One thing was suggested here earlier is a computer that can send data out but is physically unable to receive. This you would use to create and sign content. An

One thing was suggested here earlier is a computer that can send data out but is physically unable to receive. This you would use to create and sign content. Another computer can only receive but never send. This one is used to read messages sent to you and verify signatures. This way you can have authenticated communication. It can probably be used for encrypted communication as well, though there are problems with entering keys of your correspondents into the computer that only sends data. I don't know how practical this is, but seems resilient against hardware back doors.