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How did Colossus help confirm that Operation Fortitude was working?

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The Colossus computer at Bletchley Park, Buckinghamshire, England, c. 1943. — Colossus Funding for this code-breaking machine came from the Ultra project.

Colossus played a critical role in the success of Operation Fortitude by decrypting high-level German military communications, known as "Tunny" messages.12

By processing these intercepted transmissions at electronic speeds, Colossus provided Allied intelligence with direct insight into the German high command's reactions to their deception efforts. Specifically, these decoded messages confirmed that the German leadership had been successfully misled into believing that the main Allied invasion would occur in the Pas-de-Calais region rather than in Normandy. This intelligence allowed the Allies to verify that their disinformation campaign—which included the use of a fictitious army and spurious radio transmissions—was effectively influencing German strategic decisions, such as the withholding of reinforcements from the actual invasion site.123