SCADA connected to networks should be protected as follows:
IF no communication is required to the SCADA,
IF the SCADA interaction rate allows for encryption AND encryption does not interfere with an FSM wrapper,
|Low||No special protection is used for the SCADA.|
Connection to external control systems:
Regardless of the technology approach, the basic options for connectivity to external systems are:
Complete independence: This is the "Do not connect" approach. In this approach, the ICS network is segregated from the rest of the world, typically by physical isolation for high surety and logical separation for medium surety.
Information only: In this approach, information from the ICS is sent to anything else with no external influences intended back on the ICS. For high surety, this is done with a digital diode, and for medium surety with a firewall or similar separation mechanism properly configured.
Shared flows with islanding and reconnection coordination: In this approach, ICS components share flows, such as Internet-based communications or other shared infrastructure, but are able to operate without the external connection, albeit perhaps with less efficiency. External command and control, changing set points, and ordering the ICS to do anything it is physically capable of are all feasible, and thus more negative consequences are attainable from external influences. In this scheme, the external networks are disconnected and the system islanded when the desired level of surety cannot be otherwise attained, and reconnect over time as surety of connectivity increases to adequate levels.
Continuous interconnect: In this approach, the ICS environment is always connected to other networks, making for higher dependency and more efficient coordination when the network is working. However, when the network fails, is taken over, or misoperates for whatever reason, the ICS cannot be disconnected, and consequences over the entire operating and failure range are often attainable from remote actions.
Encryption: Encryption takes substantial time. For a SCADA that has to interact in real-time with feedback times on the order of milliseconds, encryption isn't fast enough in most cases to allow both the necessary SCADA computations and the encryption to take place in time to meet the demands of the SCADA signal timing.
Restricted access network zone: Such a zone reduces the sources that can be used to directly influence and observe SCADA inputs and outputs. When such a zone is available, it should be used unless there is a reason not to use it.
Use a custom FSM wrapper for the SCADA input: A custom FSM for the input of a SCADA provides a means by which all inputs can be checked for validity in the context of the expected machine state. This provides a high degree of certainty that unauthorized and unanticipated input sequences cannot appear at the SCADA input.
Use a digital diode to exfiltrate SCADA data: A digital diode can be used to prevent output channels from being used for input to a high degree of certainty. This will normally require protocol alterations, such as TCP to UDP and UDP to TCP proxies on sending and receiving sides of the diode in order to interface with technologies that depend on 2-way transport.
Do not connect the SCADA to the network: When the situation has consequences too high to risk connection, it should be run in an isolated manner.