preserve continuity of breaking. In 31 day months there was normally one "triplet" i.e. 3 days on the same W.O. and Clips.
"Wheel Order Rules"
18. There were a number of ways in which the German key maker quite unnecessarily restricted his choice of wheel orders e.g. the W.O. always contained a 6, 7 or 8 in it. Restrictions of this kind were known as Wheel Order Rules and could on occasions narrow the choice from 336 to as little as 10 or 20 which was of enormous value to us. [See Chap. for fuller discussion].
19. The "Stichwort" was a device used when the enemy suspected that the keys had been captured, or wished to protect himself from the danger of this happening. An example will make its nature and method of use clear.
20. All holders of the machine (at the time of this example) held a sealed envelope labelled PERSEUS. An order was sent out "STICHWORTHEFEHL PERSEUS". The holders opened their envelopes and found the word DANZIG inside (the "Kennwort"). They then "applied" DANZIG to the key as follows (1) D = 4 was added to the number of each wheel in the W.O. (2) A, N, Z respectively were added to the three letters of the Ringstellung. (3) I = 9 was added to each letter in the Stecker. This change was applied to all keys until further notice.
21. Suppose for instance the key for the day had been W.O. 275, Rings. BYL, Stecker A/F, B/J, D/X.... . The modified key would then be W.O. 631, Ring. CML, Stecker J/O, K/S, M/G .... .
22. This is a security measure with some "nuisance value" but is clearly not equivalent to a new key. Given that a key has been captured (the assumption on which the Stichwort is based) then - once we have discovered what is happening - there are 8 W.O.'s and 26 sets of Stecker to try; this might certainly be tiresome but does not compare with the 336 W.O.'s and 140 million million stecker on a completely unknown key.
C. The Message Setting.
23. Various procedures were used to disguise the actual message setting. The best and most widespread of these (which ultimately became almost universal) was that using the "Kennbuch" (K book) and the "Bigram Tables".