You may be interested to know that RFID cards are also 'smart', although less so. They have hardware circuits that can perform some cryptographic routines, and receive enough power to perform many calculations. So an RFID reader will send a challenge, the card will perform some operations based on a secret key, and send back the result. The reader is connected to a central database, that also knows this secret, so it knows if the response is valid. As such you can't just 'copy' an RFID card, or even replay a valid request (since the challenge is unique).
