Cryptographic History of Work on the German Naval Enigma

the values of the bigrams in the indicators from the bigram table use on the day and therefore Banburismus is impossible unless the bigram table used is wholly or largely known. At least one of the nine bigram tables had therefore to be worked out for any Banburismus to be possible.

(3) A considerable amount of man power - or rather girl power - and also of Hollerith machinery is needed. (a) All the day's traffic has to be sorted and examined for cases where the same group of 4 letters occurs more than once; this was a job done by Mr. Freeborn's Hollerith section. (b) A large number of messages have to be compared with each other by hand in a number of different ways and agreements between them recorded, which means a considerable amount of clerical labour.

(4) The effect of a successful Banburismus is to give us a "test" for correct machine key for the day and to reduce the number of W.O's which have to be examined from 336 to perhaps 18.

17. Turing was now faced with the following dilemma. There were only two ways of getting into a key (1) Cribbing (2) Banburismus. Cribbing required some detailed knowledge of the traffic since otherwise one could not predict what a message would say; it therefore seemed necessary to break a few days on Banburismus first. Banburismus needed no knowledge of the content of the traffic but needed at least one known bigram table; it therefore seemed necessary to break a few days on cribbing first. A further difficulty was that the bombes - essential to complete the break on modern keys - did not start to arrive until the summer of 1940 and the German Air and Army section working on Enigma (Hut 6) also needed these machines. Thus the testing of even one crib, supposing this to be available, presented a considerable problem.

18. The only really satisfactory solution to the problem was (1) a pinch either of the key sheets for a month or (rather less valuable) of the set of bigram tables, combined with (2) maximum bombe production to enable such a pinch to be exploited. Failing a pinch or a really large number of bombes there was little hope of any progress on up to date material.


< previous

next >