Researchers uncover lost Nazi cipher manuals for SG-41 machine in Prague

Get you up to speed: Researchers uncover lost Nazi cipher manuals for SG-41 machine in Prague

Nazi cipher manuals for code machine more advanced than Enigma found | News World

The SG-41 cipher machine used a code that was more advanced than Enigma (Picture: E.Antal/Cryptologia/Cover Media)

Lost Nazi cipher manuals relating to a code believed to be more advanced than the famous Enigma cipher have been discovered in Prague after more than 80 years.

The original wartime manuals for the secret German cipher machine were discovered in archives.

Researchers say the documents relate to the Schlüsselgerät 41 (SG-41), a highly classified encryption device used by the German Wehrmacht in the final years of the war.

The papers were uncovered during an academic investigation by cryptography researchers Eugen Antal, Carola Dahlke and Robert Jahn. They were found in two Czech institutions; the Military History Institute in Prague and the country’s Security Services Archive.

The files include operating instructions, encryption rules and original key tables used during the closing weeks of the war in 1945.

A sheet of encrypted ciphertext letters from the rediscovered manual (Picture: E.Antal/Cryptologia/Cover Media)

As with Enigma, British cryptographers did succeed in breaking some SG-41 due to errors by the German operators of the machines in October 1944, but did not fully understand what they were dealing with until the final months of the war when machines were captured and analysed.

However, historians say there are still remaining questions about how the SG-41 operated due to the lack of surviving documentation – ones that these new documents can answer.

‘The recently found documents provide insightful information on several levels,’ the researchers, led by Eugen Antal of the Slovak University of Technology in Bratislava, wrote.

‘Firstly, the extent of cryptanalysis in Central Europe and its influence becomes clearer in a broad historical context, since the elaborate Czechoslovak investigations were not known to this extent.

‘Secondly, for the first time it is possible to view complete instructions on how to use the machine in the field. Especially, the very precise instructions on how to use and maintain the machine and solve mechanical problems are very interesting in this respect.

‘The documents on key design are particularly exciting, confirming the previously assumed cipher operation and answering other previously unanswered questions.’

During the Second World War, the German military relied heavily on encrypted communications.

The Enigma machine became the best known of these systems after Allied codebreakers succeeded in deciphering its supposedly unbreakable code.

A glossary of the SG-41 parts. The machine is seen opened, left, and ready to be used, right. (Picture: E.Antal/Cryptologia/Cover Media)

The SG-41, designed in 1941 by German engineer Fritz Menzer, was widely regarded as a more sophisticated device.

Unlike Enigma, which used electrically powered rotors, the SG-41 relied on a fully mechanical system based on a ‘pin-and-lug’ principle first developed by Swedish cryptography pioneer Boris Hagelin.

The machine used six rotating wheels fitted with movable pins. These pins could be set to active or inactive positions according to daily encryption settings.

When an operator typed a letter on the keyboard, the internal mechanism analysed the pin positions and generated a pseudo-random number. That number was added to the original letter to produce the encrypted text. To decrypt the message, the receiving machine had to be configured identically so the same value could be subtracted.

The research suggests the SG-41 incorporated several innovations that made it difficult to analyse.

One was an irregular stepping mechanism. In many cipher machines, internal wheels rotate in predictable sequences.

In the SG-41, however, the wheels influenced each other’s movement, creating irregular patterns that complicated attempts to identify repeating sequences in encrypted messages.

A second feature was a ‘negation’ function built into the sixth wheel. When activated, it reversed the state of pins on the other wheels, instantly changing the machine’s behaviour and increasing its unpredictability.

Diagrams show the construction of the SG-41, which relied on a fully mechanical system consisting of six rotating wheels fitted with movable pins (Picture: E.Antal/Cryptologia/Cover Media)

Together, these features made the SG-41 one of the most advanced mechanical cipher systems developed during the war.

At the Military History Institute, researchers located a folder titled Wehrmacht Encryption Guidelines containing several original German documents in good condition. Among them was the official operating manual for the SG-41 and its variant, the SG-41Z, dated 2 September, 1944

The archive also contained a field manual for operators, a regulation known as Vorschrift Nr. 90 explaining how encryption keys were generated, and monthly key tables used between 16 and 31 March, 1945.

These tables are considered particularly valuable as they reveal how the machine was configured during the final weeks of the conflict.

Researchers also found a 41-page document written in Czech describing the machine’s technical design along with a post-war cryptanalysis carried out by Czechoslovak intelligence. The manuals also shed light on the machine’s physical design.

Although intended for field use, the SG-41 was far from lightweight. The device itself weighed around 10kg and about 17kg when fully assembled with its protective lid and base plate.

To make it usable in the field, German engineers developed a padded wooden support known as a Knieplatte, or “knee plate”. This allowed operators to rest the machine on their knees while typing messages, in a manner similar to using a laptop computer.

The SG-41 transformed into a backpack and ready for transportation (Picture: E.Antal /Cryptologia/ Cover Media)

The board could also be converted into a backpack frame so soldiers could carry the heavy device during transport.

The newly found documents also clarify how the machine’s complicated key system worked.

Operators used a monthly table containing 26 possible pin configurations, corresponding to the letters of the alphabet.

Each day they received a six-letter daily key that determined which configuration from the table would be used for each of the machine’s six wheels.

Additional settings included a ‘camouflage key’, designed to disguise the starting position of messages, and a two-digit identification number assigned to each communication station.

Before sending a message, operators had to set all of these values correctly.

Despite the discovery, several questions remain unanswered – as few working machines exist today, with some of the best examples thought to be possibly languishing in Russian state archives. As a result, manuals like these could be key to carrying out simulations to get a feel for how th SG-41 actually worked.

‘Unfortunately, there are only a few intact machines in the world,’ the authors write. ‘Most of them are unlikely to be in perfect working order, as regular maintenance of the mechanics is absolutely essential.

‘Cryptanalytic studies such as these can therefore only be carried out with the help of simulations today.’

For now, the Prague discovery represents one of the most significant breakthroughs in understanding the SG-41.

After decades of mystery, researchers say the forgotten documents are finally helping to reveal how one of the Second World War’s most complex cipher machines actually worked.

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