  The increase in network connectivity has also resulted in several
high-profile attacks on cyber-physical systems. An attacker that manages to
access a local network could remotely affect control performance by tampering
with sensor measurements delivered to the controller. Recent results have shown
that with network-based attacks, such as Man-in-the-Middle attacks, the
attacker can introduce an unbounded state estimation error if measurements from
a suitable subset of sensors contain false data when delivered to the
controller. While these attacks can be addressed with the standard
cryptographic tools that ensure data integrity, their continuous use would
introduce significant communication and computation overhead. Consequently, we
study effects of intermittent data integrity guarantees on system performance
under stealthy attacks. We consider linear estimators equipped with a general
type of residual-based intrusion detectors (including $\chi^2$ and SPRT
detectors), and show that even when integrity of sensor measurements is
enforced only intermittently, the attack impact is significantly limited;
specifically, the state estimation error is bounded or the attacker cannot
remain stealthy. Furthermore, we present methods to: (1) evaluate the effects
of any given integrity enforcement policy in terms of reachable
state-estimation errors for any type of stealthy attacks, and (2) design an
enforcement policy that provides the desired estimation error guarantees under
attack. Finally, on three automotive case studies we show that even with less
than 10% of authenticated messages we can ensure satisfiable control
performance in the presence of attacks.
