Arduino/DigiKey USB comms (car immobiliser)

I'd like to build a circuit as an immobiliser for a kit car that will prevent the engine starting unless a dongle is plugged in.

I'm thinking I could programme a Digispark to output a number/string like it's a keyboard (this is fairly trivial). This data would be read by the Arduino which would then authenticate it and control a relay or something.

Now, is this just a case of connecting a USB female cable to RX/TX on the Arduino (Uno/Nano) and reading the serial data or is there a better way of doing it?

You could use an iButton. It has the security number built in.

Simply touch the button to its reader and enable the ignition. Program to require the read before each restart of the engine.