Tunny code-breaker rebuilt at Bletchley Park

Tunny code-breaker rebuilt at Bletchley Park

Summary: Engineers at the National Museum of Computing at Bletchley Park have rebuilt the Tunny machine, a key device used in decoding German High Command messages during the Second World War

TOPICS: After Hours

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  • The Tunny machine

    The Tunny machine turned the encrypted message into cleartext.

    At the top of the machine were three banks of relay switches called 'uni selectors', which represented the 12 rotors of the Lorenz machine.

    Below the uni selectors was a plug jack-board, which was the control panel for the uni selectors. Lamps above the jacks light up depending on the settings of the rotors: setting one has one lamp, setting two has two, and so on up to 27, as Lorenz machines had 27 rotor settings per rotor.

    The original Tunny machine was built after some ingenious mathematical and logical deduction by a number of British people.

    Photo credit: Bletchley Park

    See ZDNet UK's gallery of Acorn computers at Bletchley Park.

  • Tunny machine jack board

    The white pegs on the Tunny machine jack-board define the wheel pattern, and they could be set to an on or off position to represent the position of the Lorenz rotors.

    In the 1940s, the Metropolitan Police came across odd radio messages while monitoring transmissions in the hunt for spies. The British worked out they had been encrypted using teleprinters, according to Whetter.

    On August 30, 1941, the British intercepted two test messages on new German radio links that had the same characters. The first message had 4,000 characters, while the second had 3,900 characters. John Tiltman, the head cryptanalyst at Bletchley, decrypted the message, and worked out the 4,000 characters were in cipher text and the 3,900 in cleartext.

    Tiltman passed the messages to William Tutte and his team, who deduced the Lorenz machine structure and its operation. They worked out how the rotors interacted with each other, and with other complex matrices, using pencils and paper.

    Tutte passed these findings to the Post Office research station at Dollis Hill, where engineers built a machine to emulate the logical structure devised by the team— the Tunny machine. The engineers built the machine mainly out of telephone exchange components.

    Photo credit: Bletchley Park

    See more photos of Bletchley Park on ZDNet UK.

  • Switches to finesse the decryption

    Underneath the jack-board was a telegraph relay. The bank of switches was used to finesse the output of the machine. Occasionally the decryted messages would turn into gobbledegook, so the switches would be used to go back and restart the machine from a specific point, without having to decode parts of the message that had already been decoded.

    Below the relay, shielded by two grey boxes, were the matrix control relays for the circuitry. There were at least 120 relays that controlled the operation of the machine, Whetter said.

    The entire machine was rebuilt using spares from BT telephone exchanges that were remodelled in the 1980s. All of the engineers that worked on the rebuild were ex-BT employees and had contacts at other museums, and so could lay their hands on the components, Whetter added.

    Photo credit: Bletchley Park

    See ZDNet UK's gallery of Acorn computers at Bletchley Park.

Topic: After Hours

Tom Espiner

About Tom Espiner

Tom is a technology reporter for ZDNet.com. He covers the security beat, writing about everything from hacking and cybercrime to threats and mitigation. He also focuses on open source and emerging technologies, all the while trying to cut through greenwash.

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  • Anybody heard of Harry Beckhough!
  • As a memberf the 'Bletchley District Gazette' during the 1960's, and living behind the 'Manor House' I enquired of my then Editor [ Carl Moser ] what went on in that area during the WWII.
    and he said in all sincerity, 'You will be an old man before details are exposed'...and how right he was.!! But I had talked to someone who had worked in the Postal communications, and he said then, that without the engineering procedures in operation at that time, the Academics, could not have succeeded....Alan Turing....God rest your Soul.
    daniel johnson-88fb3
  • went to 'Bletchley Parl a few years ago, fantastic place - i feel a return is in order :)
  • Great pics , better than the ones I took. I agree with Will above , I had a lovely visit to Bletchley when I was on holiday in England and look forward to going back some day. It brought some dusty old history to life for me.