TNMOC showcases role of Wren Colossus operators in D-Day

Colossus was used to decypher German Lorenz encrypted messages, one of which appears to have given the Allies the green light

In the past few months, researchers from the National Museum of Computing (TNMOC) have uncovered detailed intelligence of Germany’s Lorenz messages decrypted with the help of the Colossus machine during the Second World War, and the messages are now being exhibited at TNMOC.

Colossus’ ability to decrypt communications between Adolf Hitler and his generals proved decisive in the success of the Allies’ D-Day operations.

Colossus was the the world’s first computer, built by a brilliant post office engineer, Tommy Flowers, to crack the Lorenz ciphers used by the Nazis.

Mathematics genius Bill Tutte worked at Bletchley Park on analysis that enabled cryptographers working on Lorenz messages to speed up decryption of the information by hand from 1941. The task of decrypting the messages was automated thanks to Flowers’ work in building Colossus.

The Colossus machine cracked its first Lorenz-encrypted message on 5 February 1944. The machine, operated by Wrens at Bletchley Park, was used by the Allied High Command to understand the morale and strategy of the Nazi generals.

One of the decrypted messages being exhibited at TNMOC concerns a communication that was sent on 5 June – the day before the invasion – in which the German command reported: “As yet, there is no immediate prospect of the invasion.”

In a new YouTube video (see below) from TNMOC, Colossus Wren Margaret Mortimer described the role played by the Wrens who operated Colossus in the run-up to D-Day. “We had no idea what we were going to do, or what it was going to do,” she said.

Wireman Margaret Bullen added: “I never knew what a computer was.”

Colossus Wren Irene Dixon, now aged 94, said: “I worked on the probability that if you set the machine at a certain place, you would come up with a hit.”

Recalling an event 75 years ago in the run-up to D-Day, Dixon added: “I was working one night with Jack Good [a Bletchley Park cryptologist]. He seemed extra happy at our Colossus readings and, without telling me why, took me into the adjacent room. This was most unusual as I had never been outside of my usual workplace room before.

“Here he used what I now know was a Tunny machine [the final stage in decryption]. As Jack used it, he was obviously ‘over the moon’ with what he had found. We must have had speedy decryptions for the essential incoming messages.”

Read more about the history of computing at Bletchley Park

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