Sometimes the most important victories in a war don’t occur on battlefields and don’t involve weaponry. On Wednesday, a very unusual group of veterans gathered at Bletchley Park, Buckinghamshire to commemorate an event that shortened the Second World War and saved countless lives. They were the men and women who built Colossus, the world's first programmable electronic digital computer, and they and their families were at the National Museum of Computing for a re-enactment of the day the famous machine began its code-breaking operations against the Axis forces.

On February 5, 1944, a switch was thrown and one of the most peculiar weapons of the Second World War went into action. As radio valves glowed in massive racks and an intricate cat’s cradle of paper tape spun in front of an electric eye, the Colossus Mark I computer took its first crack at a German Lorenz-encrypted message used by Hitler and his High Command to send their most secret and important messages. It was an act that would lay bare the secrets of the Nazis and even become a decisive factor in launching the D-Day invasion.

Many people have heard of the German Enigma cipher that Alan Turing and others at the British code-breaking center at Bletchley Park managed to crack, allowing the Allies to read German communications. But there was another, more complicated cipher called Lorenz. Generated by the SZ40/42 teletypewriter in-line cipher machine (code named “Tunny” by the British) built by C. Lorenz AG in Berlin, the machine produced what is called a Vernam cipher using 12 rotor wheels linked in an eye-wateringly intricate manner based on the Boolean XOR function. The upshot of this is that a message encrypted by the Lorenz machine was incredibly difficult to decrypt without knowing the wheel settings used to write it.

The British first got wind of the new cipher in 1940 when it was heard by police officers manning a listening post on the south coast of England. They puzzled over this unreadable traffic until August 1941 when a German cipher clerk sending a message from Athens to Vienna made a mistake. When a message was not received correctly and a request was sent to repeat it, the clerk did so with the same wheel settings, but with minor changes to the message. This blunder provided cryptanalyst John Tiltman with “depth” and other clues that allowed him to crack the cipher, enabling mathematician Bill Tutte to reverse engineer the machine, which the British never laid eyes on until the war was almost over.

The only snag in this bit of good news was that in order to read a Lorenz-encrypted message, it was necessary to work out the original wheel settings. The rest was easy, but that first step was a bit like hunting for a white rabbit in a snowstorm. The number of possible settings for the machine were staggering and trying to work them out by hand took up to eight weeks, which is next to useless for messages that need to be read within hours.

Bletchley Park turned to machines to automate and (hopefully) speed up the process. The first attempt were the Heath Robinsons – extremely complicated electromechanical devices named after the mad inventions drawn by cartoonist Heath Robinson. They worked, but they were temperamental because they used a pair of paper tapes that had to be synchronized, but rarely were, and they were still much too slow to be practical.

Enter Thomas “Tommy” Flowers, a General Post Office (GPO) telephone electrical engineer who’d been developing a digital electronic telephone switching system using radio valves. He was brought in to improve the Robinsons or, failing that, come up with a viable replacement.

Based on his telephone experience, he believed that a digital electronic computer would be much faster, but the powers that be at Bletchley dismissed the idea because the required valves were expensive, made of fragile glass, and prone to breaking down. Flowers countered that the GPO had solved the reliability problem by making sure that the valves ran in a stable environment and that by leaving them on they didn’t burn out for a long time. It’s a bit like the difference between an entry hall light that has to be replaced every couple of months and one left on forgotten in a cupboard that burns for years.

Regardless, Flowers ended up not only designing the new computer, but supplying parts for it from GPO stocks and paying for much of it out of his own pocket. However, after many months of work, by 1943 Bletchley Park had the world’s first electronic codebreaker and true computer, the Colossus Mark I, and the Mark II was already on the way.

Colossus didn’t look anything like a modern computer and not much like the mainframes of the 1950s and '60s. It was a giant collection of racks, wheels, valves, and printers weighing five tonnes (5.5 tons), measuring 7 x 17 x 11 ft (2.1 x 5.1 x 3.3 m), and consuming 8 kW of power. Built out of standard GPO telephone exchange parts, it was programmed through plugboards and jumper cables, used 1,600 valves, thyratron rings to simulate the Lorenz rotors, about 100 logic gates, 10,000 resistors, 4.3 mi (7 km) of wiring, a printer, and one paper tape drive running at 30 mph (48 km/h) containing the message for decrypting that was read by a photoelectric cell. It could read 9,000 characters a minute, but at that speed the tape disintegrated, so it was limited to 5,000 characters a minute.

When the Mark I began operations on February 5, 1944, the Mark II with 2,400 valves was already in the works and the original Mark I prototype would eventually be upgraded to the same specifications. These Colossi were five times faster than the Robinsons and could deduce Lorenz rotor settings in about four hours. As more of the computers were built and set working together, the Allies were soon reading cipher messages almost as fast as they were sent and some thought that Bletchley Park read the messages before the German High Command did.

In all, ten Colossi were built and an eleventh was planned. The machines deciphered 63 million characters and shortened the war by keeping the Allies appraised of the Germans' plans and knowledge. They even told the Allies that Hitler believed the false story that the D-Day invasion would land at Calais instead of Normandy, which gave the assault forces a huge advantage.

Unfortunately, the reward for the Colossi and their builders was three decades of obscurity. When the war ended, the team was unceremoniously disbanded and the decryption effort was seen as a vital secret, so all records regarding it were sealed under the 30 years rule and the people who knew about Colossus were bound by the Official Secrets Act to remain silent.

According to Flowers in a television interview for the US PBS network, the outbreak of the Cold War saw two of the computers go to Britain's Government Communications Headquarters (GCHQ) in Cheltenham and continued decryption work until 1960. The other eight machines and the documents that went with them were destroyed on the grounds that the best way to keep a secret is to burn it. The GPO components were removed and sent back to become part of the telephone system and the rest were smashed with hammers and tossed down a disused coal mine.

It was a sad end, but though sworn to secrecy, the men and women who worked on Colossus gained valuable new insights, training, and experience that helped in the postwar development of computers. The logic gates developed for Colossus are in every computer today, and the expiration of the 30 years rule in the 1970s meant the story of Colossus could finally be told.

In 1994, Tony Sale, co-founder of the National Museum of Computing, started the effort to rebuild Colossus. Based on bits of information released by the government starting in the 1970s, eight photographs of the original machines, and surviving circuit diagrams kept by engineers, a team managed to reconstruct the computer, which went operational at the museum on November 15, 2007, and on March 6, 2012, became the centerpiece of the Colossus gallery, where it is a working exhibit.

"The achievements of those who worked at Bletchley Park are humbling," said Tim Reynolds, Chair of The National Museum of Computing. "Bill Tutte's ingenuity in deducing out how the Lorenz machine worked without ever having seen it, the skill of those in the [code-breaking unit] who broke the cipher by hand, and Tommy Flowers' design of the world's first electronic computer Colossus to speed up the code-breaking process are feats almost beyond comprehension."

He went on to say that, "February 5 will be a proud day for the Museum to host the Colossus and Tunny veterans who are able to make the journey today. This day is in honor of all the men and women who worked on breaking the Lorenz cipher."

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