Crib (cryptanalysis)
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In cryptanalysis, a crib is a sample of known plaintext; the term originated at Bletchley Park, the British codebreaking operation during World War II (WWII).
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.
Examples include stereotyped salutations, endings, titles, routing codes, etc. In the case of WWII German traffic, one site regularly reported to headquarters each morning using precisely the same phrase, albeit encyphered using the current Enigma key.
Bletchley Park sometimes arranged operations to provoke messages from the Germans to which they knew the plaintext; this was termed gardening.