Appendix I

Cribs. Menus, and Bombe Set-Up1

In order to set up the U.S. Navy Bombe, cryptanalysts first had to determine a 'crib.' A crib is the unenciphered text that is assumed, or known, to appear in the message.

Cribs could come through a variety of methods. Some of the best cribs came from errors made by the Germans themselves. On more than one occasion, a German signal clerk sent the same message twice in two different codes. If the code for one was known, it provided a crib for the unknown system.

Another frequent German mistake came in standardized messages. For example, a shore weather station in the Bay of Biscay sent out a message every day at 7:00 A.M. which began, 'The weather in the Bay of Biscay will be....' 2 Knowing the exact wording of a message made a perfect crib for the Allies, so it became a high priority to intercept the daily message from this weather station.

A final example of a common German error involved the practice of submerged U-boats. When the submarines resurfaced after extended periods of time under water, they requested all the important messages they had missed while below the waves. The transmissions that followed inevitably involved communications previously sent and deciphered. Cryptanalysts merely checked the back files for messages with the same number of letter groups and used them as cribs for the new message.3 Since the resulting message would be identical to the previous one, it helped reveal the Enigma setting for the current day. With the daily setting, all the current day's messages could be read.