40. A very important aid to the correct placing of a crib is the non-crashing property of the machine i.e. the fact that a letter cannot encypher as itself. Thus if we write out a crib underneath a stretch of message with which we think it belongs any case of a letter coming underneath itself (known as a CRASH) at once proves the crib to be wrong. Suppose for instance we thought that a certain message with encyphered text A J U L N Y T T G S B C ..... was the "Bereich Sieben" message. Writing out encyphered text and crib we get
A J U L N Y T T G S B C .....
V O R H E R S A G E B E
and see at once that we are wrong. On long cribs and particularly on R.E's (re-encodements) this is very powerful test; given a 200 letter R.E. there is only 1 chance in 2500 of avoiding a crash in a wrong position.
41. For this method to be applicable we require (1) a fairly large volume of traffic - at least 300 messages - otherwise we do not get enough pairs of messages starting close to each other (2) that the 3 wheel and not the 4 wheel machine should be used, since the latter - with 26 times as many possible starting points - spreads the traffic out too much.
Banburismus and Cribs in combination.
42. In the course of "Banburising" one automatically discovers which wheels are in the middle and right hand positions; owing to the different position of the turnover notches different results are obtained according to which wheels are being used. We can therefore use Banburismus to reduce our choice of W.O's from 336 to (say) 18 and then use a crib as the actual testing material. Moreover, when the relative positions of a number of messages have been discovered it is frequently possible by examining them in conjunction with each other to work out the contents of some or all of the messages. This process is known as DEPTH CRIBBING and cribs produced in this way give cross checks from one message to the other of such a kind that one can be virtually certain of their correctness. [See Chap. X]
43. We have outlined above how the necessary material to test the correctness of a position is produced. The BOMBE is a highspeed mechanism to do this testing. An "extract" or "menu" suitable for the bombe is