It's a very strange key. Designed to prevent pirates and rogues from lockpicking. I would say the lock this goes to can't be lockpicked by conventional means, of course up to the GM. Letting players see the key might make them understand why it isn't conventional. In my Campaign this Key has a special addition to it. Inserting the key into it's proper lock won't unlock it. It's kinda a red herring seems kinda obvious which way it should be insurted especially with the key ring on the other side. Upon closer inspection players can notice that on the key ring there is a snake eating it's own tail etched around the entire loop. Knowledge Arcana will tell players it's an alchemical symbol called Ouroboros. Upon uttering this phrase the key ring will turn into a metalic snake slithering up the arm and coiling around the wrist of the person who muttered the word. It can't be physically removed at this point. However, this frees the back side of the key and allows it to be used in the lock backwards. Unlocking it. The metal snake will bite the player if the phrase "Honest" is said within earshot of the metalic snake for 1d4 damage. It can be released with the phrase "Absolution," in which the snake will attempt to return to the key wherever it may be. You can see how fun of a situation this puts the players in especially if they are trying to get somewhere they really shouldn't be. If you wan't to give clues on how to free the player I recomend the snake begin to wiggle or feel warm when players say words that are synonumous with freedom. You might also do the same for the biting phrase.