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How did German changes to Enigma in May 1940 affect Polish code-breaking methods?

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Ultra The British government established a code-breaking effort, known as Ultra, at Bletchley Park in Buckinghamshire, England, during World War II.

In May 1940, the German military implemented a radical change to the Enigma system that eliminated the specific loophole previously exploited by Polish mathematician Marian Rejewski.1

Rejewski’s original method, which utilized the Polish bomba, relied on identifying the starting positions of the machine's wheels based on the repeated transmission of message keys. By altering the system, the Germans effectively rendered the Polish approach—and the bomba itself—obsolete for those specific decryption tasks.12

While the Polish methods were neutralized by this change, the Allied intelligence project known as Ultra at Bletchley Park successfully adapted. Alan Turing developed a more powerful, crib-based method that did not rely on the loophole the Germans had closed. This new approach, implemented through the Bombe, allowed code breakers to continue deciphering German communications despite the increased complexity of the Enigma system.12