http://www.eng.tau.ac.il/~yash/kw-usenix06/index.html </a>
Check it Out…
How to Build a Low-Cost, Extended-Range RFID Skimmer
Also some of the supporting documents.
A Practical Relay Attack on ISO 14443 Proximity Cards
S4100 Multi-Function Reader Module Data Sheet
Security Analysis of a Cryptographically-Enabled RFID’s