Channel scrambling for secrecy

We investigate the fundamental limits of secure communication over a wiretap channel in which the legitimate receiver is able to scramble the state of the channel. We provide simple single-letter bounds for secrecy capacity, which are useful in several instances. For the full-duplex Gaussian case, we show that jamming with Gaussian noise yields rates within at most 0.5 bits per channel use of secrecy capacity. For the half-duplex binary symmetric channel, we show that jamming is strictly suboptimal in certain regimes.