152 | 14 TUNNy: HITlER’S BIGGEST fISH
Nevertheless, in January 1942, a young member of Bletchley Park’s Research Section, William
Tutte (Fig. 14.5), was able to deduce the internal workings of the Tunny machine simply by
studying the key that Tiltman had retrieved. It was one of the most astonishing pieces of crypt-
analysis of the war.
Tutte deduced that the machine produced key by means of adding two separate streams of
letters. He inferred further that each of these two streams was produced by a different set of five
wheels (five because there are five bits in the teleprint code for each character), and he made up
the names ‘chi-wheels’ and ‘psi-wheels’. The chi-wheels, he deduced, moved regularly, whereas
the psi-wheels moved in an irregular way under the control of two further wheels, to which he
gave the name ‘motor wheels’. Tutte’s secret deductions would save untold lives.
There are two separate steps in breaking a new cipher system. First comes what is called
‘breaking the machine’: the codebreakers must discover enough of the design of the cipher
machine, and this Tutte had now done. Second, a fast and reliable method must be found for
breaking the intercepted messages on a day-to-day basis. This method must work quickly
enough to allow the codebreakers to decipher the messages in a timely fashion, before the intel-
ligence they contain goes out of date.
Turing devised the first method used against the daily Tunny message traffic. It was known
simply as ‘Turingery’.^4 Turingery depended on a technique that Turing invented, a process of
‘sideways’ addition called ‘delta-ing’ (after the Greek letter delta, Δ). To delta the four characters
figure 14.5 The great codebreakers John Tiltman (left) and Bill Tutte.
Left-hand image reproduced with permission of Barbara Eachus and Government Communications Headquarters, Cheltenham.
Right-hand image with permission of Jack Copeland.