The project is going to be implemented on an FPGA because that gives me the necessary flexibility in the design. I have chosen the ICE40HX1K that requires 34112 bytes of configuration data, so that it fits into a simple 24LC512 eeprom. I have built an Arduino based eeprom programmer earlier, that will serve as an starting point for my project:

I will implement the communication via an ultrasonic link. Ultrasonic is advantageous because it requires a real physical contact between the communication partner. That is due to the big difference in acoustic impedance between water (and therefore human) and air, it causes mostly everything to be reflected when crossing between these media. It makes it almost impossible to fake a key exchange without real physical contact.

I have done some initial experiments with high voltage piezos (right) with a resonance frequency of about 100kHz, but they performed very bad, in fact I was not able to transmit any ultrasonic. I thought about what other piezos I could use and came up with ultrasonic misting devices. I got some (left), if they are suitable is still a point of research.