>but that'll require power source and some interaction interface. Oh, I dunno - here's an RFID transmitter which only uses 2 pins on an ATTiny, neither of whic

>but that'll require power source and some interaction interface. Oh, I dunno - here's an RFID transmitter which only uses 2 pins on an ATTiny, neither of which are power or ground: https://trmm.net/AVR_RFID https://trmm.net/AVR_RFID Okay, technically the power source is the external RFID reader and the interaction interface is hardcoded, but that would work fine for something like a crypto key.