Science Article

June 23 marks the 100th birthday of Alan Turing. If I had to name five people whose personal efforts led to the defeat of Nazi Germany, he helped decipher the messages created by the Nazi's Enigma coding machines.

My, my, my Delilah

Turing is often associated with breaking codes, but in 1943 he also spent time making them. At the secret UK government laboratory at Hanslope Park in Buckinghamshire, Turing led the development of a portable device for speaking securely with another person. It was named Delilah and could be used to scramble a telephone or radio conversation.

Delilah was revolutionary because the scrambling system was very hard to break, yet was portable. By contrast, the secure phone system that linked the British prime minister at 10 Downing Street to the US president in the White House was so large that it had to be installed in the basement of the Selfridges store on London's Oxford Street. Called SIGSALY, it weighed 50 tonnes and required thousands of watts of power. Other, smaller voice scramblers were insecure and easily decoded by the Nazis.

Delilah worked by combining the speech to be scrambled with what sounded like random noise, similar to an untuned radio. When put together, all an eavesdropper would hear was noise, but by careful synchronisation between two Delilah machines at either end, it was possible to filter out the random noise and hear the original speech.

Delilah generated a sequence of numbers that both the sender and receiver could reproduce from a secret key (a way of setting the machine known only to them). By sampling the waveform of the voice to be transmitted - just as modern computers sample music to produce numbers that are stored on a CD or in an MP3 music file - the voice became a stream of numbers. These could then be enciphered by adding each number to a corresponding number from the random sequence. At the other end, a subtraction of the same random number would reveal the original number, which could then be used to reproduce the voice waveform.

Cracking Nazi Codes

When studying at Princeton University in 1936, Turing wrote to his mother saying that he had discovered a way to use mathematics to encrypt messages. Three years later, he was back in the UK using his skills to break Nazi codes. His efforts at the British military intelligence base Bletchley Park significantly affected the course of the second world war.

At Bletchley, Turing was known universally as "The Prof". He quickly became arguably the most important code breaker there. Most famously, he helped decipher the messages created by the Nazi's Enigma coding machines. To do so, he and his colleagues had to use a combination of insight, intelligent guesses and clever engineering.

An Enigma machine employed rotating wheels that assigned each typed letter to a different coded letter of the alphabet. A second machine at the receiver's end would reverse the process, deciphering the message.

Enigma messages were impossible to crack by brute force, using either raw manpower or simple mechanical calculators, because of the huge number of "keys" used. A key is the crux of any code system: it is a secret "password" agreed on by two people communicating in code. In the case of Enigma, the key consisted of the way each machine was set up before a message was transmitted. Both sender and receiver would arrange their wheels in a pre-agreed fashion, as well as arranging a plugboard similar to that used in a telephone exchange. Depending on whether three or four wheels were used, that meant there were 26x26x26 (17,576) or 26x26x26x26 (456,976) possible keys. And the additional variations in the plugboard settings made messages even more secure.

Alan Turing is my favorite Scientist, (23 June 1912 – 7 June 1954). Maths genius Alan Turing honoured with an interactive Google doodle on 100th birthday in 23 June 2012.

A new short film about Alan Turing’s contribution to the birth of the modern computer and his work in Britain following the war is now available. Based on Turing’s work with the National Physics Laboratory (NPL), the film features Tom Vickers, who knew Turing and worked on the Pilot ACE at the Laboratory.