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The method of Zygalski sheets was a cryptology technique used by the Poland Cipher Bureau before and during World War II, and during the war also by British cryptologists at Bletchley Park, to decryption messages cipher on German .
The Zygalski-sheet apparatus takes its name from Polish Cipher Bureau mathematician–cryptologist Henryk Zygalski, who invented it about October 1938.
The 26 × 26 matrix represented the 676 possible starting positions of the middle and right rotors and was duplicated horizontally and vertically: a– z, a– y. The sheets were punched with holes in the positions that would allow a "female" to occur.
Polish mathematician–cryptologist Marian Rejewski writes about how the perforated-sheets device was operated:
Like Marian Rejewski's "card-catalog" method, developed using his "cyclometer", the Zygalski-sheet procedure was independent of the number of plugboard plug connections in the Enigma machine.Marian Rejewski, "Summary of Our Methods for Reconstructing ENIGMA and Reconstructing Daily Keys...", Appendix C to Władysław Kozaczuk, Enigma, 1984, p. 243.
On that date, the Germans introduced rotors IV and V, thus increasing the labor of making the sheets tenfold, since ten times as many sheets were now needed (for the now 60 possible combinations of sequences, in an Enigma machine, of 3 rotors selected from among the now 5).
On 25 July 1939, five weeks before the outbreak of World War II, the Polish General Staff's Cipher Bureau disclosed to their France and Great Britain allies, at Warsaw, their cryptologic achievements in breaking Enigma ciphers.Władysław Kozaczuk, Enigma, 1984, p. 59. Part of the disclosures involved Zygalski's "perforated-sheet" method.
The British, at Bletchley Park, near London, England, undertook the production of two complete sets of perforated sheets. The work was done, with the aid of perforators, by a section headed by John R. F. Jeffreys.Ralph Erskine, "The Poles Reveal their Secrets: Alastair Denniston's Account of the July 1939 Meeting at Pyry," Cryptologia 30 (4), December 2006, pp. 294–305.Ralph Erskine, "Breaking Air Force and Army Enigma," in Action this Day, edited by Ralph Erskine and Michael Smith, 2001, p. 53. The sheets were known at Bletchley as Netz (from Netzverfahren, "net method"), though they were later remembered by Gordon Welchman as "Jeffreys sheets"; the latter term, however, referred to another catalog produced by Jeffreys' section.
The first set was completed in late December 1939. On 28 December part of the second set was delivered to the Polish cryptologists, who had by then escaped from German-overrun Poland to PC Bruno outside Paris, France. The remaining sheets were completed on 7 January 1940, and were couriered by Alan Turing to France shortly thereafter. "With their help," writes Marian Rejewski, "we continued solving Enigma daily keys." The sheets were used by the Poles to make the first wartime decryption of an Enigma message, on 17 January 1940.Władysław Kozaczuk, Enigma, 1984, pp. 84, 94 (note 8).
In May 1940, the Germans once again completely changed the procedure for enciphering message keys (with the exception of a Norwegian network). As a result, Zygalski's sheets were of no use, though the Herivel tip could still be used.Marian Rejewski, "Summary of Our Methods for Reconstructing ENIGMA and Reconstructing Daily Keys...", Appendix C to Władysław Kozaczuk, Enigma, 1984, pp. 243, 245.
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