An example of the puzzle. The example code is 'EIHC', thus the final decoder strip must be placed on the C, revealing that the number is 2.
The Decoder puzzle is a puzzle-type door lock used in The Curse of Arrav, Dimension of Disaster: Defender of Varrock, Dimension of Disaster: Curse of Arrav, and Ritual of the Mahjarrat quests. It was released on 29 July 2009 with The Curse of Arrav quest. The puzzle must be solved using a code obtained from a code key, and decoded using decoder strips.
As with other quests involving Zemouregal, you must use decoder strips to unlock a door; in this case, the main entrance to his fort. Begin by reading the code key, which will give you a code of four letters; for example 'EIHC' (see the image above). Click the main entrance door to begin decoding the lock. Within the puzzle interface, you'll have your decoder strips shown on the right side, and a series of code numbers organized by letters on the left. Beginning with decoder strip 1, drag-and-drop (you may also select a strip and use the four-way arrows to move) the decoder strip onto the first letter of the code revealed in the Code key. (For example, using 'EIHC' as our code, you would place decoder strip 1 on the 'E' row).
The first code number needed to unlock the door will show through the slot in the decoder strip (for example, the number 6 in the image above). Use the 'up' and 'down' arrows in the middle of the interface to cycle through available numbers, and use the 'right' and 'left' arrows to respectively submit or delete a number from the combination slots at the top of the interface. Repeat this process using all of the decoder strips to decode the lock and enter the fort.
If you have two of the same letter in your code (for example: 'CCGH'), you'll want to place one decoder strip on the row of numbers, input the code number, and then replace the current decoder strip with the next one in the sequence.