(3) Probably therefore either (a) the message setting is encyphered at a Grundstellung and the resulting letters replaced by bigrams, or (b) the message setting is replaced by bigrams and the resulting bigrams encyphered at a Grundstellung.
14. Theory (a) was strongly supported by the following set of messages.
For on theory (a) EX and XX should be equivalent to the same single letter viz. the encypherment of V at the first position of the Grundstellung, say "x" and in that case when they both occur as second bigrams should again give a repeated letter - the encypherment of "x" at the second position of the Grundstellung - and in fact we do get a repeat.
15. The theory was finally confirmed by the discovery that if the indicators VCP, VUE, NUM were decoded at the Grund obtained from the AFÄ messages the V in position one and the U in position two decoded as the same letter. The problem of the indicating system was fundamentally solved, and the same day Turing thought of the method of Banburismus though as he says himself "I was not sure that it would work in practice".
(2) The Beginning of Banburismus Jan.1940 - Mar. 1941.
16. Banburismus played such a large part in the life of the section from now until June 1943, that it will be much easier for the reader to appreciate our activities from here on if he will now read Chap. IX. For those readers who are unwilling, or in their own opinion, incapable of this the following salient points about Banburismus should be borne in mind.
(1) It needs fairly large traffic - at least 300 messages.
(2) Those messages are no use unless their trigrams (Verfahren kenngruppe) are known. To find the trigrams we have to substitute