The rebuilding of Colossus

Rebirth of the Colossus

Prior to the 1970s, very few people had any idea that an electronic computer was running during the Second World War, but information finally started to resurface about Colossus.

In 1989, Tony Sale (30th January 1930 - 28 August 2011) was an electronic engineer working as senior curator at the Science Museum in London restoring some early British computers. He, and a number of colleagues started, in 1991, the campaign to save Bletchley Park from property developers who were wanting to demolish it. He became aware of the scant information about Colossus and started to gather information about it

By 1993, he had gathered together eight 1945 photographs taken of Colossus as well as some fragments of circuit diagrams which were kept by some of the engineers, even though this was against the secrecy act. He started to believe that it would be possible to rebuild Colossus from this information, although, he states that nobody believed that this would be possible - just like Tommy Flowers before him!

After three months of work, transferring all the information from the photos and circuit diagrams to a CAD system, he found he had trouble working out exactly how the optical tape reader worked on the bedsteads. Fortunately, he was able to contact the original designer of the system, Dr Arnold Lynch, who originally designed it in 1942. Together, they managed to re-engineer the reader to the original's specifications. He also, along with Harry Fenson, one of the original Colossus engineers, visited Dr Allen Coombs who engineered the Mk 2 Colossus from Tommy Flowers' Mk 1. He gave Tony his wartime notes and some circuit diagrams.

Using his, and his wife Margaret's own funds, he started the huge task rebuilding the Colossus computer. He put together a team of ex-Post Office and radio engineers to help the rebuild. In 1995, the American NSA was forced by the Freedom of Information Act to release around 5000 World War II documents into the National Archive. He quickly obtained a copy of the list and found it contained a number of documents written by American service men seconded to Bletchly Park with descriptions of Colossus code breaking. Using these reports enabled many more of the functions, circuits and switches of Colossus to be rebuilt.

On 6th June 1996, His Royal Highness the Duke of Kent came to Blethley Park to switch on the basic two-bit working Colossus rebuild, an occasion where Dr Tommy Flowers attended as well as a number of people who worked at Knockholt, in the Testery and Newmanry during the war.

Colossus Mark 2, 1996 - 1998

By 1996, the basic working functions of a Mk 1 Colossus were working, although at a 2 bit level out of the five coming from the paper tape. This showed that not only was it possible to recreate the 5,000 character a second tape reading, but also that 45-50 year old thyratron values could still be coaxed into functioning

His team managed by 1999 to get a number of other functions working including all five tracks and the lamp panel to show the counter outputs and wheel start positions

Virtual Colossus begins

At this time, there were still large areas of the code breaking work that were classified, but finally in 2000, the Newmanry report was declassified which gave details of many more circuit boards. How they were all connected together though remained a mystery.

Tony Sale therefore, in 2001, wrote the first version of Virtual Colossus to try to reproduce how Colossus ran it's code breaking procedures to test how everything worked together. He wrote it in Javascript so it could run in a browser rather than as a seperate application in the availble version of Internet Explorer at that time. It was written to and run on a 600Mhz PC and was at the time, much slower still than Colossus running the real code breaking!

The huge task of wiring up the K2 switch panel and all the thyratron rings that run the Chi wheels was completed by 2003 along with the additions required for the main speed up of the Mk 2 against the Mk 1, namely the "R" signals or remembering circuits.

Virtual Colossus was upgraded again by Tony to a MK 2, complete with all 12 rings and patch panels which enabled him, along with the Walter Fried weekly reports in the NSA archives to work out what algorithms were needed to break the Chi wheel settings on a real cipher text. This meant they could then try to reproduce these on the real rebuilt colossus.

In December 2003, the team were finally getting releable counts on the (1+2)=. algorithm for setting K1 and K2 and on the 4=5/1=2 algorithm for setting K4 and K5 and the slash (/) count for K3 using a re-encipher of a real German decrypt.

They managed to go for a pretty much fully working Colossus Mk 2 by 1st June 2004, the 60th anniversay of the first running of a Colossus Mk 2 in Bletchley Park in 1944.

Tony Sale sadly died on 28 August 2011, but his legacy can be seen running and still maintained in H Block in The National Museum of Computing within Blechley Park, the original room where Colossus No.9 stood in World War II. It's an amazing site and well worth a visit - you can see how massive a machine it is, feel the heat coming off the numerous valves and hear the clicking of the relays and tape as it runs.

Virtual Colossus - The rebuild of the virtual rebuild

I am a computer programmer, working and living locally in Milton Keynes, fortunately for me, close to Bletchley Park

In 2016, after a number of visits to the National Museum of Computing at Bletchley Park to see Colossus - I was excited to find that there was a website to run a virtual version of this amazing machine. Unfortunately, it didn't really work very well on currently browsers. I managed to download the files and made a few adjustments to get it running on an older version of Internet Explorer, but the interface has a few issues and takes a while to update when selecting switches.

I was interested enough in the amazing story behind Colossus to want to find out more about how exactly this machine actually broke codes, so I bought a number of books on the subject (see further reading) and started to work through the Javascript code to find out what exactly it was doing to decipher the German cipher text.

Tony, fortunately I think with foresight, had written the code to be availble to look at - he says in his documentation "The Javascript is all there for anyone to see in the download." so I was able to work through the details of how it was working.

At this point, I started to worry that as this didn't seem to be working any more in current browsers, there could come a point where someone said "This page isn't working any more, let's remove it from the website". The Virtual Colossus code could possibly be lost and I felt it was just as much a part of the story behind the rebuild as the physical machine itself and all the stories of the breaking of the Lorenz.

I decided to try for a rebuild project myself and, using the main logic engine written by Tony as a basis, started to bring the interface up using today's faster internet and graphics.

I have kept the code available and not minified to keep with Tony's original vision. There are a few switches and buttons which I have no idea what they actually do yet and I'm sure I have a few things not quite as the original, but it's enough to run the main algorithms and to get a feel for what this amazing machine does. I hope it brings it to life for you if you're not able to see the real rebuilt Colossus in person.