The Enigma was considered to be the Unbreakable Code of WW2. But there were a few flaws with the code with one being that the letter could not be coded with the same letter. This eliminated a few thousand possibilities on how you could crack the Enigma Code.
The Allies saw this mistake and constructed a machine called “The Bomb”, which was an electromagnetic device which was held in Bletchely Park. The prime purpose of this machine was to crack the German Enigma code. It would do this by pairing up letters with a the 25 letters but itself. This would eventually eliminate the possibilities for the code and in the end, the Allies cracked the code.