On February 5, 2014, on the occasion of the 70th anniversary of the activation of the Colossus computer, at the National Museum of Computing at Bletchley Park there was a gathering of a group of veterans who worked to decode nazi encrypted messages During World War II. The story of the first electronic computer was also celebrated by re-enacting the decoding process from the interception to the decipherment using a replica of Colossus.

The story of the German Enigma machine and the work done by the British to decipher its encrypted messages is quite known. However, the Nazis had another cryptographic code generated by the Lorenz SZ40/42 machine that for many months the British couldn’t decipher because at the time it was very sophisticated.

In 1941, however, an encrypted message wasn’t received correctly in a transmission between Athens and Vienna. The receiving station asked to repeat it and that operation was carried out without changing the settings of the encoding with only a few changes in the message. The interception of the two copies of the message provided the British cryptanalyst John Tiltman and mathematician Bill All vital clues about the structure of the Lorenz SZ40/42 machine.

A long process, also based on theoretical and practical concepts developed by the great Alan Turing in deciphering the messages of the Enigma machine, led to the development of a more advanced computer, Colossus. It was the first computer to use vacuum tubes in a large equipment that altogether weighed about 5 tons and measured 2.1 x 5.1 x 3.3 m (7 x 17 x 11 ft).

Colossus Mark 1 was tested on December 8, 1943, with satisfying results. It was then disassembled and sent to Bletchley Park, where it was reassembled. On February 5, 1944, it was activated and used for the first time to decode a message. The time taken to recognize the encoding settings used for the message could reach four hours.

At that point, a more advanced version of Colossus, the Mark 2, was assembled and a total of ten computers was built and used until the end of World War II. Once finished their use in the war, almost all were dismantled except two, which were transferred to GCHQ (Government Communications Headquarters) and the project remained top secret for many years, so much so that its documentation was destroyed. It’s for this reason that for decades the American computer ENIAC was considered the first electronic computer.

Despite all this, when the story of the project Colossus became known, it turned out that there was sufficient information, especially thanks to the notes of the engineers who designed it, to build one. On November 15, 2007, a working replica of a Colossus Mark II was revealed to the crowd at the National Museum of Computing, where is now on display and where the 70th anniversary of its activation was celebrated.

The use of computers to decipher Nazi messages gave the Allies a crucial strategic advantage. Without them, the war would’ve been longer and its outcome uncertain. Today there are no more secrets around them so we can celebrate them together with the people who designed them and operated them, names that for decades have been known only by very few people.

This story was told for example in the essay “Colossus: Bletchley Park’s Greatest Secret”, available on Amazon USA, UK, and Canada.