Physical Layer Secrecy in Large Multihop Wireless Networks

Wireless networks provide flexible ubiquitous user communications but, at the same time, provide the same access to an attacker or eavesdropper trying to intercept private messages. Hence, security has become a primary concern in wireless networks, and there has been a resurgence of interest in physical layer forms of secrecy based on Shannon’s original information-theoretic formulation and Wyner’s consideration of the wiretap formulation of such. In this chapter, we venture beyond the prototypical three-node Alice-Bob-Eve security scenario to consider physical layer forms of secrecy in wireless networks, where the need to protect the message over multiple hops opens up system vulnerabilities, but the addition of system nodes beyond the communicating parties also offers opportunities. In particular, the scaling of secrecy in (asymptotically) large multihop networks will be considered, and we will focus, in particular, on how the flexibility offered by the large number of system nodes in the network can facilitate secure communication in the face of a large number of eavesdroppers, even if the location of those eavesdroppers is unknown. The transmission techniques that are employed in the conclusive results for secrecy scaling in large networks with noncollaborating eavesdroppers also have clear analogs that should prove effective in finite networks, but there are challenges to be addressed in such an adaptation.