This article refers to a term used in cryptanalysis. For the article of furniture, see cot.

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

Bletchley Park (BP) cryptographers obviously had something of a sense of humor, for their usage was adapted ('cribbed' even) from the student term (both verb and noun) meaning a bit of a cheat. Thus, 'I cribbed my answer from your test paper', or 'That copy of Foo's old Greek grammar has answered exercises in each chapter that Master Smythe has been using in his tests. We can use it as a crib'. The application to cryptanalysis of German messages is clear.

Examples include stereotyped salutations, endings, titles, routing codes, etc. In the case of WWII German traffic, one site, well beloved by BP, was quite bored. It reported this 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.

Crib is a term used to describe a rudimentary holiday home in the South Island of New Zealand. Bach is used for the North Island equivalent.