The ubiquity of wireless connectivity between an ever-increasing number of devices has confirmed dire past predictions of widespread eavesdropping (among other security concerns) in wireless networks, further promoting the need for secure communications. Of particular interest are physical layer techniques which exploit error-correction coding intrinsic to digital transceivers and which offer two clear advantages: (i) no need for key distribution; and (ii) secrecy that is not conditioned on a computationally limited adversary. Present techniques, however, assume some knowledge of the channel connecting the transmitter to the eavesdropper; if the eavesdropper is truly passive, this critical parameter is unavailable. We therefore examine secure communication schemes that do not assume knowledge of the eavesdropper's channel, yet remain keyless in their operation. Two schemes are offered, with behavior intermediate between cryptographic and information-theoretic solutions.
The ubiquity of wireless connectivity between an ever-increasing number of devices has confirmed dire past predictions of widespread eavesdropping (among other security concerns) in wireless networks, further promoting the need for secure communications. Of particular interest are physical layer techniques which exploit error-correction coding intrinsic to digital transceivers and which offer two clear advantages: (i) no need for key distribution; and (ii) secrecy that is not conditioned on a computationally limited adversary. Present techniques, however, assume some knowledge of the channel connecting the transmitter to the eavesdropper; if the eavesdropper is truly passive, this critical parameter is unavailable. We therefore examine secure communication schemes that do not assume knowledge of the eavesdropper's channel, yet remain keyless in their operation. Two schemes are offered, with behavior intermediate between cryptographic and information-theoretic solutions.
Wireless networking has kept evolving with additional features and increasing capacity. Meanwhile, inherent characteristics of wireless networking make it more vulnerable than wired networks. In this thesis we present an extensive and comprehensive review of physical layer security in wireless networking. Different from cryptography, physical layer security, emerging from the information theoretic assessment of secrecy, could leverage the properties of wireless channel for security purpose, by either enabling secret communication without the need of keys, or facilitating the key agreement process. Hence we categorize existing literature into two main branches, namely keyless security and key-based security. We elaborate the evolution of this area from the early theoretic works on the wiretap channel, to its generalizations to more complicated scenarios including multiple-user, multiple-access and multiple-antenna systems, and introduce not only theoretical results but practical implementations. We critically and systematically examine the existing knowledge by analyzing the fundamental mechanics for each approach. Hence we are able to highlight advantages and limitations of proposed techniques, as well their interrelations, and bring insights into future developments of this area.
We are in the midst of wireless revolution, and increasing demand continues for wireless applications.This explosive growth, of wireless communications and services, inevitably renders security into a challenging quality of service constraint that must be accounted for in the network design.The state of the art methods in combating the security threats are usually founded on cryptographic approaches.These techniques typically assume limited computational resources at adversaries, are usually derived from unproven assumptions, and most of the time do not offer a measurable security notion.Information theoretic security, on the other hand, eliminates the aforementioned limitations of the cryptographic techniques at the physical layer of communication systems.In this thesis, we concentrate on both the theoretical and the practical aspects of physical layer security.We first start by analyzing elemental interference networks, in particular, two-user channels with an adversary.The problem here is to characterize the fundamental limits on secure transmission rates.Towards this end, we devise coding schemes, forming inner bounds to the capacity region, and compare the achievable rates with outer bounds.This analysis is useful to explore microscopic gains that can be leveraged by the different coding schemes, and our analysis shows that the inherent interference can in fact be exploited to cooperatively confuse the eavesdropping node.4.4 There are ⌈δ log n⌉ number of disjoint highways within each rectangle of size (κ log nǫ) × √ n.The legitimate users in the slab, denoted by dotted lines, of the rectangle is served by the highway denoted with red bold line. . . . . . . . . . . . . . . . . . .
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.
The ability to exchange secret information is critical to many commercial, governmental, and military networks. Information-theoretic security-widely accepted as the strictest notion of security-relies on channel coding techniques that exploit the inherent randomness of the propagation channels to strengthen the security of digital communications systems. Motivated by recent developments in the field, we aim to characterize the fundamental secrecy limits of wireless networks. The paper is comprised of two separate parts. In Part I, we define the intrinsically secure communications graph (iS-graph), a random graph which describes the connections that can be securely established over a large-scale network. We provide conclusive results for the local connectivity of the Poisson iS-graph, in terms of node degrees and isolation probabilities. We show how the secure connectivity of the network varies with the wireless propagation effects, the secrecy rate threshold of each link, and the noise powers of legitimate nodes and eavesdroppers. We then propose sectorized transmission and eavesdropper neutralization as viable strategies for improving the secure connectivity. Our results help clarify how the spatial density of eavesdroppers can compromise the intrinsic security of wireless networks. In Part II of the paper, we study the achievable secrecy rates and the effect of eavesdropper collusion.
Information theoretic approaches to security have been examined as a promising complement to current cryptographic techniques. Such information theoretic approaches establish reliable communication and data confidentiality directly at the physical layer of a communication network by taking the properties of the noisy channel into account leading to unconditional security regardless of the computational capabilities of eavesdroppers. The provision of accurate channel state information is a major challenge particularly in wireless communication systems, especially information about the channels to eavesdroppers. In addition, there might be malevolent adversaries who jam or influence the channel of the legitimate users. This paper surveys different models for secure communication under channel uncertainty and adversarial attacks and reviews the corresponding secrecy capacity results, which characterize the maximum rate at which information can be sent to legitimate receivers while being kept perfectly security from eavesdroppers.
Wireless communications are naturally exposed to important security risks due to their broadcast nature. In contrast with wired communications, no physical medium is isolating the transmission surroundings to a specific path. Traditional methods to reach the goals of communications security rely on algorithms at protocol stack level, which are based on mathematically derived cryptographic protocols. With the tremendous proliferation of wireless devices, and the increase in computational power available for decryption, traditional cryptographic measures may be compromised. To protect communications at the lowest level, physical layer (PHY) security methods have been proposed recently. These techniques aim to ensure secure communications by exploiting the unique characteristics of the physical medium between communicating nodes, effectively reducing the decryption capabilities of an eavesdropper to intercept transmissions. This thesis explains within a general framework the most important PHY layer security schemes. First we explore approaches which utilize the knowledge of the unique channel between communicating nodes to distort a transmitted signal with various kinds of randomizations, in such a way that the intended receivers are unaffected by the distortion, but any other observer will be exposed to it. These techniques, however, introduce several trade-offs between transmission resources and the effectiveness in achieving secrecy. Of particular interest for this investigation is a class of methods which can be referred to as "security enhancing techniques", in which minimal priors about the potential eavesdroppers locations and capabilities are assumed. In this thesis, a wave-based approach to the communication secrecy problem is introduced. An interesting perspective is provided by studying the relation between electromagnetic theory and information theory, the so-called electromagnetic information theory. This allows to properly identify the unavoidable limitations imposed by the laws of electromagnetics to our ability to communicate securely. The proposed model can be extended to the analysis of different signal distortion methodologies and different channel conditions. The resulting approach is highly relevant to the study of PHY layer security methods since it is physically fundamental as well as architecture independent, which leads to a consolidating point of view of the secrecy problem. In addition, the thesis presents computer simulation results which shed insight on the security possibilities and limitations of three different security scenarios, which are termed MISO (multiple-input, single-output) (with single eavesdropper), MISOME (multiple-input, single-output, multiple eavesdropper), and MIMOME (multiple-input, multiple-output, multiple-eavesdropper).