Enter the Option menu, then enter the Controller menu. Now, hold R2 and press Right, Down, Right, Triangle, X. If done right, you will hear someone coughing and the cheat menu will activate. You can access it after you start the game.
After activating the cheat menu, press START and go to Cheats > Keys. The following defines the element for each key:
Note: The keys are given a numeric designation, as follows:
1: Steel Key
2: Cave Key
3: Axe Key
4: Fire Key
5: Emerald Key
6: Dungeon Key
7: Silver Key
8: Rusted Key
9: Horn Key
10: Swamp Key
11: Castle Key