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Hitler's "unbreakable" encryption machine - and the Bletchley Park devices which cracked the code

The story of how Bletchley Park codebreakers decrypted top secret Nazi communications

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Topic: Security
the-lorenz-sz42-770.jpg
1 of 10 Image: Charles Coultas

The Lorenz SZ42

It was a huge secret during World War II but now the crucial work done by the codebreakers of Bletchley Park is very well known - even to the extent that Benedict Cumberbatch played Alan Turing in 2014 blockbuster movie 'The Imitation Game'.

But it wasn't only the Allies who had access to cutting-edge technology; Hitler's "unbreakable" cipher machine - the Lorenz SZ42 - was used to send top-secret messages of Nazi Germany's High Command.

However, the machine, despite being more complex than the Engima, wasn't as unbreakable as they thought and Lorenz cipher could be broken - thanks to Bill Tutte's deduction of the architecture of the Lorenz machine without ever having seen it. This achievement is credited with helping to shorten the war and save millions of lives.

It's thought there were 200 Lorenz machines in existence during the war but only four are known to have survived and now one of them is on display at The National Museum of Computing at Bletchley Park, giving visitors the opportunity to see both Allied and Axis technology used for encryption and decryption during World War II.

the-lorenz-sz42-serial-number-and-details.jpg
2 of 10 Image: Charles Coultas

Lorenz SZ42 seal number and close updetails

The particular Lorenz SZ42 machine to be displayed at The National Museum of Computing has the serial number 1137 and was used at the German HQ in Norway at Lillehammer, north of Oslo. Since Norway was occupied by the Nazis at the end of the war, it's likely to have received the final surrender instruction message at 24:00 hrs on 8 May 1945, marking the end of World War II.

the-lorenz-sz42-wheels.jpg
3 of 10 Image: Charles Coultas

Wheels used to produce cipher text

Using two sets of five wheels, the machine produced a cipher text which obscured and altered characters to encrypt traffic. In order to decode and understand messages, both the sending and receiving Lorenz machines had to be configured to the exact same starting position in order to decrypt the transfer.

ablesetafel-40-or-spruchtafel.jpg
4 of 10 Image: John Robertson for TNMOC.

Ablesetafel 40 or Spruchtafel

The Ablesetafel 40 or Sprunchtafel was used by German cryptographers to identify the wheel settings that the operator should use on the Lorenz machine. Only two such devices are known to exist today and visitors can see one for themselves at Bletchley.

lorenz-teleprinter.jpg
5 of 10 Image: John Robertson for TNMOC.

The Lorenz teleprinter input

A teleprinter was connected to the Lorenz SZ42 cipher machine, in order so that the operator could use the keyboard to type messages in plain text before they became encrypted.

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6 of 10 Image: Robert Dowell

Reconstruction of Knockholt receiving station with undulator

British intelligence was able to intercept messages using a Knockholt Y station. This is a partial reconstruction of Bletchley Park's Knockholt station, featuring an undulator to record the incoming encrypted traffic on tape for translation.

the-colossus-rebuild.jpg
7 of 10 Image: National Museum of Computing

The Colossus

Built in 1944, the Colossus computer reduced the time to decrypt German code from weeks to just hours, a speed which helped undermind any operation Hitler's troops were attempting to carry out. Colossus was used to find the wheel start positions used by the Lorenz machine from the five-hole punched paper tape. This rebuild of the Colossus now resides in Bletchley Park.

the-tunny-reconstruction.jpg
8 of 10 Image: Robert Dowell

The Tunny

The reconstructed Tunny Gallery shows the entire World War II code-breaking process of the Lorenz-encrypted messages - known as Tunny in the UK - from German territories. The original Tunny machine represented a rengineering of the Lorenz machine, despite British forces having never seen one.

There were between 12 and 15 by the end of the war and they produced the final decrypts of enciphered teleprinter communications of the German High Command during World War II.

teleprinter-to-produce-plain-text-decrypts.jpg
9 of 10 Image: Robert Dowell

The teleprinter to produce plain text decrypts of top-secret German orders

The decrypted text was printed in plain-text German, ready to be translated into English and reveal the top secret operations of Nazi high command - something credited with bringing about a much quicker end to the war.

the-tunny-gallery.jpg
10 of 10 Image: Robert Dowell

Reconstructed Tunny Gallery at the National Museum of Computing.

The German Lorenz SZ42 cipher machine is available to see at the National Museum of Computing in Bletchley park, along with the British machines which decrypted the messages.

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