car starting is something that John already does, using an arduino, and something I plan to do.
You don't necessarily have to bypass any security. I plan to still require the key to be inserted and turned to the usual running position, mostly because I don't want to remove the steering wheel lock, but the key just wont be used to crank the engine, the NFC ring, arduino and a relay will do all of that for me.
Essentially, im guessing the arduino outputs 12v on 1 wire when an authorised tag is present, if this is the case the 12v from the RFID system above goes to Pin 85, Pin 86 is grounded, Pin 87 connects to the ignition wire and Pin 30 is connected to the starter wire in the ignition circuit. The Pin numbers are obviously on the relay.
Should be simple really, but I know NOTHING about arduinio's im assuming john has a NFC shield attached to it, but as for code, anyone's guess.