I wrote an article about the security of passive RFID tags at school about a year ago: http://kimjoar.net/security-passive-rfid-tags.html http://kimjoar.net/sec

I wrote an article about the security of passive RFID tags at school about a year ago: http://kimjoar.net/security-passive-rfid-tags.html http://kimjoar.net/security-passive-rfid-tags.html. There you might find some interesting stuff regarding your question. RFID is very cool, but there are still a lot of (unsolved) security problems with them, especially the passive tags.