Bill Tutte has been described as one of the most talented code breakers of World War 2. His analysis enabled the cryptographers working on Lorenz messages, known as Tunny, to speed up decryption of the information by hand from 1941, and later to design decryption machines.
Tunny dealt only with messages between Hitler, his high command and the generals in charge of the different theatres. "Hitler signed some of the messages. I know because I decrypted one or two myself. There was no higher value traffic," says says Jerry Roberts, the last surviving member of the Testery, part of Station X.
The Germans believed the Lorenz system was unbreakable. And it was, says Roberts, except for an extraordinary piece of luck. A German signals operator in Greece was unhappy with a message and asked that it be sent again. The sending officer obliged, but without changing the settings on the machine, as per standard operating procedure.
Tiltman gave this long stretch of keystream to Tutte. This gave Tutte an identical copy of the original message, or in code-breakers' jargon, a "depth". "Breaking a code with a depth is very much easier than doing it with only a single copy," Roberts.
From this, Tutte worked out the complete logical structure of the Lorenz cipher machine. This enabled the eventual complete decryption of more than 90% of Tunny.
This was first published in January 2010