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.
We secondly focus on large networks. As the users interfere, the problem here is to identify the optimal ways of distributing network resources among users under the secrecy constraints. Initially, we consider a network with finite number of nodes in the high signal to noise ratio regime, where the noise impairing each receiver is deemphasized compared to the signal strengths. In this asymptotic scenario, we propose efficient interference alignment techniques along with secrecy pre-coding allowing each user to achieve positive secure degrees of freedom. Next, we investigate the secrecy capacity scaling, i.e., the secure rate per user as the number of users gets large. We propose a novel multi-hop forwarding scheme, and, utilizing tools from the percolation theory, we show that each user can achieve the optimal secure rate scaling under a mild condition on the eavesdropper density.
The final attempt is on the design of practical coding schemes for physical layer security. Specifically, we show that polar codes achieve non-trivial perfect secrecy rates for a fairly large set of channels with a low encoding and decoding complexity. We also propose a secret key agreement scheme over fading channels. This technique essentially combines physical layer coding schemes with cryptographic methods, and, establishes key agreement by only requiring the statistical knowledge of the eavesdropper channel state information.
These results are encouraging in the sense that a) interference can be exploited to enhance security, b) not only small sized networks, but also large networks can achieve information theoretic security, c) there exists practical coding schemes achieving secrecy for a fairly large set of channels, and d) physical layer security can be implemented together with higher layer cryptographic protocols, and hence, allows for cross-layer security solutions.