That was our approach. Build it into the keyboard, do all the various secrets management (passwords, TOTP, et al) and message encryption behind a data diode (sorta air gapped). And RPi isn't the ideal compute module for it as you don't want a wireless chip on the inside of the "airgap". https://www.anomie.tech/
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Using something like an RPI Zero and carrying this around would make more sense if the device had a display and could be airgrapped.
That was our approach. Build it into the keyboard, do all the various secrets management (passwords, TOTP, et al) and message encryption behind a data diode (sorta air gapped). And RPi isn't the ideal compute module for it as you don't want a wireless chip on the inside of the "airgap". https://www.anomie.tech/
I’m not sure a RPI makes sense at all for stuff like this. They are so heavyweight compared to a little microcontroller board.