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There're different devices. You put online some but not the others.

Temperature sensor is a sensor. Refrigeration compressor motor is an actuator.

GPS positioning of a car is a sensor. Motor to turn the wheel is an actuator.

Aircraft altimeter is a sensor. Motors to control control surfaces are actuators.

And so on.

Sure, but as stated in reply to a sibling comment, you then just hack the read-only system to mis-report sensor state and interfere with operations that way instead. Or use one of a million other approaches. Attacks won't follow your rules.

I think this comes down to the implementation of a particular system. But in general, since control system (the one that controls things through actuators) is NOT online, one can't interfere with it through online channels. They can interfere with sensors and fake readings reported online for example, probably, but not the ones used by the control system as it's offline.

Indeed, I agree that it's better to have the control systems separate from the telemetry systems and that absolutely does prevent certain types of attack. It's dangerous to think that prevents all types of attack, however.

Reply on news.ycombinator.com