The Bombe machine, designed by British mathematician Alan Turing at Bletchley Park during the early stages of World War II, was crucial to cracking German communications encoded by the Enigma machine.

Turing’s machine, which is a precursor to what we now think of as a computer, was able to rapidly speed up the rate at which intercepted messages were decoded, allowing allied forces to react accordingly within hours rather than weeks.

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The story of Turing and the Bletchley Park ‘codebreakers,’ for years kept secret, has now become part of national folklore. Information about the work at Bletchley was declassified in the 1970s, while Robert Harris’s novel Enigma, 2014 movie The Imitation Game and the Andrew Hodge book that inspired it have also all shed light on the ground-breaking work.

Much has been written about Turing’s genius, troubled private life and tragically early death. But how much do you know about the machine he built, the code it helped crack and the impact it had on the course of the War?

The Bomba before the Bombe

The Bombe machine, envisioned by Turing, enhanced by fellow code breaker Gordon Welchman and built by the British Tabulating Machine Company, followed up on the work of Polish mathematicians Marian Rejewski, Henryk Zygalski and Jerzy Rozycki.

These Polish Bomba machines succeeded thanks to a flaw in German encryption, which double-encrypted the first three letters at the beginning of each message, allowing codebreakers to search for patterns.

Much of how those Bomba machines worked remains a mystery, but by using six Bomby in parallel, the Enigma Ringstellung (the positioning of the coding ring) could be detected in a couple of hours.

Once the Germans discovered and eradicated the double encryption weakness, a more advanced solution was required. Enter Turing and his team at Bletchley Park.

The British Bombe

Using intelligence provided by the Poles, Turing set about cracking the Enigma messages with his own computer. His methods were based around the assumption that each message contained a crib - a known piece of German plaintext at a familiar point in the message.

In one example the Atlantic weather forecast, which was written in the same format each day, was crucial. Location-detecting equipment in listening stations allowed codebreakers to find where a message was originating from and, if it matched up with the positioning of a weather station, it was likely that the word “wettervorhersage” (weather forecast) would be both present and in a similar place in every message.

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Another clue was Enigma’s inability to code a letter as itself – an S could never be an S. That way, the encrypted message could be lined up with the crib until no letter lined up as itself.

However, even with this information, there was not enough time or manpower to work through all the possible combinations. This was because individual letters were encrypted in a different way every time they were entered into the Enigma machine.

So, even though guessing one key word offered clues, cracking the code required smashing odds of 158,962,555,217,826,360,000-to-1 - the exact number of ways Enigma machines could be configured.

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Not only that, a different code had to be cracked every single day to account for the Germans’ change in settings at midnight.

How the Bombe worked

The Bombes were 7ft wide, 6ft 6in tall and weighed a ton – literally. They had 12 miles of wiring and 97,000 different parts. Turning’s prototype was built on a budget of £100,000, which is around £4m today.

Essentially, the Turing Bombe was an electromechanical machine comprised of the equivalent of 36 different Enigma machines, each one containing the exact internal wiring of the German counterpart.

When the Bombe was switched on, each of the Enigmas is allocated a pair of letters from the obtained crib text (for example, when a D becomes a T in the guessed word).

Once the machine is switched on, each of the three rotors moves at a rate mimicking the Enigma itself, checking on approximately 17,500 possible positions until it finds a match.

The machine only stops when each of the Enigma machines finds what it believes to be the correct pair of letters at the same time and opens up its electrical circuit.

So rather than guessing the key, the Bombe used logic to dismiss certain possibilities. As Arthur Conan Doyle said: “When you have excluded the impossible, whatever remains, however improbable, must be the truth.”

This method, though successful, still provided a number of possible correct answers for the German ring settings, so further work needed to be done to narrow it down to the right one. With the help of a checking machine, the process could be repeated until the correct answer was discovered.

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This gave the codebreakers part of the key, but not all of it. It was then down to the codebreakers to use the knowledge provided by the Bombe and figure out the rest of the key.

Once the code was cracked, the workers at Bletchley Park could set up an Enigma machine with the correct key for the day and reverse the code for every message intercepted on that day.

Impact on World War II

Once the Enigma machine was cracked, 211 Bombe machines were built and ran around the clock. They were stationed at different locations across Britain, in order to reduce the threat of bombings wiping out these highly complex and expensive pieces of kit.

Because of a shortage of captured Enigma machines, British cipher machines called Typex were converted into working Enigma machines. Fully deciphered messages were then translated from German to English before being passed on to British intelligence.

At its peak, the Bombe was able to help crack 3,000 German messages per day. By the end of the war that amounted to 2.5 million messages, many of which gave the Allies vital information about German positions and strategy.

It’s estimated that this knowledge played such important roles in key battles that the work of the Bombe and the team at Bletchley Park shortened the war by two years.

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