  This paper addresses the problem of output consensus in linear passive
multi-agent systems under a False Data Injection (FDI) attack, considering the
unavailability of complete state information. Our formulation relies on an
event-based cryptographic authentication scheme for sensor integrity and
considers FDI attacks at the actuator end, inspired by their practical nature
and usages. For secure consensus, we propose (i) a passivity-based approach for
detecting FDI attacks on the system and (ii) a Zeno-free event-triggered
observer-based switching controller, which switches between the normal and the
defense modes following an attack detection. We show that the closed-loop
system achieves practical consensus under the controller's action in the
defense mode. Simulation examples are provided to support the theoretical
findings.
