In answer to Hitler's demand for a machine that produced unbreakable code, the electrical engineering firm Siemens designed and built the Lorenz SZ40.
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The Enigma machine, which had been on sale since 1923, had three or four wheels. According to cryptographer Jerry Roberts, it was pretty well known, despite its secret activity.
The Lorenz SZ40 was much more complex. It used the standard five-element or bit Baudot teleprinter code to represent characters, but it used two sets of five wheels known to Station X as the chi and psi wheels. Each wheel had a different prime number of teeth. Two "motor wheels" imparted a "stutter" or semi-randomised effect to the codes.
Hitler and his generals were convinced the Lorenz machine produced unbreakable code and trusted their most secret information to it.
Roberts says the code should have been unbreakable, and would have been if not for a lapse of discipline between operators in Berlin and Athens in August 1941. This allowed John Tiltman, Bletchley Park's chief cryptographer, to break one message. From then on, the search for decipher speed quickened.