I guess even some sort of "signed device protocol" will not work. An attacker can just create a device that guesses the device identifier (or whatever is used to create the signature). Then, the attacker device can just keep guessing until it gets it right. Chances are, some serial number or similar will be used for this, so continuous guessing is feasible.<p>Will the solution to this, then, be to have some sort of "smart card enabled device"? For example, assuming TOFU, you manually accept all device's public keys (and all devices, including cables and stuff will have one of these). Then, the computer will have to verify all actions done by those devices by sending a challenge for each action. But this seems impractical and inefficient...<p>Perhaps physical security is the only way for this...