In recent years, researchers have explored using unique radio propagation characteristics between two devices for extracting symmetric keys. However, the state-of-the-art has following limitations: (i) paying more attention to only when the devices are in communication range, and (ii) considering only the devices that are mobile. Secret key generation for devices which are not in communication range and for stationary nodes is quite a challenging task. In this work, we study the feasibility of generating secret keys between two devices which do not possess any direct link with the help of a trusted relay. We propose a novel key generation protocol for unreachable nodes, and a multi-level quantization mechanism. We implement our solution using off-the-shelf commercially available resource constrained devices suitable for IoT and wireless body area networks. We conduct an extensive set of experiments in indoor environments for various scenarios involving stationary and mobile nodes. Our
results reveal that an eavesdropper cannot obtain sufficient useful information about the shared keys when at least any one of the three legitimate devices is mobile. The entropy of shared keys obtained between the legitimate devices ranges from 0.97 to
0.98 bit, which is very close to the highest entropy of binary key.
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This work is partially supported by Australian Research Council Discovery grant DP150100564.