A set of long-missing documents has shed new light on one of Nazi Germany’s most secret encryption devices from World War II. Researchers in Prague have uncovered detailed manuals for the Schlüsselgerät 41, or SG-41, a cipher machine that puzzled historians and cryptography experts for decades.
The discovery of the manuals for the cipher machine came nearly 80 years after the end of the war. The newly found material explains how the device was operated, how its complicated key settings were managed, and how it was deployed in the field. For years, experts knew the machine existed but lacked clear instructions describing its full operation.
The documents were identified in two Prague archives by researchers Eugen Antal, Carola Dahlke, and Robert Jahn. Their study reports that the files include official German operating manuals, instructions for field use, technical guidance on key settings, and wartime key tables dated March 1945.
Among the materials is also a 41-page Czech report describing the machine’s design, along with a postwar cryptanalysis produced by Czechoslovak intelligence services.
A complex cipher device is finally explained
The SG-41 was designed in 1941 by German engineer Fritz Menzer. Unlike the more widely known Enigma machine, the SG-41 relied on a different mechanical system known as a pin-and-lug mechanism.
The device used six rotating wheels filled with movable pins. These pins generated numerical values that were added to letters during encryption and then reversed during decryption.
Researchers say two features made the machine particularly difficult to analyze. First, the wheels did not rotate in simple or predictable steps. Instead, they moved in irregular patterns, increasing the complexity of the cipher.
Second, the sixth wheel included a special “negation” function. This mechanism could reverse the active and inactive pin positions on the other wheels, dramatically altering the behavior of the encryption system.
The newly discovered documents also clarify how operators used the machine in daily communications. Messages relied on a layered key system that included a monthly table with 26 possible pin arrangements, a daily key consisting of six letters, and a disguise key that concealed the starting point of each transmission.
Wartime records reveal field use
The archives also provide details about the machine’s physical design and how it was used during wartime operations.
According to the records, the SG-41 weighed about 10 kilograms (22 pounds) by itself. With its lid and base plate attached, the total weight reached roughly 17 kilograms (37.5 pounds).
Engineers modified the device so it could be carried and operated in the field. A padded plate allowed operators to rest the machine on their knees while working, and the same structure could serve as a frame for carrying it as part of a backpack.
A postwar Czech intelligence analysis identified a minor weakness in the encryption system. Some output characters appeared slightly more often than others. However, researchers noted that the pattern was extremely subtle and would have been difficult to exploit in real wartime communications.
The study concludes that the SG-41 was likely a highly secure encryption system during the war, though some gaps in the historical record remain.