self-interference cancellation). We compare our proposed method with a conventional approach to physical layer security based on directional antennas. In particular, we show, using theoretical and measurement results, that our proposed approach provides significantly better security measures, in terms bit error rate (BER) at Eve's location. Also, it is proven that in our novel system, the possible eavesdropping region, defined by the region with BER-1, is always smaller than the reliable communication region with BER-3.
In existing physical layer security (PLS) and key generation protocols, major assumptions, including channel reciprocity, localization, and synchronization between the legitimate parties, are often considered. However, these assumptions are arguable in practice leading to major barriers in building systems based on PLS protocols. To overcome these barriers, we proposed, designed, and implemented a novel embedded architecture for distributed Internet-of-Things (IoT) networks that utilize a master-slave full-duplex communication to exchange a random secret key. In the proposed architecture, an IoT node generates a phase-modulated random key/data and transmits it to a master node in the presence of an eavesdropper, referred to as Eve. The master node, simultaneously, broadcasts a high-power signal using an omnidirectional antenna, which is received as a jammer signal or interference by Eve. This results in a high bit error rate (BER) making the data undetectable by Eve. The two legitimate nodes communicate in a full-duplex fashion and, consequently, subtract their transmitted signals from the received signal (self-interference cancellation). Our proposed protocol does not require any knowledge of the node locations. In particular, we show, using theoretical and measurement results, that our proposed approach provides significantly better security measures, in terms of the BER at Eve's location, compared to a conventional method based on directional beamforming antennas. Also, it is proved that in our novel system, the possible eavesdropping region, BER-1, is always smaller than the reliable communication region, BER-3.
With the advent of mobile telephony, complemented by the availability of economic infrastructure devices has provided users the freedom to communicate and perform everyday tasks wirelessly. The access to information anywhere anytime provide users more flexibility to communicate, nonetheless it also makes communication more vulnerable to eavesdropping. Security is now considered as an important issue in the design of modern wireless communication system. In contrast to the conventional systems where security issues are dealt in the upper\nlayers of the protocol stack, in this thesis we focus our attention to construct security schemes by exploiting the attributes of the physical layer. \nIn the first part of the thesis, we consider a two-user multiacces channel where the two encoders, Encoder 1 and Encoder 2, are allowed to communicate with each other over a unidirectional noiseless bit-pipe of finite capacity in the presence of an adversary. In this model, the role of Encoder 2 is restricted to only help Encoder 1 in securing information from adversary. For this model, we\nestablish fundamental performance limits on secure communication and characterizes the secrecy capacity in some extreme cases of cooperation between the encoders. The second part of the thesis considers an elementary\nnetwork, which consists of four nodes – a source, a relay, a legitimate receiver and a passive eavesdropper. In this network, the link between two nodes consists of multiple parallel channels as subchannel. Towards this end, we establish bounds on secure communication. We show that for the case in which the relay does not listens to the source, under certain conditions secrecy capacity is established. In the last part of the thesis, we studied a two-user multi-input multioutput X-channel with local feedback and delayed channel state information\navailable at the transmitter in high signal-to-noise ratio regimes. In this setting each transmitter transmits independent messages to both receivers that need to be kept secret from each other. We characterize the optimal sum secure degrees of freedom region of this model.
A novel framework for sharing common randomness and generating secret keys in wireless networks is considered. In particular, a network of users equipped with pulse oscillators (POs) and coupling mechanisms in between is considered. Such mechanisms exist in synchronized biological and natural systems, and have been exploited to provide synchronization in distributed networks. We show that naturally-existing initial random phase differences between the POs in the network can be utilized to provide almost identical common randomness to the users. This randomness is extracted from the synchronization time in the network. Bounds on the entropy of such randomness are derived for a two-user system and a conjecture is made for a general n-user system. Then, a three-terminal scenario is considered including two legitimate users and a passive eavesdropper, referred to as Eve. Since in a practical setting Eve receives pulses with propagation delays, she can not identify the exact synchronization time. A simplified model is then considered for Eve's receiver and then a bound on the rate of secret key generation is derived. Also, it is shown, under certain conditions, that the proposed protocol is resilient to an active jammer equipped with a similar pulse generation mechanism.
