Most applications of RFID by authentication (think door locks) use only unique ID (address) of the card and nothing else. And the communication protocol used by

Most applications of RFID by authentication (think door locks) use only unique ID (address) of the card and nothing else. And the communication protocol used by reader works like this: Is there anyone with address starting with 0? ... Starting with 1? Yes. Starting with 10? ... Starting with 11? Yes. ..... So you only have to listen to reader side of communication and guess the last bit.