In the 1970s information about Colossus began to emerge. Professor Brian Randell of Newcastle University started undertaking some research. Dr Tommy Flowers and some of the other design engineers presented papers in the 1980s describing Colossus in fairly general terms.
In 1991 when Tony Sale and some colleagues started the campaign to save Bletchley Park from demolition by property developers, he was working at the Science Museum in London restoring some early British computers. He believed it would be possible to rebuild Colossus (although nobody believed him at the time). In 1993 he gathered together all the information available. This amounted to the eight 1945 wartime photographs taken of Colossus plus some fragments of circuit diagrams which some engineers had kept (quite illegally). The first stage was to produce accurate machine drawings of the frames for Colossus (all the original machine drawings had been burnt in 1960). This involved three months of eyestrain poring over the photographs and using 3D projections to transfer the details to a CAD system. The next problem was the optical paper tape reader system. The details of this are not shown in any of the eight photographs. However Tony managed to locate Dr Arnold Lynch who designed the reader system in 1942. Although well into his eighties Dr Lynch came to my house and using my CAD system we re-engineered the reader system to his original specifications. In July 1994 His Royal Highness the Duke of Kent opened the Museums in Bletchley Park and inaugurated the Colossus Rebuild Project. At that point Tony had not managed to obtain any sponsorship for the project but in 1993 he and his wife Margaret decided to put their own money into it to get it started. Over the next few years various private sponsors came to their aid and some current and ex-Post Office and radio engineers formed the team that undertook the Rebuild.

Colossus first worked at two-bit level (out of the five-bit channels from the paper tape). HRH the Duke of Kent returned to the Park on 6 June 1996 to switch on the basic working Colossus. In the past eleven years since that first switch on, Tony and his team have worked solidly towards completing the full rebuild of the Colossus Mark 2 at Bletchley Park.

The Mark 2 machine implements Bill Tutte’s method of rectangling that enables the Colossus to recover the Lorenz wheel settings using ciphertext only. This is the cryptanalysts holy grail.

The rebuild is marvellous tribute to Tommy Flowers, Allen Coombs and all the engineers at Dollis Hill and a great tribute to Bill Tutte, Max Newman, Ralph Tester and all the code breakers involved at Bletchley Park. We must also remember all the WRNS who operated and supported Colossus and the interceptors at Knockholt without whom there would have been no messages to break.

Performance of the Colossus
Colossus is not a stored-programme computer. It is hard-wired and switch-programmed, just like ENIAC5. Because of its parallel nature it is very fast, even by today's standards. The intercepted message, punched on to ordinary teleprinter paper tape, is read at 5,000 characters per second. The sprocket holes down the middle of the tape are read to form the clock for the whole machine. This avoids any synchronisation problems: whatever the speed of the tape, that's the speed of Colossus.

Tommy Flowers once wound up the paper tape drive motor to see what happened. At 9,600 characters per second the tape burst and flew all over the room at 60 mph! It was decided that 5,000 cps was a safe speed.
At 5,000 cps the interval between sprocket holes is 200 microseconds. In this time Colossus will do up to 100 Boolean calculations simultaneously on each of the five tape channels and across a five character matrix. The gate delay time is 1.2 microseconds which is quite remarkable for very ordinary valves.

It demonstrates the design skills of Tommy Flowers and Allen Coombs who re-engineered most of the Mark 2 Colossus. Colossus is so fast and parallel that a mid-range modern PC programmed to do the same code-breaking task takes as long as Colossus to achieve a result.