A Note on the Information-Theoretic-(in)Security of Fading Generated Secret Keys

In this work we explore the security of secret keys generated via the electromagnetic reciprocity of the wireless fading channel. Identifying a new sophisticated colluding attack, we explore the information-theoretic-security for such keys in the presence of an all-powerful adversary constrained only by the laws of quantum mechanics. Specifically, we calculate the reduction in the conditional mutual information between transmitter and receiver that can occur when an adversary with unlimited computational and communication resources places directional antenna interceptors at chosen locations. Such locations, in principal, can be arbitrarily far from the intended receiver yet still influence the secret key rate. We show how, in principal, the key rate can be driven to zero. We then investigate how assumed limitations on an adversary's knowledge of transceiver positions can potentially restore some level of information-theoretic security. Finally, we compare our new results with the secret key rates anticipated from quantum-technology implementations that are deployed in next generation wireless networks under the assumption of an all-powerful adversary.