Crib (cryptanalysis)

For other uses, see Crib.

Crib, in cryptanalysis, is a known or suspected plaintext. The term originated at Bletchley Park, the British World War II decryption operation.

History

The usage "crib" was adapted from a slang term referring to cheating—thus, "I cribbed my answer from your test paper." A "crib" originally was a literal or interlinear translation of a foreign-language text — usually a Latin or Greek text — that students might be assigned to translate from the original language.

The idea behind a crib is that cryptologists were looking at incomprehensible ciphertext, but if they had a clue about some word or phrase that might be expected to be in the ciphertext, they would have a "wedge"—a test to break into it. If their otherwise random attacks on the cipher managed to sometimes produce those words or (preferably) phrases, they would know they might be on the right track. When those words or phrases appeared, they would feed the settings they had used to reveal them back into the whole encrypted message, to good effect.

In the case of Enigma, the German High Command was very meticulous about the overall security of the Enigma system, but nonetheless understood the possible problem of cribs. The day-to-day trench operators, on the other hand, were less careful.

Enigma messages would often be broadcast, every day, with the same stereotyped introduction. For example, an officer in the Africa Corps helped greatly by constantly sending: “Nothing to report.” Other operators too would send standard salutations or introductions. Standardized weather reports were also particularly helpful.

When cribs were lacking, Bletchley Park would sometimes ask the Royal Air Force to “seed” a particular area in the North Sea with mines (a process that came to be known as gardening, by obvious reference). The Enigma messages that were shortly sent out would most likely contain the name of the area, or the harbour threatened by the mines.

When a captured German revealed under interrogation that Enigma operators had been instructed to encode numbers by spelling them out, Alan Turing reviewed decrypted messages, and determined that the number “eins” ("1") appeared in 90% of messages. He automated the crib process, creating the Eins Catalogue, which assumed that “eins” was encoded at all positions in the plaintext. The catalogue included every possible position of the various rotors, starting positions, and keysettings of the Enigma.

See also

• Enigma machine
• Cryptanalysis of the Enigma
• Polish Cipher Bureau
• Bletchley Park
• PC Bruno
• Cadix
• Ultra

References

• Welchman, Gordon (1982), The Hut Six Story: Breaking the Enigma Codes, Harmondsworth: Allen Lane, ISBN 0713912944
• Smith, Michael (2006), "How It Began: Bletchley Park Goes to War", in Copeland, B. Jack (ed.), Colossus: The Secrets of Bletchley Park's Codebreaking Computers, Oxford: Oxford University Press, ISBN 978-0-19-284055-4