RFID hacking preamble: designing an FPGA IIR filter for the proxmark3

Introduction At work, they recently replaced the coffee vending machine for a new one. One detail quickly piqued my interest though: you could now ask the front desk for an RFID token, pay your coffee with it instead of regular coins, and also deposit cash into the token at the vending machine. How does it work? Which RFID IC does it use? Can we play with it? The next few posts will narrate this journey.