To many, the Tunny machine was just as vital to breaking the codes of the German army during World War II as was Colossus; the device's main purpose was to deliver the final decrypts of enciphered communications of the German High Command.
Building the machine itself - which was the British version of the German Lorenz S42 rotor cipher machine - proved to be an incredibly challenging feat given that it had to be reverse engineered based on outputs from the original Lorenz and a mistake made by a German operator in August 1941.
It took nine man-years to build the machine and involved three engineers, including John Pether and John Whetter. To some extent, they had to follow in the footsteps of their illustrious predecessors given the fact that the original parts disappeared and that circuit diagrams and plans were lost after the war.
The Tunny machine worked in tandem with the Colossus which is widely regarded as the first modern computer. By the end of the war, up to 15 Tunny machines were deciphering on average 20 messages each per week from the German army. Bletchley Park is now home to a fully functional unit of Colossus and Tunny.
BT donated decommissioned analogue telephone exchanges to the project; parts were salvaged from these to build the Tunny. 200 relays had to be sourced along with complex wiring.