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Nigel de Grey's cracking of German codes and his decipherment of the Zimmermann telegram showed that the codes used in the First World War were fundamentally flawed. The problem with codes is the codebook, a vast tome consisting of perhaps 1,000 pages, which has to be constructed, printed and distributed. Because of its size, a codebook is rarely changed, which means that the enemy codebreaker has several months or even years to crack the code.

In the wake of the First World War, cryptographers searched for a more secure and more flexible method of encryption. Ciphers, as opposed to codes, fit this model. A cipher consists of a general system for encryption, involving the substitution of individual letters or groups of letters, which is inherently flexible. The system is specified for each particular message by choosing a key, an element that is added to the encryption system to define exactly how the message is encrypted.

For example, the substitution cipher is a general system that substitutes each letter with another letter (or symbol). The key in this case would be the list of 26 letters (or symbols) that specify exactly how each letter is substituted. The key is flexible and can be changed regularly so that each message is encrypted according to the same cipher but according to a different key. It is this ability to change regularly that makes a cipher more secure than a code.

Fathering Enigma

Because of the security failures of the First World War, such as the decipherment of the Zimmermann telegram, the German inventor Arthur Scherbius developed the famous Enigma cipher machine. This consisted of a box containing something akin to a typewriter. The operator typed in the message letter by letter, which resulted in the illumination of other letters on a lampboard. So typing in 'hello' might illuminate 'JJKPM'. The operator would then send JJKPM to the receiver, who would then type it into his Enigma machine in order to get back to the original message.

The exact details of the encryption and decryption were defined by the key, which told the operators how to arrange the internal settings of the machine. With a different key, the message 'hello' would be encrypted in a completely different way. The key could be regularly and easily changed, and there were millions of billions of possible keys, making the codebreaker's job almost impossible.

Of course, we now know that British codebreakers at Bletchley Park did break the German Enigma cipher and that the intelligence gathered from this brilliant breakthrough played a major part in the Allied war effort. The British success depended on exploiting errors in the way the Germans operated the machine and errors in the machine's design. For example, a subtle flaw in the encryption mechanism is that a letter can never be encrypted as itself.

Scherbius, the father of the Enigma, did not live long enough to learn that his cipher machine was flawed. By 1925 he was mass-producing Enigmas, which went into military service the following year and were subsequently used by the government and state-run organisations such as the railways. Over the next two decades, the German military would buy over 30,000 Enigma machines. But in 1929, while driving a team of horses, he lost control, crashed into a wall, and died from internal injuries.