Open Vault Challenge - Level 4


Open Vault Challenge: Level 4

Challenge reminder :

Be sure you understand all previous levels before attempting this one: level 1, level 2, and level 3.

Keep the following in mind before starting:

  • The principle for typing a code is always the same: the user clicks on the button that is the same colour as the current digit s/he wants to type. The machine then proceed by elimination to identify the digit the user wants to type.
  • The user decides what the colour/meaning of each button is. For example, on level 2 the left button means “yellow” and the right button means “gray”, irrespectively of the actual colour displayed on the button that kept changing. On level 3, there are 9 buttons that were initially all black but their colour appeared progressively but were not predefined.

In this level 4, we do not show any colour. How can you find out the code?

Demonstrations videos

In the 3 videos below, I am typing the exact same code into the interface. It is the code of the vault for level 4.

Try it online

If you think you have cracked the code, you can verify it online:




Make sure you understand level 3 first:

Click to show the explanation of level 4. We encourage you to reveal the solution only as last resort or for confirming your solution.

How to find the code

This is the same principle than level 3 but this time we do not reveal the colours of the button. The colours remain in the head of the person typing the code and becomes a shared secret between the user and the machine, yet this secret is built on the fly during the interaction.

The key to solve this level is to not focus on the buttons but to focus on the number. At each stage, there is only 10 possible digits the user could be typing [0 to 9].

We can use the following reasoning: “if the user is trying to type a 0, then when he pressed this button, he meant that this button was of the colour of the digit 0”. By repeating this process for all digits [0 to 9], we can build 10 different configuration of button colours.

By doing this, you will find out that very quickly some button colour configurations are inconsistent, meaning that, if the user was typing the digit X, then he would be using some of the associated button inconsistently. Inconsistent means that the same button would be used for yellow and gray which is not possible. Hence the digit X is not the one the user is typing. After a few iteration, only one digit will remain, which is the one the user has in mind.

Once you have found the first digit, you also learned the colour of the button which will vastly speed up the identification of the subsequent digits.

We will release a video explaining this in a more visual way at a later date.

Take Home Message

The only reliable indicator is the consistency of the user. The user will always use the same button to mean the same thing. In other words, a button can only be used to mean yellow or gray, and never both at the same time.

Check you got it

You can train by visualising the code as you type it here: