In this talk from the 2013 DEFCON conference, Francis Brown and Bishop Fox provide practical insight into how RFID proximity badge systems work. They highlight the real world difficulties encountered in reading RFID prox cards with conventional readers, and what you’ll need to build out your own RFID physical penetration toolkit. They use an Arduino microcontroller to convert commercial RFID badge readers for long range hacking.
Hardware details, schematic, code links and more can be found at Bishopfox.