It is really hard to find good information online about proximity cards. I’m sure some companies consider this obscurity a form of security, but Jonathan Westhues demonstrates how severely flawed that notion is. He found a data sheet mentioning a 125kHz carrier frequency. He was off and running from there, eventually building a device that can read and repeat a proximity card’s data. It’s been argued that RFID technology is safe because of the short read distance. Jon actually found it easier to passively read cards that were being energized by a legitimate reader than by the device alone. Make sure you check out his home built PCB mill while you’re at the site.