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    I Successfully created a hopper and comparator based key system. Basically, a redstone torch is set up next to one side of the comparator, while the other side has a hopper. If the hopper is not full of items completely, then there will be a stronger signal from the redstone torch, and the comparator will emit no signal. However, If the hopper IS full, then the hopper will have a stronger signal, causing the comparator to emit a redstone signal to the iron door.

     The hopper is currently filled with tripwire hooks, except for the fact that one stack has only 63 so the iron door stays closed. if any item BESIDES a tripwire hook is thrown in, nothing will happen, because the hopper cant fit it. if a tripwire hook is thrown in, the hopper will accept it into the remaining stack of 63, making the stack 64 and making the hopper completely full. this will make the comparator emit a redstone signal to the iron door, and cause it to open! 

    This system would best be used for adventure maps, and secret bases. If you would like to make a secret base, you would simply hide the hopper, and make a hidden piston door home system. the hopper would for example, be underwater, you would simply throw the proper item in, and your secret base would open up.

  If you are looking to prevent other players who happen to have the same item from getting in, you can simply use a renamed item, (Go to an anvil, throw the item in, and type in a new name, like "Key To My Secret Base") and only that specific block WITH that specific name will be able to open the door.
 
                                                        Simply put, i made a working lock and key system!

Link Here:  https://www.dropbox.com/sh/ldabhvsccejqqip/AAC8mgUvZjDszKpzj7dbBrwpa?dl=0 (For some reason instead of just giving you the folder, this gives you everything IN the folder. im gonna have to fix this, but for now, make a new folder (name this what you want your world to be named) then put everything from the link into that folder. then, put that folder in your saves for minecraft.) im sorry about this, ill fix it asap.