So what was Colossus? On behalf of Digital Planet, Gareth Mitchell talks to Tony Sale about The Colossus Rebuild Project at The National Museum of Computing (based in Bletchley Park, UK).
Colossus was designed in World War II to break the German Lorenz cipher. And the Lorenz SZ42 cipher machine was used literally by Hitler to his generals, so very important messages being sent enciphered on that, and it was absolutely top grade, top secret stuff and it needed Colossus to be able to find the settings the German operator had used on an intercept that we’d got, because they transmitted their cipher text by teleprinter, by radio, and we intercepted those signals but then we had to find the settings the German operator had used on his Lorenz machine before we could decipher the message.
Then when you’d found those settings, you then plugged up those settings on our equivalent of the Lorenz SZ42 cipher machine. Now we never had a Lorenz machine here in World War II, we never even saw one until right at the end of the war, but because of a horrendous German mistake, 30th August 1941, we managed to find out how the Lorenz machine worked and we were then able to build our equivalent, which was called Tunny.
This is the intercepted message punched onto paper tape in the Baudot-Murray code and then loaded onto Colossus as an endless loop and being analysed over and over again by Colossus, because Colossus has got no memory, it’s a machine which is switched programme direct logic, very, very fast and very parallel. It’s got no memory, so it has to read the cipher text over and over again, analysing it statistically and teasing out the wheel settings used to encipher it.
You’ve got five different counters here, five sets of lights, and they show the counts of the match down the tape in thousands, hundreds, tens and units, for each of those five algorithms you're trying on there, and then the lower panel shows the actual wheel start positions that you're using at this moment to do that comparison. And it’s those wheel start positions that you're changing, looking for the maximum scores, which tells you you’ve found the right positions.
And if that whetted your appetite about the Colossus, here's another way of looking at it. Once again, the Digital Planet team made use of Microsoft's experimental Photosynth service, and turned a bunch of snapshots into something much more than the sum of its parts. The original photos are all courtesy of Digital Planet's resident commentator, Bill Thompson.
Want to try making your own Photosynths? You can do it with a pair of free programs. It only works on Windows XP and Vista, and you'll need to have a Windows Live ID. Download the Photosynth applications.
If you don't have a Windows PC, but do have an iPhone or an iPod Touch, you can create similar images with iSynth, which brings Photosynth to your mobile. Find out more about iSynth.
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If digital photography is your thing, why not check out the Open University short course Digital photography: creating and sharing better images (delivered online and available worldwide).