The idea here wasn't to make the smallest one possible. I decided to make a device that was practical and useable. And something that looks like it was from WW2. Something that could actually be used in the field in place of a real Enigma machine. Obviously there were some limitations. I could have a 26 key keyboard for a start so I had to come up with a UI that would work with a minimal number of keys. I bought a small 128x64 OLED, a suitable battery and started breadboarding it all up. With it working on a normal Arduino I bought an Arduino Pro Mini (or a good replica!) and started looking at getting it running on that.