  This paper considers the problem of optimal zero-delay jamming over an
additive noise channel. Early work had already solved this problem for a
Gaussian source and channel. Building on a sequence of recent results on
conditions for linearity of optimal estimation, and of optimal mappings in
source-channel coding, we derive the saddle-point solution to the jamming
problem for general sources and channels, without recourse to Gaussian
assumptions. We show that linearity conditions play a pivotal role in jamming,
in the sense that the optimal jamming strategy is to effectively force both
transmitter and receiver to default to linear mappings, i.e., the jammer
ensures, whenever possible, that the transmitter and receiver cannot benefit
from non-linear strategies. This result is shown to subsume the known result
for Gaussian source and channel. We analyze conditions and general settings
where such unbeatable strategy can indeed be achieved by the jammer. Moreover,
we provide the procedure to approximate optimal jamming in the remaining
(source-channel) cases where the jammer cannot impose linearity on the
transmitter and the receiver.
