Comment by mikewarot

The story referenced as part of the motivation for the project[1] is pretty chilling. The laws of physics can put a lower limit on things for you if you have an old school analog oscilloscope handy to watch for network packets.

If you have old school TTL, EPROMs, RAM, and time, you could built a CPU you can test all the parts of, and trust. You could even work your way up to floppy disks, and an analog CRT display.

Once you want to ramp up the speed and complexity, things get dicey. I have ideas that would help, but nothing provably secure.

[1] https://www.teamten.com/lawrence/writings/coding-machines/