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In Hut 6 or Hut 8 the bombe’s hypotheses were examined and tested on a replica Enigma.
The codebreaker typed ciphertext at the keyboard and watched hopefully for fragments of plain
German in the output. If fragments of plaintext appeared, the bombe had succeeded, but if
no plaintext was uncovered then the information from this stop was discarded and another
telephone call told the bombe operators to continue the search. Hut 8 had a room dedicated
to testing the bombe’s guesses, the ‘Machine Room’.^67 The menus that the bombe operators
received also came from the Machine Room, although sometimes the menus were drawn up in
the Crib Room itself, depending on the difficulty of the job.^68 The sheets containing the menus
travelled by a pneumatic tube that connected the Machine Room to the Bombe Hut.
Later, as the numbers of bombes and outstations grew, a control room was set up to allocate
the menus from Huts 6 and 8 to specific bombes. The controllers, recruited from the Wrens,
routed jobs using a ‘wall chart of all machines giving their peculiarities and capabilities’.^69
A complication glossed over so far is the question of which three wheels the sender had
used to encrypt the message. At the start of his shift, the sender selected three wheels from his
wooden box of wheels, in accordance with his instructions for the day, and placed these into
the machine in the order specified in the instructions. German Army and Air Force Enigmas
came with a box of five wheels, offering 5 × 4 × 3 = 60 different choices of three, while the box
accompanying German Naval Enigmas contained eight wheels, giving a total of 8 × 7 × 6 = 336
choices. Each wheel had a different wiring pattern inside it, so, if Turing’s method was to deliver
correct answers, it had to use chains of Enigmas containing the same wheels in the same order
as the sender’s machine.
One way of dealing with this problem was to let the bombe test a number of different possible
sets of wheels simultaneously. If, say, a dozen of the bombe’s thirty-six replica Enigmas were
required to deal with the loops in the crib, then the remaining twenty-four could be used to run
the same search using two other possible sets of wheels. But because testing all the possible sets
of wheels, in all their possible orders, required too much precious bombe time, a hand method
called ‘Banburismus’ was applied before the job was put on the bombes. Banburismus, another of
Turing’s inventions, was designed to eliminate as many sets of wheels as possible. Banburismus
is described in detail in Chapters 13 and 38. It was used only against naval messages.
Another complication not mentioned so far is that, from time to time, the Enigma’s mid-
dle and left-hand wheels would ‘turn over’ a notch, as explained in Chapter 10. The search
procedure described so far assumes (possibly incorrectly) that no ‘turnovers’ occurred during
the enciphering of KEINEZUSAETZEZUMVORBERIQT. In practice, additional bombe runs
would be made to test out the various possibilities for the occurrence of a turnover. Apart from
the fact that more bombe time was used up, turnovers presented no great difficulty.
Simultaneous scanning and the diagonal board
There is an obvious way to speed up the bombe’s search procedure. The procedure as described
so far takes unnecessarily long since it requires the drums to move through all their positions
each time a new input letter is tested.
The search can be conducted much more quickly if, instead of making a complete run
through all the drum positions every time a new input letter is tested, all twenty-six letters are
tested simultaneously at each position of the drums. This was called ‘simultaneous scanning’. As
Turing put it, in simultaneous scanning ‘all 26 possible Stecker values of the central letter [are]