This is a minimal AtTiny AVR firmware that will listen to a TWI (I2C) bus and store all data on EEPROM. It's simple, but will probably require some getting to know before any modifications can be done. I'm sorry for the assembly, but I really needed fine control over execution and timing.
###Motivation
While taking a closer look at a hardware password manager from a friend, It was noted that sometimes critical data is going on physical busses of devices. The design in question would gladly read an encryption key from protected memory over TWI without encrypting it in transit. By implanting the device with a small microcontroller, the keys to decrypt the password store can be obtained.
Essentially this implant will listen to a TWI bus and store any captured data on the EEPROM of the chip, from which it can later be read. An attacker would need to implant the chip, have the owner do the operation that will generate the wanted traffic, and finally retrieve the device for collection and post-processing of the data.