The bomba ( s.

The German Enigma used a three-letter key (cryptography) (for example, NJR ) to indicate the way the operator was to set the machine. German Enigma operators were issued lists of these keys, one key for each day. For added security, however, individual messages were not broadcast using these keys. Instead, the operator randomly selected a completely new key for each message (for example, PDN ). This message key would be repeated once ( PDNPDN ) and encrypted, using the daily key. At this point each operator would reset his machine to the message key, which would then be used for the rest of the message. Because the configuration of the Enigma s rotor set changed with each depression of a key, the repetition would not be obvious in the ciphertext since the same plaintext letters would encrypt to different ciphertext letters. (For example, PDNPDN might become ZRSJVL. )

This procedure, which might appear secure, was nonetheless a cryptographic error. Using the knowledge that the first three letters of a message were the same as the second three, Polish mathematician Marian Rejewski was able to determine the internal wirings of the Enigma machine and thus reconstruct the sight-unseen device. That accomplished, in order to break an encrypted message ( ciphertext ), it was necessary to check each of the potential daily keys. With many thousands of such possible keys, and with the growing complexity of the Enigma machine and its keying procedures, this was becoming an increasingly daunting task.

In order to mechanize and speed up the process, Marian Rejewski, a civilian mathematician working at the Polish General Staff s Cipher Bureau in Warsaw, about October 1938 invented the bomba (bomb). It was an electro-mechanical device — essentially an electrically powered aggregate of six Enigmas. The bomb method was based, like the Poles earlier grill method, on the fact that the plug connections in the commutator did not change all the letters. But while the grill method required unchanged pairs of letters, the method of the bombs required only unchanged letters. Hence it could be applied even though the number of plug connections in this period was five to eight. In mid-November 1938 the bombs were ready, and the reconstructing of daily keys now took about two hours. (Rejewski, in Kozaczuk s Enigma 1984, pp. 242, 290.)

Just how the machine came to be called a bomb has been an object of intense fascination and speculation. One, most likely apocryphal, version originated by the Polish engineer and army officer Tadeusz Lisicki (who knew Rejewski and his colleague , p. 63, note 1.)

Up to July 25, 1939, the Poles had been breaking Enigma ciphers in secret from their France and Britain Allies of World War II. On December 15, 1938, two new rotors, IV and V, had been 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 ten-fold 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.

It has been speculated that the Poles decided to share their Enigma-breaking equipment and techniques with the French and British in July 1939 because they had encountered insuperable technical difficulties. Rejewski explains, in the same critique: No, it was not [cryptological] 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.

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