In this thesis, we consider secure communication in the presence of an eavesdropper. With the explosion in the growth of the data produced and communicated, sensitive information such as financial transactions, health records, and control signals for cyber-physical systems, has to be securely exchanged. Today, the ever-increasing computational power of adversaries is challenging the state-of-the-art cryptographic encryption mechanisms, as these mechanisms assume adversaries with limited computational power. Thus, with the advent of the quantum computing era, we require new mechanisms to guarantee a secure exchange of information. Moreover, the growing number of small and energy constrained connected devices involved in data exchange calls for lightweight encryption schemes, as low-complexity devices cannot implement complex schemes.
We consider three different scenarios and exploit specific opportunities present in each of these scenarios; we develop lightweight encryption schemes that do not require sharing large keys in advance and are secure against eavesdroppers with unlimited computational capabilities.
The first scenario we consider is multiple unicast traffic over wireline networks. In these networks, a single source is connected to m destinations interested in different messages. In designing encryption schemes, we exploit the fact that although the eavesdropper is computationally super-powerful, it might not have capabilities to eavesdrop the entire network. We use the multi-path diversity to securely communicate against the eavesdropper without requiring any pre-shared key.
The second scenario we consider consists of millimeter wave (mmWave) networks. mmWave communication requires deploying networks of relays that communicate through directional beams to compensate for the high path-loss and the high blockage. Since we need to use beamforming and align beams to activate links, we cannot use all the underlying links of the network simultaneously. However, the degree of freedom in choosing the links to activate can be leveraged for secure communication against an eavesdropper. We show that we can achieve a secure capacity that in some cases, can be very close to the unsecure capacity. Here, capacity refers to the maximum flow of information over the network.
For the third scenario, we consider cyber-physical systems and propose a distortion based security framework where the distortion measures the distance between the eavesdropper's estimates and the ground truth. The primary motivation for this framework is that the messages exchanged in these systems are embedded in a metric space having a notion of distance, and securing raw bits as in traditional encryption schemes might not be necessary. Instead, we show with an example of a linear dynamical system that a carefully designed encryption scheme can significantly distort the eavesdropper's view with just one bit of the pre-shared key.
