Couldn't you reverse-engineer the master key from a lock anyway? Or is there some clever design preventing that?
You have one lock, and one key (not the master) for that lock, and a bunch of blanks.
You take the first blank. You cut a key that is identical to your key, except you vary the depth of a single cut. You repeat this until this new key works in your lock. That gives you the master key cut depth of one part of the key. You repeat this process forthe rest of the positions. You end up with a master key.
Edit: MrJones' comment here: https://news.ycombinator.com/item?id=10186309