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The method of perforated sheets was a cryptological technique used to decrypt ciphers produced using German Enigma machines (see Cryptanalysis of the Enigma). The method involved superposing a series of sheets — each containing a grid of holes in various positions — and shining a lamp underneath. Using this procedure, a large number of possibilities for the Enigma daily keys could be eliminated. Like the Polish Cipher Bureau's "card-catalog" method, developed using their "cyclometer ," it was independent of the number of commutator plug connections. The "perforated sheets" were invented in late 1938 in Poland by Henryk Zygalski, and accordingly are sometimes known as Zygalski sheets.
In late July 1939, a month before the outbreak of World War II, the Polish General Staff's Cipher Bureau disclosed to their French and British allies, at Warsaw, their cryptological achievements in breaking Enigma ciphers. Part of the disclosures involved Zygalski's "perforated-sheet" method.
The perforated sheets were subsequently produced at Bletchley Park, in England, by John Jeffreys. In late 1939 or early 1940, the British delivered a precious complete set (60 x 26 sheets) to the Polish cryptologists, by then escaped from German-overrun Poland to France. "With their help," writes Polish cryptologist Marian Rejewski, "we continued solving Enigma daily keys." (Rejewski, in Kozaczuk's Enigma 1984, p. 243; more from him about the perforated sheets, on pp. 287-89 and elsewhere.)
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