  The vast majority of today's critical infrastructure is supported by numerous
feedback control loops and an attack on these control loops can have disastrous
consequences. This is a major concern since modern control systems are becoming
large and decentralized and thus more vulnerable to attacks. This paper is
concerned with the estimation and control of linear systems when some of the
sensors or actuators are corrupted by an attacker. In the first part we look at
the estimation problem where we characterize the resilience of a system to
attacks and study the possibility of increasing its resilience by a change of
parameters. We then propose an efficient algorithm to estimate the state
despite the attacks and we characterize its performance. Our approach is
inspired from the areas of error-correction over the reals and compressed
sensing. In the second part we consider the problem of designing
output-feedback controllers that stabilize the system despite attacks. We show
that a principle of separation between estimation and control holds and that
the design of resilient output feedback controllers can be reduced to the
design of resilient state estimators.
