Perforated sheets

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The Enigma cipher machine Enigma machine *Enigma rotor details Cryptanalysis of the Enigma *Cyclometer *Perforated sheets *Bomba *Bombe Ultra

Cipher Bureau [http://encycl.opentopia.com/ edit ] Cryptologic methods and technology: Lacida • Grill • Clock • Cyclometer • Card catalog • Bomba • Zygalski sheets Location: Saxon Palace • Kabaty Woods • PC Bruno • Cadix Personnel: Maksymilian Ciężki • Jan Graliński • Jan Kowalewski • Gwido Langer • Stanisław Leśniewski • Stefan Mazurkiewicz • Wiktor Michałowski • Antoni Palluth • Franciszek Pokorny • Marian Rejewski • Jerzy Różycki • Wacław Sierpiński • Piotr Smoleński • Henryk Zygalski

Zygalski sheet

Demonstration of two sheets at Bletchley Park Museum

The method of perforated sheets was a cryptologic technique used by the Polish Cipher Bureau before World War II, and during the war by British cryptologists at Bletchley Park, to decrypt messages enciphered on German Enigma machines. The method involved superposing a series of sheets — each containing a grid of holes in various positions — and aligning them in the proper manner with respect to each other, while shining a lamp underneath. Using this procedure, a large number of possibilities for the Enigma daily keys could be eliminated. If the settings were correct and sufficient data were available, a single aperture would remain, which indicated the solution.

Like Marian Rejewski's "card-catalog" method, developed using his "cyclometer," the "perforated-sheet" procedure was independent of the number of commutator plug connections.

The "perforated sheets" were invented about October 1938 by Polish Cipher Bureau cryptologist Henryk Zygalski, and accordingly are sometimes known as Zygalski sheets.

The Cipher Bureau's manual manufacture of the sheets, which was done by the mathematician-cryptologists themselves, was very time-consuming; by December 15, 1938, only one-third of the job had been completed. 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).

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 cryptologic achievements in breaking Enigma ciphers. Part of the disclosures involved Zygalski's "perforated-sheet" method.

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 PC Bruno outside Paris, 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.)

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 rendered completely useless.

See also

• punch card
• Cryptanalysis of the Enigma

External links

• [Javascript demonstration of Zygalski sheets]
• ["Polish Enigma Double"]
• [About the Enigma (National Security Agency)]
• ["The Enigma Code Breach" by Jan Bury]
• [The „Enigma” and the Intelligence]
• [www.enigmahistory.org]
• ["Codebreaking and Secret Weapons in World War II" By Bill Momsen]
• [A Brief History of Computing Technology, 1930 to 1939]

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