This is a cool thing to do on a multiplayer world I am not sure if it is a glitch or not
It works on every minecraft version (as far as I know)
You need a door and a painting

Step 1: Door

Place a door and open it make sure it is open now because it will be hard to open later from outside it has to be against the wall you want to hide the door on

Step 2: Painting

Put the painting near the door
It might not cover the door on the first try if it doesn't, break the painting and replace it until it does (the door is between the torches)