By Mark Ward
Technology Correspondent, BBC News website
Bletchley's code-breaking effort shortened the war by many months
For the first time in more than 60 years a Colossus computer is cracking codes at Bletchley Park.
The machine is being put through its paces to mark the end of a project to rebuild the pioneering computer.
It is being used to crack messages enciphered using the same system employed by the German high command during World War II.
The Colossus is pitted against modern PC technology which will also try to read the scrambled messages.
Colossus is widely recognised as being one of the first recognisably modern digital computers and was developed to read messages sent by the German commanders during the closing years of WWII.
It was one of the first ever programmable computers and featured more than 2,000 valves and was the size of a small lorry.
The re-built Colossus will be put to work on intercepted radio messages transmitted by radio amateurs in Paderborn, Germany that have been scrambled using a Lorenz SZ42 machine - as used by the German high command in wartime.
The German participants in the code-cracking challenge will transmit three enciphered messages - one hard, one very hard and one ultra hard.
Speaking to the BBC, Andy Clark, one of the founders of the Trust for the National Museum of Computing, said radio problems had stopped the challenge getting under way on time.
"The radio path has not been particularly good between Germany and here," he said. "We are at a bad point in the sunspot cycle."
Signals had improved throughout the day, he added, and he hoped to get 100% of the ciphertext - the code - through soon.
The Colossus machine will be pitted against modern computer technology that will also be used to decipher and read the transmitted messages.
Tony Sale, who led the 14-year Colossus re-build project, said it was not clear whether the wartime technology or a modern PC would be faster at cracking the codes.
"A virtual Colossus written to run on a Pentium 2 laptop takes about the same time to break a cipher as Colossus does," he said.
It was so fast, he said, because it was a single purpose processor rather than one put to many general purposes like modern desktop computers.
Mr Sale it could be Friday before the teams find out if they have managed to read the enciphered messages correctly.
Re-building the pioneering machine took so long because all 10 Colossus machines were broken up after the war in a bid to keep their workings secret. When he started the re-build all Mr Sale had to work with were a few photographs of the machine.
In its heyday Colossus could break messages in a matter of hours and, said Mr Sale, proved its worth time and time again by revealing the details of Germany's battle plans.
"It was extremely important in the build up to D-Day," said Mr Sale. "It revealed troop movements, the state of supplies, state of ammunition, numbers of dead soldiers - vitally important information for the whole of the second part of the war."
This, and the other information revealed by the code-cracking effort at Bletchley, helped to shorten the war by at least 18 months, said Mr Sale.
The Cipher Challenge is also being used to mark the start of a major fund-raising drive for the fledgling National Museum of Computing. The Museum will be based at Bletchley and Colossus will form the centre-piece of its exhibits.
Colossus has a place in the history of computing not just because of the techniques used in its construction. Many of those that helped build it, in particular Tommy Flowers and Tommy Kilburn, went on to do work that directly led to the computers in use today.
The Museum said it needed to raise about £6m to safeguard the future of the historic computers it has collected.