  In this paper, we investigate detectability and identifiability of attacks on
linear dynamical systems that are subjected to external disturbances. We
generalize a concept for a security index, which was previously introduced for
static systems. The generalized index exactly quantifies the resources
necessary for targeted attacks to be undetectable and unidentifiable in the
presence of disturbances. This information is useful for both risk assessment
and for the design of anomaly detectors. Finally, we show how techniques from
the fault detection literature can be used to decouple disturbances and to
identify attacks, under certain sparsity constraints.
