  In this paper, we introduce the notion of Plausible Deniability in an
information theoretic framework. We consider a scenario where an entity that
eavesdrops through a broadcast channel summons one of the parties in a
communication protocol to reveal their message (or signal vector). It is
desirable that the summoned party have enough freedom to produce a fake output
that is likely plausible given the eavesdropper's observation. We examine three
variants of this problem -- Message Deniability, Transmitter Deniability, and
Receiver Deniability. In the first setting, the message sender is summoned to
produce the sent message. Similarly, in the second and third settings, the
transmitter and the receiver are required to produce the transmitted codeword,
and the received vector respectively. For each of these settings, we examine
the maximum communication rate that allows a given minimum rate of plausible
fake outputs. For the Message and Transmitter Deniability problems, we fully
characterise the capacity region for general broadcast channels, while for the
Receiver Deniability problem, we give an achievable rate region for physically
degraded broadcast channels.
