If your job is installing locks on people's houses and fixing their broken locks, you can call yourself a locksmith.
A locksmith knows all about locks; she makes them, puts them on doors, and repairs them. If you've ever locked yourself out of your house, the person to call for help is a locksmith. When you need a new copy of your apartment key, you can also visit a locksmith. The word comes from lock and smith, from the Old English smið, "one who works with metal."