Other cribs came from knowing the current activities of the enemy. If, for example, a battle occurred, it could be assumed that messages following the attack reported on the battle. It was more difficult for the cryptanalysts to build cribs for these types of messages since it involved guesswork.

Cryptanalysts also needed to know

• how the Enigma worked,

• what was not possible on the machine, and

• the Germans' standard practices.

Because the Enigma rotors moved with each keystroke, a letter typed twice usually enciphered to two different letters. Also, the Enigma could not encrypt a letter to itself. Finally, the Germans indicated a space between words with the letter X and spelled out numbers.

Knowing these details played an important role in ultimately breaking the Enigma's daily settings.

To better understand the process of developing a crib and a Bombe menu, we'll use an example. The cryptanalyst knows that two Allied pursuit planes attacked U-boat 66, commanded by Kapitanleutnant Friedrich Markworth, at 2130 hours. Shortly afterwards, Markworth sent an encrypted message. The cryptanalyst assumes the message includes the following: Assumed text:

(To make it easier to follow the example, the assumed plain text will be in English. Cryptanalysts during World War II would have used cribs in German.)