  We investigate physical-layer security of the full-duplex (FD)
amplify-and-forward (AF) relay channel. We provide a new perspective on the
problem and show that the processing time (delay) at the relay can be exploited
to improve the system's security. We show that the FD AF relay channel can be
seen as an intersymbol-interference (ISI) channel, hence, the discrete-Fourier
transform (DFT) can be used for data modulation and demodulation to convert the
frequency-selective channel into flat-fading channel per
sub-channel/sub-carrier. By exploiting the fact that the channel memory needs
to be cleared by inserting the cyclic-prefix, Alice injects an artificial-noise
(AN) signal that hurts the eavesdropping nodes only. The strength of this AN
signal and its interference rank are controlled by the relay's processing time.
