History
The usage "crib" was adapted from a slang term referring to cheating: for example,"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 and understood the possible problem of cribs. The day-to-day trench operators, on the other hand, were less careful. The Bletchley Park team would guess some of the plaintext based upon when the message was sent. For instance, a daily weather report was transmitted by the Germans, at the same time every day. Due to the regimented style of military reports, it would contain the word Wetter (German for "weather") at the same location in every message and knowing the local weather conditions helped Bletchley Park guess other parts of the plaintext as well. Another example was an officer in the Africa Corps who helped greatly by constantly sending, "Nothing to report." Other operators too would send standard salutations or introductions.
At Bletchley Park in World War II, strenuous efforts were made to use and even force the Germans to produce messages with known plaintext. For example, 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 soon 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") was the commonest string in the plaintext. 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.
The Polish Cipher Bureau had likewise exploited "cribs" in the "ANX method" before World War II (the Germans' use of "ANX," German for "to," followed by "X" as a spacer.)
Classical ciphers are typically vulnerable to known-plaintext attack. For example, a Caesar cipher can be solved using a single letter of corresponding plaintext and ciphertext to decrypt entirely. A general monoalphabetic substitution cipher needs several character pairs and some guessing if there are fewer than 26 distinct pairs.
Read more about this topic: Known-plaintext Attack
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