LockiesChests

Mod is small and consists of a few keys, a lock, and a couple of blocks. You can craft a Key Cutter, blank keys, and a basic lock. Using the Key Cutter you can cut a key for the lock which generates a unique code that the lock and key share, so no duplicate keys made by 2 random players. Upon right clicking a normal vanilla chest with the lock, it will lock the chest and be only openable with the corresponding key.

The chests can still be destroyed by normal means, however this allows for chests to be uniquely locked without worry on worlds where you can't destroy the blocks. A skeleton key which can be bounded to a player for use is also available in creative mode to unlock any chest.