Wiktionary
vb. (context computing English) To take as a controlling input datum
Usage examples of "key off".
Flynn pulled his keys out of his pocket, worked the house key off the chain.
So he tapped into one of the many skills he acquired during a life of petty crime, duped the locker key off Moogey's key ring, and by process of lengthy elimination, found the locker.
She picked up the ring of keys and took the old black key off the ring.
I went back to one of the motels, went into the office, turned on the light, picked a key off the desk and located a cabin by myself.
You want in one of these cases, you go to the appropriate office and get the key off a hook by the desk and sign for it.
She didn't, either, but took a key off a hook and dropped it into his hand, her fingers crooked, as if to make sure that she shouldn't touch him.
Carefully she peeled the key off the dead man's flesh, dropped it into Nangi's palm.