The 125kHz proximity technology has been around since 1990 its basically a physical RFID chip and when you present it to an EMI field the antenna inside is excited and responds with a signal that is identified by the reader. Theres a 26-bit format, which comprises of a site/facility code and the card number site code from 0-255 and card number from 0-65500 which is 16,702,000 possible permutations. It works well, but in the face of modern computing power, its a vulnerable technology.