The idea is to add an Intel Edison with an NFC reader to the door's (electric) lock to open it with the smartphone (and without the old metal keys). Intel Edison is the best bet for this project because of its size, maybe it can be even embedded into the door.
The management of the users database could be done using the wifi or bluetooth connection and a PC with a specific application.
In this way, the door will become smart enough to even interconnect with other domotic systems.
I have already built a modell but with a RPi which is much bigger.