Physical Layer Security over OFDM-Based Links: Conjugate-and-Return

We describe a novel technique that can be used to allow two parties to exchange a secret key over an orthogonal frequency division multiplexing (OFDM) channel with perfect secrecy, provided subcarriers are subject to independent fading and a malicious attacker is passively eavesdropping. Our approach is shown to be robust against the eavesdropper´s channel being correlated with that of the legitimate users, and also active attacks in which the eavesdropper injects fraudulent messages into the system. For the active attack, we show that when the eavesdropper injects fraudulent messages into the system, they will also inadvertently allow the legitimate users to calculate a lower bound on the secrecy capacity of the channel. This allows them to establish whether secrecy has been achieved. A consequence of our approach is that the degrees of freedom within the channel are halved.