The United Kingdom’s National Museum of Computing has finished its laborious rebuild of a Tunny machine, a system that produced the final decryptions of intercepted Axis communiques during World War II. The Tunny machines—built by a team led by Bill Tutte—were an effort to re-engineer the Germans’ Lorenz SZ42 cipher machines used in teleprinter communications. Unlike the famous Enigma machines, no Lorenz units were captured by Allied until World War II ended, so Tutte and his team reverse-engineered the processes of the Lorenz machine using only German encrypted communications, a bit of information about the Lorenz, and a handful of communiques that had been successfully decrypted by hand.
Tutte and his team, after thousands of man hours, managed to get the first Tunny machine operational in 1942, well before the famous “Colossus” computer was completed and put to work decrypting German communications. Built largely from existing telephone equipment, the British had 12 to 15 Tunny systems running by the end of the war at the famous Bletchley Park facility, decrypting about 300 messages a week.
The National Museum of Computing’s rebuild of a working Tunny system is also a remarkable achievement: the working Tunnys were dismantled for parts after the war, and no comprehensive circuit diagrams or other engineering records of their design were ever compiled. Nonetheless, a team led by John Pether and John Whetter managed to reconstruct a Tunny system based on photographs, partial circuit diagrams, and consultations with the small number of original Tunny operators who are still alive. Some of the telephone system components were rather easy to locate: some identical parts remained in use by British Telecom through the 1980s.
“We’ve succeeded in rebuilding Tunny with scraps of evidence, and although we are very proud of our work it is rather different from the truly astonishing achievement of Bill Tutte’s re-engineering of the Lorenz machine,” said Pether, in a statement. According to Pether, the most difficult part of hte rebuild was getting the systems’ six timing circuits operating in unison.
From a technological point of view, the Tunny systems weren’t as significant as the later Colossus system, which is now generally regarded as the world’s first modern computer. However, the Tunny systems’ role in World War II cannot be underestimated, as they were functional long before Colossus came online. The Colossus system significantly sped up the process of finding wheel settings used in the Lorenz machines; that information would then be fed to the Tunnys, which would decipher communiques.
The rebuilt Tunny means the National Museum of Computing can now demonstrate the complete codebreaking process performed by the British during World War II, starting with an intercepted communique and proceeding all the way through to a final decrypted message.