Logic locking
Beginner
Building a secret key into a chip, like a password. Without it, a stolen copy gives wrong answers.
Novice
Adding extra gates controlled by a secret key, so the chip only works correctly once the right key is loaded after manufacture.
Expert
Aimed at overproduction and design theft by an untrusted factory. The simplest form inserts XOR/XNOR gates on internal wires, each fed by a key bit. The SAT attack recovers keys by using a working chip as an oracle; later schemes trade resistance to it against other attacks and overhead.
Explained in Defense (Industries).