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chain in a particular position against the true alphabet was either wholly eliminated— indicating
that some speculative alignment was wrong—or a triumphant survivor. The outcome was
the set of thirteen pairs of letters, each to be substituted for the other, constituting the cur-
rent ‘alphabet’ for that wheel. But because the setting of the wheel’s ring was still unknown,
that was not enough to determine which wheel it was. Another feature had to be brought
into play.
The wheels had notches that governed when they turned their neighbour over (see
Chapter 10), and the building of chains also revealed where the turnovers occurred. It was
the finding of the positions of its notches that enabled a wheel to be identified. The five wheels
that were common to the Enigma machines of all three military services had their notches in
different places, but the three additional wheels in naval machines had them all the same. If all
eight had been notched in the naval fashion, Banburismus would have been far less valuable.
As it was, finding the turnover point of a wheel could identify which wheel belonged in that slot
for those two days. With an accompanying crib, this paved the way for sending a menu to the
bombes, as described in Chapter 12.
The construction and testing to destruction of chains just described—the second stage of
Banburismus—started from a speculative alignment of two messages of which nothing more
was known. A still more sophisticated method (of aligning messages correctly) started from
possible but unconfirmed alignments of several messages for which there were possible, but
unconfirmed, cribs, usually for the beginnings of the messages. With the knowledge that if A
enciphered into Q, then Q enciphered into A, and that no letter ever enciphered into itself, and
working backwards and forwards between the messages and looking out for contradictions on
the one hand and the appearance of German words on the other, the Banburist could perhaps
confirm both the alignments and the cribs. Such an outcome provided a more powerful menu
for the bombes than one that came from building chains. Falling short of their usual flair for
‘-ismus’ names, Hut 8 called this ‘depth cribbing’.
Comic strips
The exhibition ‘Codebreaker: Alan Turing’s Life and Legacy’, which ran at the Science Museum,
London, in 2012–13, had two exhibits described simply as ‘cryptanalytic working aids used at
Bletchley Park’. I have identified one of them as the ‘comic strips’ described by Turing in his
‘Treatise on the Enigma’, colloquially known as ‘Prof ’s Book’.^11 As Turing wrote:
For demonstration purposes it is best to replace the machine by a paper model. We replace
each wheel by a strip of squared paper 52 squares by 5 squares. The squares in the right hand
column of the strip represent the spring contacts of the wheel . . . The machine itself is repre-
sented by a sheet of paper with slots to hold the ‘wheels’ . . .
and much more of greater complexity. Time after time his explanation of the attack on Enigma
is illustrated by annotated drawings of the strips in appropriate positions (Fig. 38.1).
Their ‘demonstration purposes’ probably excluded operational use. After visiting the Navy
Department in Washington in November 1942 (see Chapter 18), Turing wrote disparagingly:^12
They were using comic strips a good deal which struck me as rather pathetic.