|
The bomba (plural bomby) was a special-purpose codebreaking machine designed by Polish cryptanalysts and used to crack the German Enigma machine prior to World War II.
A bomba was designed to exploit an obscure but fatal weakness in the Enigma cipher. The Enigma used a three-letter key (for example, "NJR") to indicate the way the operator was to set the machine. German Enigma operators were issued lists of these keys, one 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 day 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 of the complexity of the Enigma cipher, the repetition would not be obvious in the ciphertext because the same plaintext letters would encrypt to different ciphertext letters. (For example, "PDNPDN" could become "ZRSJVL".)
However, this method, which appears to be quite secure, nonetheless is a cryptological mistake. Using the knowledge that the first three letters of a message were always the same as the second three, Polish mathematician Marian Rejewski was able to determine the internal wirings of the Enigma machine. However, even with this knowledge, because of the complexity of the machine, each of the possible day keys needed to be checked. With 263 = 17,576 possible keys, this was not a task that could be accomplished by hand.
Working for the Polish cryptologic agency, Rejewski invented the bomba, a machine to work through each of the possible day keys using brute force. It decrypted the message using each key, checking to see if the first six characters contained the repetition that indicated a possible match. It printed a list of these decryptions, of which there were hundreds for each message, only one of which was the correctly decrypted text. However, a list of hundreds was far easier to sort by hand than a list of seventeen thousand, and the machine was able to generate this list in only about two hours. The machine was called a bomba (Polish for bomb) for debatable reasons. Some historians claim that it was named after an ice cream dessert; others say that the name came from the fact that the sounds made by the machine as it worked resembled that of a ticking bomb. Rejewski wrote they used the name for lack of a better one, suggesting to bomb i.e. destroy the cryptography-armour in message traffic, exposing the unshielded plaintext.
Up to August 1939, the Poles had been working on breaking the Enigma code in secret, not informing their French or British allies of their efforts. However, the Germans made a fundamental change to the design of the Enigma, adding two additional rotor components. If the basic principles of the Bomba were to be updated, its complexity would be increased tenfold. The Bomba was no longer a viable option. The Polish, at a dead end, finally shared their work with the French and British, who refocused their efforts.
Before that, the french were (according to Gustave Bertrand) pretty desparate and even considered telling the germans they read enigma so as to provoke another code-change, which they planned to monitor very carefully and exploit the mistakes the germans hopefully would do again in its course.
Until 1939 the Polish company AVA had manufactured about 70 replica of Enigma, they built the first batch of 15 Enigma-E in Feb.1933 . AVA also delivered 6 bomby in Nov. 1938 to Pyry, Poland. Each consisted of 6 polish enigma-cores with attachments for cryptanalysis, such as coincidence detection circuits.
See also
- Bombe, the device for deciphering the Enigma by Allies.
|
