Marian Adam Rejewski ; 16 August 1905 – 13 February 1980) was a Polish mathematician and cryptologist who in late 1932 reconstructed the sight-unseen Nazi German military Enigma cipher machine, aided by limited documents obtained by French military intelligence. Over the next nearly seven years, Rejewski and fellow mathematician-cryptologists Jerzy Różycki and Henryk Zygalski developed and used techniques and equipment to decrypt the German machine ciphers, even as the Germans introduced modifications to their equipment and encryption procedures. Five weeks before the outbreak of World War II in Europe, the Poles shared their technological achievements with the French and British at a conference in Warsaw, thus enabling Britain to begin reading German Enigma-encrypted messages, seven years after Rejewski's original reconstruction of the machine. The intelligence that was gained by the British from Enigma decrypts formed part of what was code-named Ultra and contributed—perhaps decisively—to the defeat of Nazi Germany.
In 1929, while studying mathematics at Poznań University, Rejewski attended a secret cryptology course conducted by the Polish General Staff's Cipher Bureau , which he joined in September 1932. The Bureau had had no success in reading Enigma-enciphered messages and set Rejewski to work on the problem in late 1932; he deduced the machine's secret internal wiring after only a few weeks. Rejewski and his two colleagues then developed successive techniques for the regular decryption of Enigma messages. His own contributions included the cryptologic card catalog, derived using the cyclometer that he had invented, and the cryptologic bomb.
Five weeks before the Nazi German invasion of Poland in 1939, Rejewski and colleagues presented their achievements to French and British intelligence representatives summoned to Warsaw. Shortly after the outbreak of war, the Polish cryptologists were evacuated to France, where they continued breaking Enigma-enciphered messages. They and their support staff were again compelled to evacuate after the fall of France in June 1940, and they resumed work undercover a few months later in Vichy France. After the French "Free Zone" was occupied by Nazi Germany in November 1942, Rejewski and Zygalski fled via Spain, Portugal, and Gibraltar to Britain. There they enlisted in the Polish Armed Forces and were put to work solving low-grade German ciphers.
In the aftermath of World War II, Rejewski reunited with his family in Poland and worked as an accountant. For two decades, he remained silent about his prewar and wartime cryptologic work to avoid adverse attention from the country's Soviet-dominated government; he broke his silence in 1967 when he provided to the Polish Military Historical Institute his memoirs of his work in the Cipher Bureau. He died at age 74 of a heart attack and was interred with military honors at Warsaw's Powązki Military Cemetery.
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