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Narrator: During the Second World War keeping information secret became incredibly important. The intelligence service inside Britian, MI5, was concerned about people sending important information to the enemy.

Posters warned people to be careful about what they said, because 'careless talk costs lives.' British spies were also working hard to confuse the Germans. Double agents even captured secret German messages, changed them and fed them back wrong.

Confusion tactics like this were critical to the success of military operations like D-Day. And so having trustworthy information was a matter of winning or losing the war. To make sure the enemy wouldn't know what was being said, people used coded messages.

This involved taking a readable message and turning it into an unbreakable code.

This ensured that messages could be sent safely to other allies, the countries who were working together to stop the Nazis. The Allies knew the code, so they could decipher the messages without the enemy understanding it.

Bernard worked in codes and ciphers for the Royal Air Force, the RAF. He remembers first joining the unit.

Bernard: There was a machine there, it was like a large typewriter. I said to the officer I said, 'what's this machine I've notseen that before.' He said, 'that's a machine we use for decoding the messages.'

This is a Typex machine, which was used to code and decode all the secret messages which we received.

Narrator: On missions, these Typex machines were carried around in a large truck called a cipher vehicle.

Bernard: The cipher vehicle was like a mobile classroom because we had two of these Typex machines and they were so bigit took four men to lift them and we had two of those in the cipher vehicle. One to set up for the previous day in case you got any late messages and one for the current day.

Narrator: Secrecy was of utmost importance for his work and Bernard took it very seriously.

Bernard: But I couldn't tell any of that information until fifty years afterwards. We couldn't tell anybody and we were the first peopleto know two days before that the war was going to finish in two days time. The message tells you nobody to be advised but we had been so used to the secrecy all through the war, we never told anybody.

Narrator: Did you know? The German encryption machine, was called the Enigma. It had one hundred and three billion trillion possible settings for encoding messages. For much of the war it was thought to be unbreakable. However, Enigma encryption had fatal flaws. A letter could not be encrypted as itself and multiple letters could not be encoded with the same letter. So A couldn't be encoded as A, nor could A be encoded as both B and C at the same time.

Knowing this, British code breakers designed a machine that could eliminate the vast majority of possible ciphers that weren't possible with Enigma's limitations. This left far fewer to be analysed by hand.

One of the main mathematicians working on this was Alan Turing, who helped to develop multiple code breaking systems. His work also created the foundations of modern computers. He has since been recognised for this incredible work and is now the face on the new fifty pound note.

Overall however, codebreaking was a team effort. Before the invention of electronic computers, computer was a job description not a machine. Both men and women were employed as computers but women were more prominent in the field.

At Bletchley Park, the centre of British code breaking during the war, teams of both men and women worked on complicated problems round the clock, hoping to crack the German codes and bring about a quicker end to the war.