encyphered so that the throw-on effect was only obtained on two of the four letters. [This was done because two of the eight letters in the indicator groups were used as discriminants leaving only six for the encypherment of the message setting]. From now on Porpoise was broken on cribs which were reasonably good and remained so until the Autumn of 1944 when allied advances in Italy resulted in the capture of most of the places sending out crib messages; from our point of view captures of enemy territory ranked as disasters rather than triumphs.
23. About this time a number of new facts were unearthed about Wheel Order Rules. The construction of key lists by the enemy had long been realized to be non-random and we kept records of W.O's and steckers used on all keys. Our analysis of these was however less thorough than it should have been and the credit for the series of discoveries now made must go to Op. 20 G; a fair number of the actual results were discovered by us, but it was on their initiative that a thorough re-examination of the whole position was made.
24. We had always used the "non crashing" rule - that the same wheel was not used in the same position in two consecutive keys. The following is a summary of the further results now discovered - interesting as an example of the extraordinary difficulty that keymakers seem to find in making a random choice.
25. (1) The keymaker evidently had a large list of W.O's from which he crossed off W.O's as used. With 276 W. O's to choose from (336 less the 60 W.O's containing no 6, 7 or 8 which were never used) and only 15 used per month this would have been of little use if he had made a fresh start each month. In fact however he would go through a period of six months or more without ever repeating a wheel order so in the later months of a period almost a third of all the possible W.O's would be automatically deleted. This rule we had used for a time in 1942 and it then ceased to be operative; when it revived we failed to spot it and it was its rediscovery by Op. 20 G that drew attention again to the
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