The story of Enigma is quite well known, given that it has received relatively wide coverage on television and radio and it has also been the subject of films and books. In contrast, there is little public awareness of Bletchley Park’s attack on Lorenz, nor even of the existence of Lorenz itself. In many respects, the breaking of Lorenz is a missing piece of history, and, to a large extent, this is because Lorenz was kept secret for approximately sixty years after the war.
Enigma was used on lower-level messages from the field, in the air and at sea. Alan Turing broke the Enigma code as used by the German Navy. His work on Enigma is widely remembered for its significance in tackling the threat from German U-boats during the Battle of the Atlantic in the middle of 1941. Lorenz was used for transmitting the highest grade of intelligence messages at the top levels of German Command. Lorenz decrypts made a major contribution to winning the Second World War. Bill Tutte’s breaking of the Lorenz system without having ever seen the machine was a phenomenal achievement, but many people have never heard of Tutte.
Enigma and Lorenz were two very different cipher systems and had very little in common. Enigma, with its three wheels, created messages using the twenty-six-letter alphabet. It could send out a code in 150 million, million different start positions. The Lorenz SZ40/42, however, was much more sophisticated, with twelve wheels and 501 pins. Enciphered messages sent by teleprinter used 5-bit punched paper tape, where a message often contained thousands of ‘places’ (characters, letters or spaces). Enigma messages often contained fewer than 300. Thus, the Lorenz could send out a code with around 1.6 quadrillion different start positions. Arguably, Lorenz was even more significant and far more complex than Enigma. It was a miracle that Bill Tutte was able to break the Lorenz system without ever having seen the machine.
The wheel settings were changed every day by the Germans, although they were changed less frequently in the early stages of the war. From January 1944, the wheel patterns changed every single day, on the five major links. Lorenz had more wheels than Enigma so the pattern was quite different and more complex. The Lorenz pattern could be changed very easily and frequently. The codebreakers in the Testery had to break the message daily by hand, in order to find out all the patterns and wheel settings at the time. Once the message was broken, we could break the rest of the traffic for that day.
It can be imagined how confident the Germans must have been in something as complex as Lorenz. They had reason to be: the number of letters before a setting would be repeated was enormous, because the number of combinations generated by ten of the twelve wheels was a prime number and their combined multiplication reached a staggering sum. As a result, the German Army entrusted their most important communications not to Enigma but to Lorenz.
In the early years of the Second World War, the Germans already had great confidence in their Enigma enciphering system, which Bletchley Park nonetheless had broken. A fourth wheel was later added, but even so Turing managed to break the naval Enigma in June 1941. He also invented the Bombe machines to greatly speed up the breaking process on Enigma – but not on Lorenz.
Turing played a vital role in deciphering the messages encrypted on the naval Enigma. He was already quite well known when I first went to Bletchley Park in autumn 1941. I used to see him from time to time at the Park; a man of medium height, 29 at that time, dressed in a sports jacket and rather baggy grey trousers (a bit like mine). He would walk along the corridor in one of the huts with his gaze averted from other people, looking at the bottom of the wall and flicking the wall with his fingers. While not very comfortable in company and certainly a shy man, he was on very good terms and sociable with other mathematicians of his kind. You would never guess that this was the most influential man in Europe at that particular time. He was no hero figure, but he made an enormous difference and saved Britain from a Nazi dark age.
Extracted from Lorenz: Breaking Hitler’s Top Secret Code at Bletchley Park