Security has become an increasingly significant and urgent issue in wireless networks. The growing computational capability of the eavesdropper (Eve) and more and more complicated secure key management in large scale and heterogeneous networks, pose more and more stringent requirement to the cryptographic protocols. As a new design paradigm, physical layer security has been recognised as a promising solution to enhance the security of wireless links relying on the channel properties and advanced signal processing, and can act as either an alternative or a complementary solution to the conventional cryptographic methods. Without complicated secure key generation and management, physical layer security is quite suitable for the large scale distributed networks. In this thesis, we aim to improve the secrecy performance in emerging wireless communication systems relying on different physical layer security techniques. Firstly, the security of an amplify-and-forward successive relaying network with multiple untrusted relay nodes is investigated, where the conventional detrimental inter-relay interference is exploited to jam the untrusted nodes without assistance of external helpers. Considering different complexity requirements, several relay selection schemes are proposed, and the closed-form expressions of the lower bound of the secrecy outage probability and the maximum secrecy diversity order are derived accordingly. It is shown that the maximum secrecy diversity order of N-1 can be achieved for an N relay nodes network. Secondly, the artificial noise (AN) aided secure transmission strategy for a multiple-input single-output multiple-Eve (MISOME) system with a secure user (Bob) and a normal user (NU) is investigated. The power allocations among Bob, NU and AN, as well as the wiretap code rates, are jointly optimized to maximize the effective secrecy throughput (EST), under the average throughput constraint of NU. An alternative optimization algorithm with guaranteed convergence is proposed to obtain the optimal parameters. It is shown that the EST increases with the transmitting power and the number of transmit antennas, and decreases with the throughput constraint of NU, and the EST can be improved through injecting AN and concurrent transmission of Bob and NU. Thirdly, physical layer security in a multi-antenna small-cell network is investigated, where the multi-antenna base stations (BSs), cellular users, and Eves are all randomly distributed according to independent Poisson point processes. Stochastic geometry is applied to analyze the connection and secrecy outage probabilities and the average achievable secrecy rate. The impact of different parameters, including power allocation, BS and Eve density, and the adaptive eavesdropping or jamming on the secrecy performance is analyzed. It is shown that the average secrecy rate is a quasi-concave function of the power allocation factor and monotonically decreases with the ratio of the Eve-BS density. Finally, we study the physical layer security in a large-scale heterogeneous network consisting of both sub-6 GHz massive multi-input multi-output (MIMO) macro cells and millimeter wave (mmWave) small cells. By considering pilot spoofing attacks from the Eves, the coverage and secrecy probabilities are derived using stochastic geometry and the conditions under which the millimeter wave tier outperforms the sub-6 GHz counterpart are discussed in terms of both coverage and secrecy.It is shown that the mmWave small cell can provide better coverage performance in the high transmission code-rate region, and the secrecy performance of the mmWave system outperforms the sub-6 GHz counterpart in the low redundant rate region, which reveals the advantage of using mmWave for secure communication.
Physical layer security (PLS) is a new paradigm aiming at securing communications between legitimate parties at the physical layer. Conventionally, achieving confidentiality in communication networks relies on cryptographic techniques such as public-key cryptography, secret-key distribution and symmetric encryption. Such techniques are deemed secure based on the assumption of limited computational abilities of a wiretapper. Given the relentless progress in computational capacities and the dynamic topology and proliferation of modern wireless networks, the relevance of the previous techniques in securing communications is more and more questionable and less and less reliable. In contrast to this paradigm, PLS does not assume a specific computational power at any eavesdropper, its premise to guarantee provable security via employing channel coding techniques at the physical layer exploiting the inherent randomness in most communication systems. In this dissertation, we investigate a particular aspect of PLS, which is secret-key agreement, also known as secret-sharing. In this setup, two legitimate parties try to distill a secret-key via the observation of correlated signals through a noisy wireless channel, in the presence of an eavesdropper who must be kept ignorant of the secret-key. Additionally, a noiseless public channel is made available to the legitimate parties to exchange public messages that are also accessible to the eavesdropper. Recall that key agreement is an important aspect toward realizing secure communications in the sense that the key can be used in a one-time pad scheme to send the confidential message. In the first part, our focus is on secret-sharing over Rayleigh fading quasi-static channels. We study the fundamental relationship relating the probability of error and a given target secret-key rate in the high power regime. This is characterized through the diversity multiplexing tradeoff (DMT) concept, that we define for our model and then characterize it. We show that the impact of the secrecy constraint is to reduce the effective number of transmit antennas by the number of antennas at the eavesdropper. Toward this characterization, we provide several schemes achieving the DMT and we highlight disparities between coding for the wiretap channel and coding for secret-key agreement. In the second part of the present work, we consider a fast-fading setting in which the wireless channels change during each channel use. We consider a correlated environment where transmit, legitimate receiver and eavesdropper antennas are correlated. We characterize the optimal strategy achieving the highest secret-key rate. We also identify the impact of correlation matrices and illustrate our analysis with various numerical results. Finally, we study the system from an energy-efficiency point of view and evaluate relevant metrics as the minimum energy required for sharing a secret-key bit and the wideband slope.