A crib is a section of an encoded or enciphered message that can easily be rendered into plain text, thus providing a tool whereby a skilled cryptanalyst can crack the entire code or message. A famous example of a "crib" from outside the world of espionage is the Rosetta Stone, used to translate Egyptian hieroglyphics.
Essentially a thank-you note from a group of priests to a magnanimous king, the stone was addressed to the second-century B.C. ruler Ptolemy V, who, like all the Ptolemies, spoke Greek rather than Egyptian. Therefore, the priests sent the note in Greek, as well as in hieroglyphics and demotic, a simplified version of hieroglyphic writing. Thus the French archaeologist Jean-François Champollion, who studied the Rosetta Stone in the early nineteenth century, was able to translate the Greek portion, and from this crack the code first of demotic, and then of hieroglyphics.
Any time a force sends out a message whose content is predictable to the enemy, this offers an opportunity for a resourceful cryptanalyst to find a crib. Thus, when the German high command in World War II sent greetings to Adolf Hitler every April 20—the Fuhrer's birthday—it was fairly easy for Allied cryptanalysts to guess the gist of the message. This would have been so no matter how carefully it had been enciphered or encoded, but the Germans sometimes made things even easier by sending the same message in plain text.
█ FURTHER READING:
Kahn, David. Kahn on Codes: Secrets of the New Cryptology. New York: Macmillan, 1983.
Konheim, Alan G. Cryptography: A Primer. New York: Wiley, 1981.
Lubbe, J. C. A. van der. Basic Methods of Cryptography. New York: Cambridge University Press, 1998.
Newton, David E. Encyclopedia of Cryptology. Santa Barbara, CA: ABC-CLIO, 1997.