Action This Day: Your Colossal Help is Needed

29 Aug 2005
Colossus was arguably* the first computer ever built. It was certainly the first digital electronic computer, and thus the forbearer of the computer on which you are reading this article and the one on which I am writing it.

Colossus was designed and built in World War II at the famous Bletchley Park for the purpose of breaking the cryptographic messages of the German Lorenz cipher machine. There were a total of ten Colossus machines built. They were so secret that eight of them were destroyed after the war when Winston Churchill ordered them to be chopped into pieces that could fit through a 9-inch hole. The other two were taken to Cheltenham, where they survived another 15 years before being reduced to scrap. All the design documents for the Colossus were destroyed as well; its designer, Tommy Flowers, personally burned them.

For 50 years, Colossus remained something of a legend. I remember huge debates about what it could have done, how fast it might have been, and so on. Because its original design work came from Alan Turing, everyone interested in computers was also interested in Colossus, but hardly anyone knew anything about it.

Then in 1994, a team led by Tony Sale started to rebuild Colossus. The word "rebuild" is not used lightly. Although all the original Colossuses were chopped into bits, not all the bits were destroyed. And although Tommy Flowers had burned all his blueprints, there were other notes that survived. Some people at Bletchley simply couldn't bring themselves to burn their own circuit diagrams. American cryptographers who were present during its use kept their notes, and the NSA declassified these in 1995. There were also eight photos of Colossus that dated from 1945.

The Colossus Rebuild Project took 10 years and 6,000 hours of effort. The resulting machine is not a replica of a Colossus, but an actual Colossus that uses some of the actual parts. The team finished a Mark II Colossus in time for the 60th anniversary of the completion of the first Mk II Colossus. They even built it in the very place that Colossus #9 was built, on the same concrete pad.

So how fast is Colossus? Colossus is fast. It decrypts at 5,000 characters per second. That may not sound fast, but the messages it decrypts are fed in on paper tape, which means it is moving at the speed of 12 meters (about 40 feet) of paper tape per second. The limiting factor on Colossus's speed is the strength of the paper tape. If you wanted to program a modern computer to do what Colossus does, you"d need a 2GHz Pentium to match it. Not bad for a machine made out of 2,500 vacuum tubes, eh? In contrast, the ABC machine could do one operation every 15 seconds; Zuse's machine could add in about 1 second and multiply in 5 seconds.

Unfortunately, the land at Bletchley Park where Colossus is located, H Block, is for sale. I went to see Colossus in May, and include here two photos I took while pressing my nose to the window. You can see the racks of tubes and the paper tape reader.

Colossus can't be moved - it wouldn't survive. It is actually built into the concrete pads for stability. Remember, it has 2,500 tubes. And more important, it shouldn't be moved. It is in the actual building and on the actual spot where Colossus #9 ran during the war. It is a piece of history - both the history of computers and the history of cryptography.

* Arguably, because people can and do argue about it. When discussing very early computers, it's hard to know what is even considered a computer as opposed to an overgrown adding machine. Other machines that are arguably the “first” computer include Iowa State"s "ABC" machine, the Harvard Mark I, and Konrad Zuse's Z1, Z2, and Z3 machines. These other early computers were primarily electromechanical. ABC was electrical, but is also shrouded in mystery.

Background Reading
“Computer Development since Colossus,” Codes and Ciphers Heritage Trust

“Encyclopedia: Colossus computer”

“The Birth of Modern Computing”

“The Colossus Mk2 Rebuild,” Codes and Ciphers Heritage Trust