Bombe

The method starts from a guessed fragment of plaintext, a crib — German traffic opened with weather reports or Keine besonderen Ereignisse with unhelpful regularity. Because Enigma could never encipher a letter as itself, you could work out where in the ciphertext the crib plausibly sat. The bombe then chained the letter assumptions into a circuit of electrical implications and spun 36 sets of rotors through the possible settings. A contradiction anywhere in the chain rejected the setting; the machine stopped only where it had failed to disprove itself.

Turing sketched the principle in 1939. Gordon Welchman's diagonal board, added the following year, multiplied the deductions drawn from each assumption and was in practice the difference between an interesting device and a useful one. The first machine, Victory, was ready in March 1940; by the end of the war some 200 were running in Britain, operated largely by WRNS personnel.

The name was inherited from the Polish bomba kryptologiczna, but the principle differs: the Poles exploited the doubled message key, a flaw the Germans corrected in 1940. Every British bombe was dismantled after the war. A working reconstruction was completed at Bletchley Park in 2007.