vrijdag 11 april 2008

Everything you ever wanted to know about the Enigma

As great a machine the Enigma was, it too could not prevent users from messing it up. Examples:

Part of the first class encryption of the Enigma was the possibility for the clerk to make up his own six-letter settings. This let to the Polish cryptanalysts occasionally being able to guess the settings. The military did not allow an obvious setting such as ABC. However, cipher clerks sometimes chose settings like QWE (the first three letters on the keyboard) or names. In the example above, if the first three letters were HIT, the cryptanalysts could guess that KOS and RLB were the ciphers to LER, spelling out HITLER. BER was usually followed by the ciphers of LIN. One particular German code clerk continually used his girlfriend’s name, Cillie, for his messages, and so these easy-to-guess indicators became known as "Cillies."

After the English had boarded the U-110 [thanks Fritz-Julius Lemp for being a pussy!] and got their hands on a working Enigma [with all dials in the correct setting for the whole month], they where able to destroy lots of U boats that where decimating the US-UK ships. Admiral Doenitz just knew something was wrong and made a change by added a thin fourth rotor between the leftmost rotor and the reflecting plate.

Bletchley Park learned of the impending change from decrypts and captured material, but until it was actually implemented there was little they could do to prepare. Fortunately, the Germans made an error. In December 1941, before the change had been made official, a U-boat sent a message using the four-rotor machine. To compound the mistake, the same message was retransmitted using only three rotors. From this seemingly innocuous error, the cryptanalysts at BP determined the wiring of the fourth rotor. :P

In order to set up the U.S. Navy Bombe, cryptanalysts first had to determine a "crib." A crib is the unenciphered text that is assumed, or known, to appear in the message.

Cribs could come through a variety of methods. Some of the best cribs came from errors made by the Germans themselves. On more than one occasion, a German signal clerk sent the same message twice in two different codes. If the code for one was known, it provided a crib for the unknown system.

Another frequent German mistake came in standardized messages. For example, a shore weather station in the Bay of Biscay sent out a message every day at 07:00 which began, "The weather in the Bay of Biscay will be. . . ." Knowing the exact wording of a message made a perfect crib for the Allies, so it became a high priority to intercept the daily message from this weather station.

A final example of a common German error involved the practice of submerged U-boats. When the submarines resurfaced after extended periods of time under water, they requested all the important messages they had missed while below the waves. The transmissions that followed inevitably involved communications previously sent and deciphered. Cryptanalysts merely checked the back files for messages with the same number of letter groups and used them as cribs for the new message. Since the resulting message would be identical to the previous one, it helped reveal the Enigma setting for the current day. With the daily setting, all the current day's messages could be read.

Other cribs came from knowing the current activities of the enemy. If, for example, a battle occurred, it could be assumed that messages following the attack reported on the battle. It was more difficult for the cryptanalysts to build cribs for these types of messages since it involved guesswork.

Because the Enigma rotors moved with each keystroke, a letter typed twice usually enciphered to two different letters. Also, the Enigma could not encrypt a letter to itself. Finally, the Germans indicated a space between words with the letter X and spelled out numbers.

Knowing these details played an important role in ultimately breaking the Enigma's daily settings.

Now why do we see these same weaknesses made over and over again?

Sometime ago there was one for sale too. Damn that would have been the hottest geek present ever. Prices have not been too extreme either...

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