have been identifiable from the card catalog. If the first six letters of the ciphertext were SZVSIK, this would be termed a 1-4 female; if WHOEHS, a 2-5 female; and if ASWCRW, a 3-6 female. The method was called Netz (from Netzverfahren, "net method"), or the Zygalski sheet method as it used perforated sheets that he devised, although at Bletchley Park Zygalski's name was not used for security reasons. 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. The set of sheets for that day's messages would be appropriately positioned on top of each other in the perforated sheets apparatus. Rejewski wrote about how the device was operated: 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. Polish bomba After Rejewski's characteristics method became useless, he invented an electro-mechanical device that was dubbed the bomba kryptologiczna or cryptologic bomb. Each machine contained six sets of Enigma rotors for the six positions of the repeated three-letter key. Like the Zygalski sheet method, the bomba relied on the occurrence of females, but required only three instead of about ten for the sheet method. Six bomby were constructed, one for each of the then possible wheel orders. Each bomba conducted an exhaustive (brute-force) analysis of the 17,576 possible message keys. Rejewski has written about the device: The cipher message transmitted the Grundstellung in the clear, so when a bomba found a match, it revealed the rotor order, the rotor positions, and the ring settings. The only remaining secret was the plugboard permutation. Major setback On 15 December 1938, the German Army increased the complexity of Enigma enciphering by introducing two additional rotors (IV and V). This increased the number of possible wheel orders from 6 to 60. The Poles could then read only the small minority of messages that used neither of the two new rotors. They did not have the resources to commission