Keyhole Mod 1.4.6 adds two items to the game.
The Keyhole Device is a block that outputs a Redstone signal when it detects the presence of its key item. To use the device, place it on the ground and put the desired key item in the slot that appears. Don’t worry, you’ll get this item back. Hit ESCAPE to close the window and the keyhole will output a Redstone pulse to indicate success. If a player throws the key item at the block or right-clicks it with the key item, it will output a Redstone pulse (about 4 seconds long) to any neighboring wires (vanilla or RedPower 2). You must be holding the key item to break the block.
The Locked Door is pretty self-explanatory. It is very similar to the Keyhole Device in behavior, except that instead of outputting Redstone it will open/close instead. Keep in mind that you cannot throw an item at it to open it; you must right-click it instead. You must be holding the key item to break the block.
Both the Keyhole block and the Locked Door (when re-added) can be found in the Redstone tab of the creative inventory.