Tunny Machine Breaking Codes Again At Bletchley

Another of Bletchley Park’s World War II secrets has been revived. The National Museum of Computing, at Bletchley’s site near Milton Keynes, has unveiled a reconstruction of one of the Tunny machines that played an important role in the run-up to D-Day and the events that followed to the end of the War and VE-Day.

Tunny machines automated the decryption of top secret German transmissions by converting radio signals into text. The devices made it possible to reveal Nazi secrets about troop movements and other plans in days rather than weeks.

An Enigmatic Puzzle To Solve

All that was known about the machines, which were destroyed after the War, was held in a few sketched details of circuitry but no complete circuit diagrams. The reconstruction team, led by John Pether and John Whetter, had to use the diagrams and the recollections of a few of the remaining workers from Bletchley to recreate the Tunny machine virtually from scratch.

Nazi Morse code dot-and-dash transmissions were printed as a modulated line by a pen recorder. This was “read” by trained operators who created a punched tape which was transmitted by landline and then fed into the Tunny. The machine applied the decryption algorithm and automatically printed the German messages onto paper for a team of translators to work on.

The messages were used to outmanoeuvre enemy troops and played a large part in the downfall of Adolf Hitler – many of whose personal messages were passed through the circuitry.

The project to recreate the machine was commenced in 2005 by a team of volunteers, primarily BT engineers, using their ingenuity, obsolete parts from BT equipment, pieces scrounged from various contacts. This was followed by 18 months of soldering and weaving the thousands of wires that bind the circuitry together.

Circuit Diagrams Down The Drain

At the end of the War, there were a dozen or so of the original Tunnys but they were all recycled in the days of rationing and make-do-and-mend that followed the conflict. Even the original circuit diagrams suffered a recycling fate.

Whetter revealed that, when the machines were being dismantled, the head designer Sid Broadhurst left a large envelope on the windowsill of the Bletchley Hall toilets. Later, all the circuit notes were found inside the envelope but not the circuit diagrams. Had it not been for rationing and a shortage of toilet paper, the circuit diagrams would probably have remained intact, Whetter claimed.

The Tunny machines and all the reconstructed electronic machines of Bletchley Parks’ war years can be viewed at the National Museum of Computing.