Originally Posted by gfunkdave

Proxy cards don't do any crypto. They just send their static serial number to the door reader (well, the reader induces a current in the antenna embedded into the card, which is enough to power a tiny chip that sends its serial number). Actually, I think they send a facility number (1-256) and a serial number (1-65536). The computer running the thing then determines if that serial number is authorized to access the door. So you need is to get the phone's NFC chip to send the correct facility and serial numbers.