CODES ARE always crackable, but depending on their sophistication, the amount of time and brain power it takes to crack them varies immensely. In the case of the Enigma machine, for example, code encryption was highly sophisticated, and it could be set up in any one of 150 million million different ways. This meant that its coding system was virtually unbreakable.

The crucial thing about the Enigma machine is that it was the world's first code that was generated by machine and therefore should have been unbreakable by the human mind. But the Enigma concept was flawed and the Germans didn't realise that a machine could be made to imitate it. The key weakness of the Enigma code was that no letter was ever enciphered as itself, ie, if the first letter in the message was an A, it could never represent an A. This meant that immediately, the possibilities were lessened and it gave the decoders an instant advantage.

The Enigma machine worked in such a way that there were various rotors to choose from and a plug board. The person receiving your message would set up their machine in exactly the same way as you had, in order to read the message. The mathematicians at Bletchley had to discover, every day, how the machine that sent the message was set up. They did that in a variety of ways. They had to narrow things down by getting cribs, whereby they could guess at what might be in the message. They called the guesses menus, and on their electro-mechanical machines, known as bombes, they set things up. They could slide a guess along under the original, to the point where there were no clashes.

It could take days or weeks as they churned over various combinations, but once you'd broken one message, you could break all that day's messages. Some of the German Enigma-users were sloppy and would often begin messages with Good Morning or Heil Hitler, which meant that the decoders at Bletchley had the code broken for five or more letters.

One of the reasons that Enigma was kept a secret for so long is that it was a great coding device and, until recently, the methods were still in use. These days, codes are generated by computers and are much more complex. However, there is no such thing as a randomly generated pattern, and therefore, ultimately, everything can be traced back to a formula.

Obviously, the key to cracking a code is to work out how it's being generated. And in order to do this, you need sophisticated decoding devices but also a logical brain. The decoders employed at Bletchley Park included mathematicians, musicians, linguists, even scholars of the papyri of ancient Egypt.

'Enigma' (15), starring Kate Winslet, opens on 28 September

© 2001 Independent Digital (UK) Ltd