By Jo Twist
BBC News Online technology staff
Colossus Mk2, a wartime code-breaker hailed as one of the first electronic computers, has been rebuilt and reunited with Bletchley Park veterans.
Tony Sale led the team to rebuild Colossus Mk2
At Bletchley, the hub of British code operations, it crucially found the keys to break the Lorenz code used by Hitler to encrypt messages to his generals.
Colossus Mk2 has been painstakingly put back together over a decade by computer conservationists for Bletchley museums.
As part of D-Day celebrations, 30 war coders gathered to see it once more.
Besides its code-breaking prowess, Colossus was one of the most significant forerunners of computing technology because it was programmable and electronic.
Colossus Mk2 was essentially an upgrade of Mk1, which went into action on 1 February 1944. It was a prototype machine which proved the concept of electronic switching.
Electronics made it a very different creature to other code-breaking machines, which carried out blind searches for text matches.
Colossus first worked at two-bit level
Intercepted message was punched into ordinary teleprinter paper
Message was read at 5,000 characters per second
Contained 1,500 vacuum valves
Could carry out 100 Boolean calculations at any one time
Source: The Colossus Rebuild Project
"The major thing was it implemented a statistical attack on a cipher and that was the first time that was done," Tony Sale, head of the Colossus Rebuild Project, told BBC News Online.
"To do that it had to repeatedly scan the messages very fast, and a large amount of messages. Because it was electronic, it was able to do that very fast."
The machine was originally built by Dr Tommy Flowers at the Post Office research labs in London.
He was a great advocate of electronic and digital systems, and thought a programmable machine could be built to automate telephone call switches.
Dr Flowers' idea was to generate the keys needed to break the code in valve circuits in and thyratron rings - gas filled triodes - explained Mr Sale.
"No one believed he could do it because it had 1,500 valves in it, and no one believed it would ever work for more than 10 seconds," he said
This was because people were used to valves in their wireless - radio - sets blowing often. But, said Mr Sale, that was because of the constant switching on and off of sets.
Colossus was crucial for D-Day operations
To get around this, Colossus was not switched off until the end of the war.
The machines worked by reading teleprinter characters of the intercepted, encrypted message from a paper tape. The tape was looped with punched holes at the beginning and end of the text.
Usually, the cipher text had been transmitted by radio. By current computing standards it was fast, according to Mr Sale.
He argues that the original Colossus was so powerful, it would take current computers the same amount of time to break codes.
Colossus was also ground-breaking because it was put into action two years ahead of its nearest US rival, the Eniac (Electronic Numerical Integrator and Computer).
Although, Mr Sale said, Eniac was thought to have been first because Colossus was kept a secret until the 1970s.
The Colossus Rebuild Project started in 1993. The team used eight old photographs and some surviving circuit diagrams to piece the machine together.
The machines filled entire rooms
Three months were spent re-drawing the machine using CAD (Computer Aided Design) software on a computer with a 486 processor. A mix of old wartime valves and new components were used to construct the machine.
By the end of the Second World War, 10 Colossus machines were in action. They cut the time to break codes used by the Lorenz cipher machine from weeks to just hours, which was vital for D-Day preparations in 1944.
This meant they were instrumental in misinformation campaigns which led to last minute changes to D-Day attack strategies.
The machines were so successful that by the end of the war, 63 million characters of German messages had been decrypted.
After the war, most of the machines were scrapped to protect their sophisticated secrets.