Excerpt from The PC Pioneers
The task of designing the first of these systems for the Germans had been given to the Lorenz Company that came up with the SZ 40/42 machine;, called Tunny by the Allies. This system used teleprinters and their 32-character Baudot code; this was the code used by the telegraph system before ASCII was developed.
The starting point for the encoding rotors was changed three times each day based upon issued code books. Sadly there was no captured device to hand or any details of its structure to help give the Allied code-breakers a headstart; they had to wait for human error to come to the aid of the decrypters.
The first British intercepts identified that these were enciphered by a modified version of the Vernam System developed in the USA. The system added characters throughout the message to hide and confuse its original content. It was concluded that they needed two versions of the same message to have any chance of cracking it. And that is precisely what they got.
On 30 August 1942 a German operator, finding a Lorenz message sent from Athens to Vienna had not been received, sent it again but vitally failed to reset the device between the two despatches. Better still he had used an abbreviation right at the beginning. This was enough to provide insight into not only decoding the message but also to using it to imply the machine’s structure.
A ‘Tunny’ Machine was built to decipher the Lorenz messages but it often took six weeks to establish the settings so the resulting decoded message was much too old to be helpful.