Kiss (cryptanalysis)

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In cryptanalysis, a kiss was a term used at Bletchley Park during World War II for occasions when the enemy sent an identical message twice, once in a breakable cipher and again in an unbroken cipher. A deciphered message in the breakable system provided a "crib" (piece of known plaintext) which could then be used to read the unbroken messages. One example was where messages read in a German meteorological cipher could be used to provide cribs for reading the difficult 4-wheel Naval Enigma cipher.