Zygalski sheets
Perforated sheets used to break Enigma ciphers.
Henryk Zygalski, a Polish mathematician and cryptologist, devised the Zygalski sheets around October 1938. This cryptologic method helped the Polish Cipher Bureau, and later British codebreakers at Bletchley Park, decipher German Enigma messages before and during World War II.
Quick Facts
- Inventor
- Henryk Zygalski
- Date of invention
- About October 1938
- Matrix size per sheet
- 26 × 26
Facts from the source article.
Method
To crack Enigma, Zygalski used twenty-six perforated sheets for each of the six possible rotor orders. Each sheet matched a specific starting position of the leftmost, slowest rotor. A twenty-six-by-twenty-six grid, repeated horizontally and vertically, held the 676 possible positions for the other two rotors. Holes were punched wherever a “female” could happen. Like Rejewski’s earlier method, this technique worked regardless of how many plugboard connections were in use.
Manufacture
The Polish mathematician-cryptologists at the Cipher Bureau had to make the Zygalski sheets by hand, using razor blades for secrecy, which was a slow process. By 15 December 1938, only one-third of the work was done. That same day, the Germans added rotors IV and V to the Enigma, making the task ten times harder because ten times more sheets were needed to cover the 60 possible ways to pick and order three rotors from the five now available. On 25 July 1939, just five weeks before World War II began, the Polish General Staff’s Cipher Bureau revealed their Enigma-breaking methods, including Zygalski’s perforated-sheet technique, to French and British allies in Warsaw. At Bletchley Park, the British made two full sets of these sheets using perforators, led by John R. F. Jeffreys. At Bletchley they were called Netz, short for Netzverfahren, though Gordon Welchman later referred to them as “Jeffreys sheets,” a name that actually belonged to a different catalog. The first set was ready in late December 1939. On 28 December, part of the second set went to the Polish cryptologists, who had fled to PC Bruno near Paris. The rest were finished on 7 January 1940 and taken to France by Alan Turing. With these sheets, the Poles kept solving Enigma’s daily keys. They used them to crack the first wartime Enigma message on 17 January 1940. In May 1940, the Germans overhauled how message keys were enciphered, except for one Norwegian network, making the Zygalski sheets useless. The Herivel tip, however, still worked.
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