The Polish Cipher Bureau Reveals Enigma Decription Techniques to the French and British (July 25, 1939)

The Biuro Szyfrów ("Cipher Bureau"), the Polish interwar agency charged with both cryptography and cryptanalysis, revealed Poland's Enigma-decryption techniques and equipment, which it had achieved using the bomba device, to the French and British. 

Poland thereby made possible the western Allies' vitally important decryption of Nazi German secret communications (Ultra) during World War II.

"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 refuted 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^{' " (Wikipedia article on Bomba (cryptography), accessed 12-21-2008).}