This paper describes the exact operation of Schlüsselgerät 41 (cipher machine 41), also known as SG-41 and Hitlermühle, and explains the complex order and timing of the events that take place when a text is enciphered. The authors believe that this is the first time that some of the properties of this machine are revealed. A comparison is made with the contemporary and similar Hagelin M-209 and BC-38 machines, and it is shown that the SG-41 was more advanced in several ways.

The principal topic is the early glowlamp Enigma machine and it cryptographic weaknesses. Development started with a few first prototypes in 1918, mainly driven by the German businessman and engineer Arthur Scherbius. It was continued after the foundation of Chiffriermaschinen Aktiengesellschaft (ChiMaAG) — 'Cipher machines joint-stock company' on 9 July 1923 with the aid of chief engineer Willi Korn in the 1920s and resulted in the German Army Enigma machine.

In this paper, some design flaws are illustrated, and the reason why they occured are explained. Possible alternatives which could have avoided or at least reduced such flaws are described. On 1 June 1930, the finally revised standard version of Enigma I was officially put into service by the German Army.

While the C-52 and CX-52 devices enter the open market and are exhibited in museums, it is hard to the untrained eye to tell the differences. There is a lot of confusion, and half-truths muddle the waters even further. It turns out that there is only a limited number of standard configurations of the C-52 and CX-52 machines. These 'models' are described in detail, to enable the reader to identify machines he/she comes across. All the configurable components are described, to aid in the understanding of their differences and of the way the devices work.