Editing Cryptanalysis of the Enigma (section)

===Perforated sheets===
[[File:Płachta Zygalskiego - decrypting Enigma.jpg|right|thumb|175px|[[Perforated sheets|Zygalski sheet]]]]
{{Main|Zygalski sheets}}
Although the characteristics method no longer worked, the inclusion of the enciphered message key twice gave rise to a phenomenon that the cryptanalyst Henryk Zygalski was able to exploit. Sometimes (about one message in eight) one of the repeated letters in the message key enciphered to the same letter on both occasions. These occurrences were called ''samiczki''<ref>{{Harvnb|Kozaczuk|1984|pp=54, 63}} note 2</ref> (in English, ''females''—a term later used at Bletchley Park).<ref>In {{Harvnb|Welchman|1997|p=72}} he suggests that this arose from the nomenclature for plugs (male) and sockets (female) because the success of this method depended on a number of overlying sheets having their apertures in register.</ref><ref>{{Harvnb|Sebag-Montefiore|2004|p=362}} cites [[Alfred Dillwyn Knox]], who attended the 25 July 1939 Warsaw conference, as having given a more frankly biological etymology, discreetly veiled in French.</ref>

Only a limited number of scrambler settings would give rise to females, and these would have been identifiable from the card catalogue. If the first six letters of the ciphertext were '''''S'''ZV'''S'''IK'', this would be termed a 1–4 female; if ''W'''H'''OE'''H'''S'', a 2–5 female; and if ''AS'''W'''CR'''W''''', a 3–6 female. The method was called ''Netz'' (from ''Netzverfahren'', "net method"), or the [[Zygalski sheets|Zygalski sheet method]] as it used perforated sheets that he devised, although at Bletchley Park Zygalski's name was not used for security reasons.<ref>Instead they were called [[John R. F. Jeffreys|Jeffreys]] sheets after the head of the Bletchley Park section that produced them.</ref> About ten females from a day's messages were required for success.

There was a set of 26 of these sheets for each of the six possible sequences ''wheel orders''. Each sheet was for the left (slowest-moving) rotor. The 51×51 matrices on the sheets represented the 676 possible starting positions of the middle and right rotors. The sheets contained about 1000 holes in the positions in which a female could occur.<ref>{{Harvnb|Welchman|1997|p=215}}</ref> The set of sheets for that day's messages would be appropriately positioned on top of each other in the [[perforated sheets|perforated sheets apparatus]]. Rejewski wrote about how the device was operated:

{{blockquote|When the sheets were superposed and moved in the proper sequence and the proper manner with respect to each other, in accordance with a strictly defined program, the number of visible apertures gradually decreased. And, if a sufficient quantity of data was available, there finally remained a single aperture, probably corresponding to the right case, that is, to the solution. From the position of the aperture one could calculate the order of the rotors, the setting of their rings, and, by comparing the letters of the cipher keys with the letters in the machine, likewise permutation S; in other words, the entire cipher key.<ref>{{Harvnb|Rejewski|1984e|p=289}}</ref>}}

The holes in the sheets were painstakingly cut with razor blades and in the three months before the next major setback, the sets of sheets for only two of the possible six wheel orders had been produced.<ref>{{Harvnb|Welchman|1997|p=216}}</ref>