Alan Turing: Codebreaking and code-making

During the second world war, Alan Turing was recruited by the Allies to crack the coded messages used by the Nazis. At first they seemed near-impossible to decipher, but using a combination of mathematics and engineering, he and his colleagues found a way, and so shaped the course of the war. During this period, Turing also devised a portable machine called Delilah that could securely encode a voice message. It was based on a special form of arithmetic and was years ahead of its time. Turing is often associated with breaking codes, but in 1943 he also spent time making them.