1938, the bombs were ready, and the reconstructing of daily keys now took about two hours. Up to July 25, 1939, the Poles had been breaking Enigma messages for over six and a half years without telling their French and British allies. On December 15, 1938, two new rotors, IV and V, were introduced (three of the now five rotors being selected for use in the machine at a time). As Rejewski wrote in a 1979 critique of appendix 1, volume 1 (1979), of the official history of British Intelligence in the Second World War, "we quickly found the [wirings] within the [new rotors], but [their] introduction [...] raised the number of possible sequences of drums from 6 to 60 [...] and hence also raised tenfold the work of finding the keys. Thus the change was not qualitative but quantitative. We would have had to markedly increase the personnel to operate the bombs, to produce the perforated sheets (60 series of 26 sheets each were now needed, whereas up to the meeting on July 25, 1939, we had only two such series ready) and to manipulate the sheets." Harry Hinsley suggested in British Intelligence... that the Poles decided to share their Enigma-breaking techniques and equipment with the French and British in July 1939 because they had encountered insuperable technical difficulties. Rejewski rejected this: "No, it was not [cryptologic] difficulties [...] that prompted us to work with the British and French, but only the deteriorating political situation. If we had had no difficulties at all we would still, or even the more so, have shared our achievements with our allies as our contribution to the struggle against Germany." See also Polish Cipher Bureau. Bombe: machine, inspired by Rejewski's "cryptologic bomb", that was used by British and American cryptologists during World War II to decrypt German Enigma ciphers. Cryptanalysis of the Enigma. Zygalski sheets: device, invented about October 1938 by Henryk Zygalski and called "perforated sheets" by the Poles, that made possible the recovery of the Enigma's entire cipher key. Notes References Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek, Frederick, MD, University Publications of America, 1984, . Marian Rejewski, "Summary of Our Methods for Reconstructing ENIGMA and Reconstructing Daily Keys, and of German Efforts to Frustrate Those Methods," Appendix C to