Appendix I. Cribs, Menus, and Bombe Set-Up
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...."[50] Knowing the exact ...
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