In this paper, we introduce the notion of Plausible Deniability in aninformation theoretic framework. We consider a scenario where an entity thateavesdrops through a broadcast channel summons one of the parties in acommunication protocol to reveal their message (or signal vector). It isdesirable that the summoned party have enough freedom to produce a fake outputthat is likely plausible given the eavesdropper's observation. We examine threevariants of this problem -- Message Deniability, Transmitter Deniability, andReceiver Deniability. In the first setting, the message sender is summoned toproduce the sent message. Similarly, in the second and third settings, thetransmitter and the receiver are required to produce the transmitted codeword,and the received vector respectively. For each of these settings, we examinethe maximum communication rate that allows a given minimum rate of plausiblefake outputs. For the Message and Transmitter Deniability problems, we fullycharacterise the capacity region for general broadcast channels, while for theReceiver Deniability problem, we give an achievable rate region for physicallydegraded broadcast channels.
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