Retired British spy Tony Sale rebuilt Colossus, the world's first programmable electronic computer.
Code-crackers and engineers who worked on the Colossus computer at Bletchley Park talk about life during war-time as part of the BBC News series on British computer pioneers and pioneering British computers.
In June 1944, the Allies were debating when to go ahead with the invasion of Europe - D Day.
Eisenhower's decision was directly influenced by information revealed by code-cracking machine Colossus.
In messages it decoded, it was discovered that the trick the Allies sought to play on the Germans had worked.
Hitler was convinced that the Allies would land near Calais and had kept two divisions in Holland and Belgium to counter this phantom attack. It meant the Allies would have time to muster their troops on the beach following the landings and dig in before reinforcements could arrive.
Colossus - the world's first large-scale, electronic programmable computer - was created to do one job: crack the codes used to conceal the messages that Hitler's generals were sending to each other. It was developed at Bletchley Park, the site of the Government Codes and Cipher School.
"We were in no doubt about how important it was," said Captain Jerry Roberts, the sole surviving member of the unit, called the Testery, that used Colossus to crack codes. "We saw a number of messages signed by Adolf Hitler himself."
The code breakers at Bletchley had been seeing messages sent by German generals since the early days of the war.
Jerry Roberts used Colossus computer during World War 2 to decipher messages sent to Hitler's generals.
"I suspect sometimes we genuinely saw the messages before the blessed Germans," said Captain Roberts.
To scramble, or encipher, the messages the Germans used the Lorenz SZ 40/42 coding machine - known as Tunny to the Allies. This was a more advanced machine than the well-known Enigma and the Allies had no captured examples on which to try out code-cracking techniques.
But they were able to break this code thanks to human error.
On 30 August 1941, a German operator sent a 4,000 character message twice, made the cardinal sin of using the same settings and made small changes to the re-sent text.
In what Captain Roberts described as the "outstanding mental feat of the 20th Century" mathematician Bill Tutte used the two messages to work out how Lorenz enciphered text.
Said Captain Roberts: "I used to see him twiddling his pencil staring into space and I wondered a few times if he was earning his corn, but he clearly was."
Tutte's crack of the system helped the Allies decode the messages but soon too many messages were being intercepted for the human code crackers to handle.
Following the success of the machines produced to crack Enigma, Bletchley decided to build another to crack Lorenz.
Overseeing it was post office engineer Tommy Flowers and much of the early design work was done at the post office's Dollis Hill research lab.
"He worked with a select band of brothers. There was about five of us initially," said Harry Fensom, who helped Flowers build the machine and oversaw its installation and operation at Bletchley.
Colossus Mark I
Tech specs: 1500 valves, 5,000 characters per second,
First operation: 5 February 1944
Creators: Tommy Flowers, Harry Fensom, Allen Coombs, Sid Broadhurst, Bill Chandler
"We didn't think of it as a computer," he said. "Because in those days a computer was simply a device for manipulating arithmetic."
Flowers wanted to use huge numbers of valves to power Colossus. The wisdom of the decision was doubted because valves were prone to breaking. But Flowers knew, from his work on telephone exchanges, that they can have a long life if they are kept running. It is switching them on and off that renders them friable.
Colossus Mark I had 1,500 valves and was delivered to Bletchley in late January 1944 and was cracking codes within a fortnight.
Flowers' belief in the longevity of valves was proved because some of the valves dating from the Second World War are in the re-built Colossus now installed back at Bletchley.
The oldest valve in the re-built Colossus dates from 1943, said Tony Sale, who oversaw the 14 year rebuild project. In total, he estimates, about 40 of its valves date from the 1940s. About 5 to 8 break every year, he said
"It can break the codes in a remarkably short time, six hours, that by other means was impossible to break," said Mr Sale.
The rhythm of the machine is dictated by the paper tape bearing the message it is working on. That feeds data into the machine at a rate of 5000 characters a second. To reading the dots on the tape it uses parts from anti-aircraft shells.
Jerry Roberts shows how he deciphered German messages while a World War 2 code cracker for Britain.
The machine had to be produced quickly and Flowers and his team managed it in 10 months. Harry Fensom remembers it as a frenetic time, when they stayed all hours to get it finished.
"That was just the way we worked," he said.
In total 10 Colossi were produced including several Colossus II machines which went twice as fast. By the end of the war it is estimated that all the Colossi had decrypted more than 63 million characters. The success of the machine spelled its doom.
"Nobody has any recognition because Winston Churchill wanted it expunged from peoples' minds," said Mr Fensom. "He didn't want any of it to be released."
This led many to claim that the American Eniac was the first programmable, electronic, digital computer.
Those who worked on Colossus claim their creation has that honour.
Captains Roberts is keen to ensure that proper credit is given to that achievement.
"I didn't realise that this was the beginning of a new world but it was," he told BBC News. "After all, the inventor of the computer which has transformed our world is a person of some importance and we should be proud of those people."
"I would have thought that a statue was the least to expect," he said.
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