So you want access control like those with a keypad, but wireless.
As I wrote already, you need a device that is continuously (actively) waiting for incoming messages. And a device that sends such a message on demand. BT communication is okay for that purpose, because it's locally restricted and also allows for bidirectional communication for safe authentication. Consider that everybody can put a simple receiver next your door, and record whatever your car is transmitting. That's why a one way transmission of a key is not sufficient.