This is one of the most secured doors which you can build in Minecraft PE. According to the creator it has more than five tredecillion combinations (and no, I’ve never heard of that number until today). It’s impossible to crack the code, but as long you’ve got the combination it won’t take more than a minute to unlock the door.
Creator: MDS Minecraft, Twitter Account
Go ahead and see if you can find a way to crack the code. Once you’ve given up (which is very likely that you will) then you can access through another door to find the combination to actually unlock the door.
Here’s a tutorial which shows you how to build a similar redstone creation.