Tunny code-breaker rebuilt at Bletchley Park

AR 88 machines were used at Y Station Knockholt on the South Downs to pick up German messages using massive rhombic aerial arrays, Whetter told ZDNet UK.

The signals were demodulated and passed through a pen recorder, which produced a trace in a square wave format. Young women were employed to physically read the square wave format, a series of ones and zeros, and translate it into letters. They had to punch out teleprinter tape for each individual encoded letter.

Teleprinter signals were then sent to Bletchley Park via landlines, where Heath Robinson, and then Colossus, worked out the rotor settings used by a Lorenz machine to encode the original message.

See more photos of Bletchley Park on ZDNet UK.

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.

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

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.

See more photos of Bletchley Park on ZDNet UK.

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.

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

The back of the Tunny machine took two people 18 months' solid work to rewire.

"That was an absolute bastard of a job," said Whetter. "It was a labour of love."

Each of the uniselectors had 25 outputs, while each switch had over 120 connectors.

Further details on how to visit the Tunny Gallery can be found at the National Museum of Computing's website.

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