Zygalski Sheets - Manufacture

Manufacture

The Cipher Bureau's manual manufacture of the sheets, which for security reasons 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).

On July 25, 1939, five weeks 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.

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. 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 December 28, 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 January 7, 1940, and were couriered by Alan Turing to France shortly thereafter. "With their help," writes 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 January 17, 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 of no use.