able to do a piece of depth - dummy depth - a remarkable reflection on our comparative ignorance of straight cribs and of their value. The policy was, however, to some extent justified as on November 30th we got a right dummy depth crib and Commander Travis, then the arbiter of bombe policy, decreed that we might have all 12 bombes to run it.
This crib and many others in the course of December duly came out and the process of EINSing, twiddling, and bigram table building, described in an earlier chapter, proceeded merrily enough and considerably more smoothly than before. By the end of December the tables were near enough to completion to begin to think of restarting Banburismus and Turing was just starting to reconstruct the K Book - almost the last theoretical problem he tackled in the Section although he remained with us for some time to come. On December 30th a pinch of keys, bigram tables, and K Book made further work unnecessary and we were able to restart Banburismus at once.
On December 28th the U Boats caused further trouble by introducing a Stichwort which was only broken after 48 hours of extremely tedious labour. As Stichworts are constantly cropping up throughout the next 3½ years, I think it best to digress and deal with the subject straight away.
Stichwoerter, or Stichworts as I prefer to call them in accordance with Hut 8 idiom, are a form of emergency procedure to be put into force if a key is believed to have been compromised. One example, the earliest I remember, will show clearly the principle upon which they worked.
All holders of the machine possess a sealed envelope labelled PERSEUS. If there is reason to believe that the key is compromised, an order goes out "STRICHWORTBEFEHL PERSEUS". All concerned open their envelopes and find the word 'Danzig' and proceed as follows; they add D = 4 to each wheel on the key sheet, thus 1 becomes 5 and 6 becomes 2. They add A,N,and Z respectively to the left, middle, and right hand clips: they add 1 to the pairings of the Stecker. G is a dummy letter.
This was an ingenious and in theory efficient system. Assuming we had captured the keys we should know no more than that a Stichwort was to come into force and, even if we possessed the envelope, we should not know what to do with the key word as instruction in Stichwort procedure was to be passed on by word of mouth only. Thus an effectively quite new key was introduced at a moments notice and cypher security would have been maintained had we not possessed cryptographic resources of which the Germans never dreamed. As it was, the average Stichwort had no more than a mild nuisance value.