Cryptanalysis of the Enigma: Crib-based decryption
Cribs were fundamental to the British approach to breaking Enigma, but guessing the plaintext for a message was a highly skilled business. So in 1940 Stuart Milner-Barry set up a special Crib Room in Hut 8.
Foremost amongst the knowledge needed for identifying cribs was the text of previous decrypts. Bletchley Park maintained detailed indexes of message preambles, of every person, of every ship, of every unit, of every weapon, of every technical term and of repeated phrases such as forms of address and other German military jargon. For each message the traffic analysis recorded the radio frequency, the date and time of intercept, and the preamble—which contained the network-identifying discriminant, the time of origin of the message, the callsign of the originating and receiving stations, and the indicator setting. This allowed cross referencing of a new message with a previous one. Thus, as Derek Taunt, another Cambridge mathematician-cryptanalyst wrote, the truism that “nothing succeeds like success” is particularly apposite here.
Boldfaced emphasis added by me.
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