Abstract
A SCADA system is a distributed network of cyber-physical devices used for instrumentation and control of critical infrastructures such as an electric power grid. With the emergence of the smart grid, SCADA systems are increasingly required to be connected to more open systems and security becomes crucial. However, many of these SCADA systems have been deployed for decades and were initially not designed with security in mind. In particular, the field devices in these systems are vulnerable to false command injection from an intruding or compromised device. But implementing cryptographic defence on these old-generation devices is challenging due to their computation constraints. As a key requirement, solutions to protect legacy SCADA systems have to be an add-on. This paper discusses two add-on defence strategies for legacy SCADA systems-the data diode and the detect-and-respond approach-and compares their security guarantees and applicable scenarios. A generic architectural framework is also proposed to implement the detect-and-respond strategy, with an instantiation to demonstrate its practicality.