There's no need for device to be made secure. Just configure your network so that the device can only talk through a specific virtual private network, or even better a separate physical network. It's not difficult, but it might be more costly. Car manufacturers do this. The entertainment system usually cannot talk directly to the engine control unit. There is a bridge device between the two that controls the conversation. Safety critical stuff is on the ECU side of the bridge not the entertainment side and the ECU side is not public.<p>I don't mean cars are perfect, just that the concept is well known and manufacturers do try to implement it (with varying degrees of success of course).