Even when they had found the Grundstellung with the help of the AFA' messages the Poles still could not read the traffic as they did not know how the indicating system worked. They set to work therefore to break individual messages on cribs - largely of the FORTY WEEPY type - not a very difficult process when stecker and wheels are known. By this method they broke out about 15 messages a day and came to the conclusion that the indicating system involved a bigram substitution but they got little further than this.
All witnesses agree that Naval Enigma was generally considered in 1939 to be unbreakable, indeed pessimism about cryptographic prospects in all fields appears to have been fairly prevalent. This attitude is constantly referred to in such letters of the period as still survive; Mr. Birch records that he was told when war broke out that "all German codes were unbreakable. I was told it wasn't worth while putting pundits onto them" (letter to Commander Travis, August 1940) and, writing to Commander Denniston in December of the same year he expresses the view that "Defeatism at the beginning of the war to my mind, played a large part in delaying the breaking of codes." When Turing joined the organization in 1939 no work was being done on Naval Enigma and he himself became interested in it "because no one else was doing anything about it and I could have it to myself". Machine cryptographers were on the whole working on the Army and Air Force cyphers with which considerable success had been obtained.
Turing started work where the Poles had given up; he set out to discover from the traffic of May 1937 how the new indicating system worked. "Prof's Book", the write-up he made in 1940 of the work he had done and of the theory of Banburismus, describes the successful conclusion of this work: (See Exhibit A immediately following the next paragraph)
The theory was further confirmed when the Grundstellung which AFA' had been using was discovered to encypher these trigrams in such a way that the Vs and Us all came out as the same letter. Turing had in fact solved the essential part of the indicator problem and that same night he conceived the idea of Banburismus "though I was not sure that it would work in practice, and was not in fact sure until some days had actually broken".
As Banburismus was the fundamental process which Hut 8 performed for the next 2 or 3 years, it is essential to understand roughly the principal on which it works.
find the keys for other days at the beginning of May and they actually found the Stecker for the 2nd, 3rd, 4th, 5th and 8th,