Skip to 0 minutes and 14 secondsThe security of a message could mean the difference between life and death. In war-time the secrecy of communications is the difference between not only winning and losing but saving and damning not just one individual but hundreds of thousands. Sometimes it is not as simple as breaking a message and announcing results. It can take a lot of time and effort to break a cipher but once a reliable method has been found, any message can be broken. Which means if a cryptanalyst announces their success, the cipher will be changed and the code-breaking process has to begin all over again. Deciding what action to take on a deciphered message then becomes a difficult decision.

Skip to 0 minutes and 56 secondsTake action and you might reveal your knowledge and lose the ability to decrypt future messages. Taking no action, the consequences could be severe. During World War I the Zimmermann Telegram was decoded by the team in Room 40, based at Bletchley Park. Room 40 evolved into the government code and cipher school. Their task was to break war-time encryption, the most famous of which was the Enigma. Designed in the 1920s and adopted by the German military in 1926. Enigma was designed to mechanise the encryption process. It was a rotor-based system that automated a system of polyalphabetic ciphers. Unlike simple systems like Vigenère, the evolution was complex and didn't rely on a short repeated key.

Skip to 1 minute and 51 secondsThe key to breaking Enigma was working out which one of the 15 billion, billion combinations was used to initialize the machine. When it was introduced in 1926, the British, French and Americans declared the code unbreakable and vital information on the build-up to war was lost. However in the 1930s an amazing feats of reverse engineering took place. Based on Rjewski's theoretical work and captured German codebooks, the Polish cipher bureau were able to reverse engineer and work out the internal wiring of the German Military Enigma. This is without even seeing one of the devices. With invasion imminent, the Polish handed their work across to the English and French codebreakers. This information was a revelation as the codebreakers had previously thought Enigma unbreakable.

Skip to 2 minutes and 54 secondsAlan Turing, a code breaker at Bletchley Park, was tasked with breaking the Enigma without using the repeated code phrases, following a method similar to the Polish but using cribs- short words that could be expected to be found in messages. Each morning a German weather report would be transmitted. This was likely to contain the term "wetter", which could be used as one of the cribs for Turing's work. Sometimes they needed to manufacture their own crib. One ingenious way of doing this was to have the RAF drop mines in areas known to U-boats your boats. The resulting reports detailing where the minefields were could then be used as a the crib. Turing's unique insights.

Skip to 3 minutes and 52 secondsinto the relationship between mathematics and electronics allowed him to develop the bomb. This was used to automate finding of the Enigma settings and was one of the first instances of machine is being used as computers. The Lorenz cipher was used by German high command. It was a rotor-based system like the Enigma but vastly more complicated. A problem with the bombs is that while they were very good at the parallel processing needed to decrypt the Enigma messages, they struggled with some of the deduction and analysis. required to break Lorenz Influenced by Turing's work, Max Neuman and Tommy Flowers designed and built Colossus, the world's first programmable computer and capable of tackling the Lorenz.

Skip to 4 minutes and 49 secondsBy the end of the war, ten Colossi were being used to decrypt messages and aid the war effort. The early 20th century brought huge changes to both cryptography and cryptanalysis. For the first-time, wide scale automation of both code making and code breaking was used. While the intelligence gained from breaking Enigma messages gave a huge advantage to the Allies, it also led to some difficult decisions. Had the Germans believed the code was broken then the system would have been changed and our ability to decrypt messages would have been lost. Allegedly information on the forth coming air raid of Coventry itself was known to the British government.

Skip to 5 minutes and 42 secondsHowever, they took the impossible decision not to warn the citizens of Coventry and many lives were lost. While the city was devastated in the following air raid, Bletchley Park retained the ability to decode German information and the war was eventually won. There are, however, several takes on this story Read them and make-up your mind.

The secret heroes of World War II

The Second World War saw cryptography being used at a greater scale than ever before.

All parties knew the importance of being able to receive information and send orders securely. Intercepting the messages of an opponent in war can give tremendous advantages. The advent of electrical communications equipment enabled more complex ciphers than anyone had seen before.

However, the same advances in technology would soon be used by cryptanalysts to create machines that could analyse messages in ways that previously would have been too time-consuming to even be considered.

In this video, we discover the methods of the cryptographers and cryptanalysts of World War II.

Your task

Discuss the importance of the messages decoded by the British and their allies in World War II. Do you think cryptanalysis changed the outcome of the war or just hastened it?

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This video is from the free online course:

An Introduction to Cryptography

Coventry University