Stealing secrets

Arthur Scherbius, a German engineer, developed his 'Enigma' machine, capable of transcribing coded information, in the hope of interesting commercial companies in secure communications. In 1923 he set up his Chiffriermaschinen Aktiengesellschaft (Cipher Machines Corporation) in Berlin to manufacture his product. Within three years the German navy was producing its own version, followed by the army in 1928 and the air force in 1933.

Enigma allowed an operator to type in a message, then scramble it by using three to five notched wheels, or rotors, which displayed different letters of the alphabet. The receiver needed to know the exact settings of these rotors in order to reconstitute the coded text. Over the years the basic machine became more complicated as German code experts added plugs with electronic circuits.

Britain and her allies first understood the problems posed by this machine in 1931, when Hans Thilo Schmidt, a German spy, allowed his French spymasters to photograph stolen Enigma operating manuals. Initially, however, neither French nor British cryptanalysts could make headway in breaking the Enigma cipher.

It was only after they had handed over details to the Polish Cipher Bureau that progress was made. Helped by its closer links to the German engineering industry, the Poles managed to reconstruct an Enigma machine, complete with internal wiring, to read the German forces’ messages between 1933 and 1938.

Ultra intelligence

With German invasion imminent in 1939, the Poles opted to share their secrets with the British, and Britain's Government Code and Cipher School (GC&CS) at Bletchley Park, Buckinghamshire, became the centre for Allied efforts to keep up with dramatic war-induced changes in Enigma output.

Top mathematicians and general problem-solvers were recruited and a bank of early computers, known as 'bombes', was built to work out the Enigma’s vast number of settings.

TThe Germans were convinced that Enigma output could not be broken, so they used the machine for all sorts of communications on the battlefield, at sea, in the sky and, significantly, within its secret services. The British described any intelligence gained from Enigma as 'Ultra', and considered it top secret.

Only a select few commanders were made aware of the full significance of Ultra, and used it sparingly to prevent the Germans realising their ciphers had been broken.

By Andrew Lycett