Crib (cryptanalysis)

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In cryptanalysis, a crib is a sample of known plaintext, or suspected plaintext; the term originated at Bletchley Park, the British codebreaking operation during World War II.

Their usage was adapted from the term meaning a bit of a cheat, for example, "I cribbed my answer from your test paper". The original sense of a "crib" was a literal or interlinear translation of a foreign language text — usually a Latin or Greek text — that students were likely to be assigned in the original language.

The whole idea of a crib is that the codebreakers were looking at difficult, incomprehensible encrypted code, but if they had a clue about some of the words or phrases they might expect to be in the message they were trying to decrypt, they would have a wedge, a test to break through some of 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 the Enigma, the German High Command was certifiably and repeatedly “irrationally exuberant” 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 sloppy.

Enigma messages would often be broadcast, every day, with the same stereotyped introduction. An officer in the Africa Corps helped greatly by constantly sending: “Nothing to report.” Other operators would send standard salutations or introductions. Standardized weather reports were 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 and interrogated German spilled 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.