Perforated sheets

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

Method

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.

Use and manufacture

The Cipher Bureau's manual manufacture of the sheets, which was done by the mathematician-cryptologists themselves using razor blades, was very time-consuming; by December 15, 1938, only 1/3 of the job had been finished. 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.

With the assistance of machinery, the production of two complete sets of perforated sheets was undertaken at Bletchley Park in England by a section headed by John R. F. JeffreysRalph Erskine, "The Poles Reveal their Secrets: Alastair Denniston's Account of the July 1939 Meeting at Pyry", pp. 294-305, "Cryptologia" 30(4), December 2006] Ralph Erskine, "Breaking Air Force and Army Enigma", p. 53 in "Action this Day", edited by Ralph Erskine and Michael Smith, 2001] . The sheets were known at Bletchley as Netz (from "Netzverfahren", "net method"), although they were later remembered by Gordon Welchman as "Jeffreys sheets"; this term, however, referred to another catalogue produced by Jeffreys's section.

The first set was completed in late December 1939, and on 28 December 1939, 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 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] The sheets were used by the Poles to make the first wartime decryption of an Enigma message on 17 January 1940.

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

* Bomba - Cryptologic bomb: a machine designed about October 1938 by Marian Rejewski to facilitate the retrieval of Enigma keys.
* Bombe: a machine, inspired by Rejewski's "Bomba", that was used by British and American cryptologists during World War II.
* Cryptanalysis of the Enigma.
* Punch card.
* Enigma (2001 film)

References

External links

* [http://www.codesandciphers.org.uk/anoraks/tools/zygalsha.htm Javascript demonstration of Zygalski sheets]
* [http://www.hut-six.co.uk/poland/ "Polish Enigma Double"]
* [http://www.nsa.gov/publications/publi00016.cfm About the Enigma (National Security Agency)]
* [http://www.armyradio.com/publish/Articles/The_Enigma_Code_Breach/The_Enigma_Code_Breach.htm "The Enigma Code Breach" by Jan Bury ]
* [http://www.ww2.pl/ The „Enigma” and the Intelligence]
* [http://www.enigmahistory.org/main.html www.enigmahistory.org ]
* [http://home.earthlink.net/~nbrass1/1enigma.htm "Codebreaking and Secret Weapons in World War II" By Bill Momsen ]
* [http://www.smithsrisca.demon.co.uk/STMsubtypes-pt3.html A Brief History of Computing Technology, 1930 to 1939]