Access 4 million hotel rooms with an Arduino →

in short: At the base of every Onity lock is a small barrel-type DC power socket (just like on your old-school Nokia phone). This socket is used to charge up the lock’s battery, and to program the lock with a the hotel’s “sitecode” — a 32-bit key that identifies the hotel. By plugging an Arduino microcontroller into the DC socket, Brocious found that he could simply read this 32-bit key out of the lock’s memory. No authentication is required — and the key is stored in the same memory location on every Onity lock..