On a special class of broadcast channels with confidential messages

It is shown that Csiszar and Korner's (1978) characterization of a discrete memoryless channel (DMC)X/spl rarr/Y as being less noisy than the DMC X/spl rarr/Z is equivalent to the condition that the mutual-information difference I(X;Y)-I(X;Z) be a convex-/spl cap/ function of the probability distribution for X. This result is used to obtain a simple determination of the capacity region of the broadcast channel with confidential messages (BCC), which is a DMC X/spl rarr/(Y,Z), when the DMC X/spl rarr/Y to the legitimate receiver is less noisy than the DMC X/spl rarr/Z to the enemy cryptanalyst and there is a probability distribution for X having strictly positive components that achieves capacity on both these channels. In particular, when these DMC's are both symmetric, then the secrecy capacity of the BCC is the difference of their capacities. It is shown further that the less-noisy condition in this result cannot be weakened to the condition that the DMC X/spl rarr/Y be more capable than the DMC X/spl rarr/Z in the sense of Csiszar and Korner.