Comment by sgc
8 hours ago
Seems like 95% of devices could be managed with a hardware filter that only allows through a handful of very rudimentary signals (more like Morse code than a full digital transmission), and then of course a properly designed on-device API that defines those signals and doesn't allow you to trigger a defrost remotely, when that should only ever be done with somebody physically present for example.
Morse code is a fully digital transmission.
How exactly would a more obscure (or even “analog”) encoding help? This seems orthogonal to reducing surface complexity and remotely executable command subsets.
The point is having the digital transmission simple enough that you can use fully *physical* filters to discard everything but the 3-4 messages you need through. Of course it is still digital, but my point is, it is a physically enforced studied, safe subset and doesn't allow for full network access. There are of course a variety of ways to further firewall that, but something needs to act like as much of an air gap as possible while allowing for remote communications.
How do you authenticate such a system? 3-4 “quasi analog” messages seem incredibly easy to replay, if nothing else. If you need to depend on something actually cryptographic to make the system replay proof, you can just as well use that system for all aspects of access control. This makes no sense.