The Lorenz SZ40 was the successor to the Enigma machine. While the Enigma machine has gained more fame since the end of World War Two, the Lorenz SZ40 was far more powerful than Enigma and capable of extremely complex encryption capabilities. The Enigma machine was seen by Adolf Hitler as being out-of-date by the start of World War Two despite its ability to encrypt a message 15 million million times. Hitler wanted an encryption machine that both he and his most senior generals could use to pass on highly sensitive and secret messages. The end result was the Lorenz SZ40. While the Enigma machine worked off either three or four internal rotors (the military version had three and the naval version had four), the Lorenz SZ40 had twelve, making it vastly more powerful than Enigma with a capability to encrypt a message 1,600,000,000,000,000 times. Such was the power of the Lorenz SZ40 that Hitler called it his “secrets writer" and the very highest military officers in the Nazi regime used it. Even Hitler himself had messages sent via the machine. It seems that no one believed that the Lorenz SZ40 could be cracked.
In early 1941 code breakers in the UK started to hear different ‘music’ on the lines that they monitored from occupied Europe and Nazi Germany. It was not the typical signal they associated with Enigma so experts at Bletchley Park decided that the Nazis had developed a new encoding machine. They called this new machine ‘Tunny’. The most brilliant minds at Bletchley Park were put to work to decode it but faced the almost impossible odds. The statistical odds of breaking a message that had been encrypted 1,600 million billion times were tiny. The breakthrough came as a result of a German officer based in Athens who broke the basic rules of code sending.
On August 30th 1941 a German operator based in Athens sent a 4,000 character coded message to Vienna. The recipient asked for a re-send as the message was so large. The sender should have resent his message using a different wheel setting on Lorenz but he maintained the same settings. This in itself would not have helped those at Bletchley Park if he had sent an identical coded message, as they simply would have received the same baffling message twice over. But on the second message, the sender made slight changes by using abbreviations and different punctuation. The sender abbreviated anything that he could presumably to save time and maybe effort second time round. This was the breakthrough Bletchley Park needed as they could see the differences between the two messages and they could build their code breaking from that. Such small details seemingly of no importance to outsiders were vital to the code breakers at Bletchley Park. While “number" changed to “nr" may have meant nothing to anyone on the outside of Bletchley Park, it was a vital start of the first stage of decoding Lorenz/Tunny.
Colonel (later Brigadier) John Tiltman decoded the coded message sent on August 30th in just ten days. Tiltman was a brilliant code breaker based at Bletchley Park. However, he could not fathom out how the Lorenz SZ40 actually worked. This task was handed over to William Tutte. In one of the most secret events of World War Two, Tutte managed to do just this. In what was later called “the greatest intellectual achievement of the war", Tutte worked out how the Lorenz SZ40 actually worked. Tutte examined the broken code and found patterns and repetitions in it. Using his own intuition and mathematical formulas, Tutte and his team worked out that the first wheel on Tunny repeated a pattern after 41 strokes/resonance’s. Therefore he concluded that the first wheel on Tunny had 41 spokes. He used the same process to work out how many spokes there were on the other 11 wheels. By doing this, Tutte found out how Tunny actually worked.
Captain Jerry Roberts who worked in the same room as Tutte and was a senior cryptographer at Bletchley Park frequently saw Tutte just looking into the distance. Roberts later said:
“I used to wonder if he was doing anything. My word, he was. (Breaking Tunny) was a most extraordinary achievement."
By finding out the way the Lorenz SZ40 worked, Tutte effectively made MI6 a fly-on-the-wall at meeting of senior Nazi officers. They sent messages to frontline generals in good faith that the Lorenz SZ40 was unbreakable. In fact, everything that they did was known to Allied intelligence. By breaking the Lorenz SZ40 the Allies knew that Hitler believed that the expected Allied invasion of occupied Europe would occur in the Pays de Calais and not Normandy. By breaking Lorenzo the Allies knew that Hitler refused to pull out German troops from Italy – despite the fact that they were desperately needed in northern France after D-day. The Lorenz SZ40 gave Allied intelligence a perfect insight into the very heart of the Nazi military elite.
When World War Two ended no one knew about Tutte’s achievement – except those in Bletchley Park. It remained secret for decades but in a recent UK television programme about William Tutte he was referred to as “one of the unsung heroes of World War Two" – the man who cracked the uncrackable – the Lorenz SZ40.