The History of Hut Eight
but little had been done in the way of analysing the traffic for routine messages. Cribbing is essentially merely a matter of guessing what a message says and then presenting the result to the bombe in the form of a menu on which the bombe has to find the correct answer. Any fool, as has recently been shown, can find an occasional right crib though some skill and judgment is required to avoid wasting time on wrong ones, or rather to waste as little time as possible.
Cribs may be divided into 3 basic groups:
1. Depth cribs.
2. Straight cribs.
3. Re-encodements.
As the history of the Hut is from this point to a large extent the history of cribbing, we must digress considerably at this point and study the 3 basic groups with some care. This process will take us far beyond August 1941 but will save innumerable minor digressions at later stages.
1. Depth Cribbing.
The concept of depth is very simple to understand. If two operators choose the same trigram they will, after transposing it at the Grundstellung, get the same starting position for their messages. Now suppose that one encyphers.
W E T T E R F U E R D I E N A C H T and the other
M I T M M M D R E I S I E B E N E I N S
it is clear that their cypher texts must have the 3rd, 9th, 12th and 13th letters in common as both hit the same letter at the same position of the machine. Let us write them under each other with the encyphered text:
Here we get the repeats - "clicks" we called them - as expected and also 'reciprocals' where the cypher text in one message equals the clear text in the other. Now let us assume that we are cribsters possessing only the cypher text but suspecting that the top message is a weather message which says ‘Wetter fuer die nacht und Morgen' - quite a likely state of
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