If one operator chooses the trigram WER and having encyphered it twice, gets the letters VQSKIN and a second operator chooses WAL, it is clear that the second operator will get V??K?? because the first operator also had W as first letter of his trigram and produced V first time and K second. Similarly an operator encyphering NEB will get ?Q??I?.
About the 8 indicating letters we have then the following information:
1 and 5 are discriminants
2 conditions 6
3 conditions 7
4 conditions 8
Any 2 messages with the same letter in position 2 must have the same letter in position 6 and if P in 2 is followed by Z in 6, then Z will always follow P in this position until the keys are changed. This phenomenon is immediately obvious after cursory examination of the indicator groups if a system of this sort is in force. We failed to observe this.
In fairness to ourselves it must be said that the existence of a separate Mediterranean K book and bigram tables was most strongly postulated by Naval Section who also maintained, on the basis of an absence of linkages, that 3 keys were involved - Black Sea, Aegean, and N. African - in this last point at least they may claim to have been prophets if not correct as to current information. It is also true to say that Turing, who was the only one of us to have come into direct contact with throw-ons, was by this time taking little part in the work of the Section and the rest of us, with a general lack of background and experience, had got it fixed into our heads that all Naval keys worked like Dolphin. At best these observations constitute a bad defence.
I am not able to say when we first started to receive Porpoise in Hut 8, but it is probably true to say that by August 1942, when it was first broken, it had been attackable for some months. The agitation started in Naval Section who thereby fulfilled what has always been one of their chief functions vis a vis ourselves. It would not be true to say of the average member of Hut 8 that he "liebt sich bald die unbedingte Ruh" and preferred life to continue unchanging and problemless from day to day, but it is none the less a fact that cryptography does not get far without encouragement derived from knowing that someone is really interested in its results. It is greatly to Naval Sectionís credit that they have always urged us to exploit any available traffic of unknown content. When they agitated about Porpoise in July 1942 the traffic was in the region of 200 a day and thinking Banburismus might just be possible at this size, we started to look at it.