I've been using serial cable with data leds and gpg ascii armored data over it. Very easy to visually inspect all data before further processing. Only attack ve

I've been using serial cable with data leds and gpg ascii armored data over it. Very easy to visually inspect all data before further processing. Only attack vector remaining is gpg ascii armored parser, signature verification & decryption. Afaik, this is quite safe. This computer has not been connected to the internet ever, and it won't be in future. Don't forget physical site security.