The Wire-Tap Channel

In this paper we study a (perhaps noisy) communication system that is being wire-tapped via a second noisy channel. Our object is to encode the data in such a way that the wire-tapper's level of confusion will be as high as possible. To fix ideas, consider first the simple special case depicted in Fig. 1 (in which the main communication system is noiseless). The source emits a data sequence SI, S2, · · ·, which consists of independent copies of the binary random variable S, where P r { S = 0 } = P r { £ = l } = ± . The encoder examines the first K source bits S* = (SI, · · ·, SK) and encodes SA' into a binary N vector 1355 NOISELESS CHANNEL Fig. 1--Wire-tap channel (special case).