Wednesday, 28 October 2009
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Building Colosuss


In the US the Freedom of Information Act, and a generally more open approach to things, makes it unlikely that any stories of "the super computer" that won the war are waiting to be revealed but in the more secretive atmosphere of the UK it is very likely.


In the mid 70s it was revealed that British Intelligence had been using machines to break the German coded communications - and one of the machines was an early electronic computer called Colossus. So was this the first working computer?



Colossus 1943 - 1945

The strange thing about keeping secrets is that when the truth if finally revealed it can be very difficult to put the past into its correct order. People actually like secrets because they help to bolster the popular conspiracy theories of the present. They also allow plenty of opportunity to over-embroider and embellish the truth when it is told years after the event.

For example, many readers may have heard of the Enigma coding machines and you may assume that the task of breaking the code was a matter of advanced cryptography and super computers. The fact that it was all kept secret for so long encourages this conclusion - after all why keep irrelevant 1940's technology secret?

The truth of the matter is rather different. To start off with the breaking of the Enigma code was as much a matter of intelligence work as cryptography and it was achieved at first by purely manual methods - see the Enigma Box at the end of this article. Only later did machines come into use and even then they were electro-mechanical devices. Although the Enigma machine and the breaking of its code is a fascinating story it has little to do with computing and certainly doesn't require a re-write of history to accommodate it.

The Germans had a very good coding machine in the Enigma but they failed to ensure that it was used properly and they believed in its invulnerability just too much. The mistakes that they made are, from a more modern and sophisticated perspective, incredible - but don't be too smug because they also read like a list of today's most common security errors!




The story of the early electronic computing device that was used in code cracking really concerns a slightly different coding machine - not the highly romantic sounding "Enigma" but the "Lorenz" machine.

Lorenz codes

Much of the German high command's communications was carried by an automatic teletype system. These teletypes transmitted data using a five-bit code read from a punched paper tape. The transmission was fast and convenient in the sense that the paper tape could be punched using a standard keyboard and the reception of the message automatically produced a paper listing. Fast it may have been but without some type of encryption it would be very vulnerable to interception. The Enigma machine could have been used to encode and decode the messages but as this was a manual machine it would have slowed things down a great deal. A better solution was to build the encryption hardware into the teletypes being used.


A Lorenz coding machine

The cipher used in the teletype machines was based on the "one time pad" invented by Gilbert Vernam. The basic idea was that each bit of the teletype code is added mod 2 to another bit which plays the role of a key.

To code a complete message you need a second tape - the key tape - which has as many characters as the message tape. The two tapes are then run through tape readers at the same time and each pair of characters is added together mod 2. The result of the addition is then transmitted as the encrypted message. To de-encrypt the message all you have to do is repeat the process but this time each character on the key tape is added to the encrypted message to produce the original text.


Notice that for the system to work both the sender and receiver have to have a copy of the key tape and this has to be as long as the original message. There is also the problem of what the key tape should actually be. If the key tape is really random then it turns out that the code is virtually uncrackable. Indeed if the key tape is truly random then the encrypted message also looks as if it was truly random and hence contains no regularities that can help the code breaker.




Last Updated ( Friday, 26 March 2010 )

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