"Mystery" is the name of the German cipher machine.

The "Mystery" cipher machine changes the input letter into another letter and outputs it, so that the enemy cannot understand what the German army is doing.

For example: I use "MI" to type in we attack the emeny in sunday. "MI" will code bu eppelf xqu uduwg nc tkwyeg. The result is an article that no one can understand. article, but as long as you enter bu eppelf xqu uduwg nc tkwyeg. into "mystery" to reverse crack, you will get we attack the emeny in sunday.. In this way, the action plan can be protected.

Because the encoding method of "Mystery" is often modified, and the British do not understand the structure of "Mystery", they have been unable to obtain the German intelligence. "Later, the British obtained the "Mystery" cipher machine. This allowed a large amount of Germany's operational plans to be leaked.