I have created a command block system that can be used to lock chests in vanilla Minecraft. To use, import the schematic into your world and click the button under the "Startup" sign. Then use the "Get Lock" button to get a lock object.

Works in both SP and MP, and in all game modes. Note that in survival, the Lock object is a one-time use (it is consumed on use), but you can have several in a stack. Also note that the key object is consumed when locking a chest, and that any object used to attempt to open a chest is also consumed. As such, there is a "cost" to locking chests.

Any item can be used as a key, as long as it can stack. This includes items re-named in an anvil, so you could have a password system that combines both the type of key object and the password itself.