COPElAND | 117
on human brains: the whole process revolved around a ‘crib’ that was carefully chosen by a
human codebreaker. A crib was a snatch of plain German that the codebreaker believed to
occur in the message.
Turing’s powerful crib-based method for breaking messages (described in detail in the sect-
ion ‘How it worked’) was unaffected by the change to the German system in May 1940—unlike
Rejewski’s method, which did not involve cribs. Moreover, Rejewski’s bomba had simply ignored
the Enigma’s plugboard.^54 This was possible in those early days, when the Germans used the
plugboard to transpose as few as five pairs of letters. Turing’s ingenious method for solving
To decrypt start hereNYfigure 12.5 How the Enigma machine decrypts.
Jack Copeland and Dustin Parry. All rights reserved.