  Consider the problem of covertly controlling a linear system. In this
problem, Alice desires to control (stabilize or change the parameters of) a
linear system, while keeping an observer, Willie, unable to decide if the
system is indeed being controlled or not.
  We formally define the problem, under two different models: (i) When Willie
can only observe the system's output (ii) When Willie can directly observe the
control signal. Focusing on AR(1) systems, we show that when Willie observes
the system's output through a clean channel, an inherently unstable linear
system can not be covertly stabilized. However, an inherently stable linear
system can be covertly controlled, in the sense of covertly changing its
parameter. Moreover, we give direct and converse results for two important
controllers: a minimal-information controller, where Alice is allowed to used
only $1$ bit per sample, and a maximal-information controller, where Alice is
allowed to view the real-valued output. Unlike covert communication, where the
trade-off is between rate and covertness, the results reveal an interesting
\emph{three--fold} trade--off in covert control: the amount of information used
by the controller, control performance and covertness. To the best of our
knowledge, this is the first study formally defining covert control.
