[mirimir]

My point is that compartmentalization allows secure communication through untrusted devices. It won't be convenient, but it's doable. There is no "computer". There are local networks of suitably isolated devices.

The device that decrypts can't send anything to the Internet, because it's behind receive-only optoisolators. The device that encrypts can't receive anything from the Internet, because it's behind send-only optoisolators. All intervening information processing may occur in your head. Or there may be other devices that are totally air-gapped, with all data transfer through single use flash storage. If you're using entirely untrusted devices, you move all crypto to such air-gapped devices.

It does help if these devices can be trusted, but that's not essential. You could, for example, do encryption manually with one-time pads. Or use that thing with decks of cards.

Maybe you claim that no trustable devices will be available. But that's unlikely. Consider how easy it is to obtain Afghani heroin in NYC. Also, if I were targeted by American adversaries, I could arguably trust devices backdoored by the Russians, or the North Koreans, etc. And vice versa.