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Turing’s Machine
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needed to get hold of Enigma machines, ideally without the Germans’
knowledge. The film U-571 merges two such capture stories into one,
taking a few dramatic liberties along the way, but it’s well worth watching.
Even with a captured machine, the codes were hard to break. You
needed a starting point – a crib to give you a clue what the machine
settings were. Helpfully, the German Army often began their messages
with a weather report. Everyone knows the German word for weather –
‘Wetter’. Decode the first 20 letters of a message until you found ‘Wetter’
and the message is unlocked. The German Navy, however, was less chatty
and avoided obvious words in their messages. One way the Allies could
find a crib was to blow something up. They would sail to some point in
the Atlantic, fill an old boat with oil drums, and set it alight. The German
Navy would get wind of this and go to investigate. The first thing they
would do is to radio a message back to base with the coordinates of
the wreckage, which, of course, the British already knew. This gave the
British a crib, and once they were in, they could decode messages for
several days in a row because the Enigma machines often cycled through
a repeating pattern.
Throughout the War, the German military never suspected the
British had cracked their codes and thought they must have traitors giving
away their secrets. The Enigma machine was an elegant compromise
between a truly unbreakable code and a simple cipher. Unfortunately for
the Germans, Turing was on the side of the Allies.
In the 1930s almost all mathematics, accounting, and code-breaking
were performed by humans using pencil and paper. It was the science
behind this process Turing sought to understand. We’ll take a step back
in time again to 1935 and Turing’s discovery of a solution to the Decision
Problem – the Entscheidungsproblem.

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