Not really computer related, but I hope someone has the answer. How do people decode messages from an Enigma machine?
I understand that if you knew the rotors, rotor positions, and plugboard the message was encrypted with, you could decrypt it, but how did other Nazis know those settings? They must have had to have a secure way of passing them around.
Here's a possible website you can use to find out more: http://www.openculture.com/2013/01/the_enigma_machine_how_alan_turing_helped_break_the_unbreakable_nazi_code_.html
A Summary:
Legitimate Method: The Nazis released a monthly code sheet with the appropriate settings for each day of that month. Once you knew the settings, you could type in the nonsense into an Enigma set that way and get the original message.
Cracking the Enigma: We knew that no letter in the Enigma ever encrypted to itself (ie, typing in "a" never yields "a"). We also knew how the Nazis sent messages (ie, their weather reports at 6 AM always had the format "The weather today is __ and _."). So we took their messages and, knowing that they had to contain certain words in certain (rough) locations, we matched them up with a limited number of slots that worked (because no letters in those slots were in a corresponding place in the word). From there, we were able to reverse engineer one of the settings by a clever brute-force attack. After that one setting, which produced by far the greatest variety, was broken, it was a simple matter to get the positions of the rotors (there were only a few thousand). The code was re-broken every day by a computer, which took about 20 minutes.
The Nazis had physical code books they would distribute. They would determine the Enigma settings for the day. Then using those settings, each message would start with a message key setting the starting rotor settings for the rest of the message. These were useful as cribs for breaking out messages since all messages the Nazis sent for a day started using the same key.
The most difficult Enigma codes to break were used by the German navy. They were highly disciplined about their use of keys, their Enigma machines had an extra rotor, and they didn't produce nearly as many messages per day as the air force or the army did. And their code books used water-soluble ink so if they were abandoning ship all someone needed to do was chuck the code book overboard, which is embarrassing because no one did that when the British captured U-110 on May 9, 1941. And because they couldn't distribute new code books to the navy without recalling the entire fleet to port, the Germans had to use compromised codes for the rest of the month, giving the British much insight into how they operated.
Enigma messages were basically broken with brute force. The British had electromechanical devices called "Bombes", which were basically devices that would tick through every permutation of Enigma key until they came up with plaintext. These were assisted by the cryptanalysts who would deduce possible plaintext for a given cyphertext, called cribs. These cribs would substantially reduce the number of keys to search if they were correct. Contrary to popular belief, the British did not invent the first electronic computer to break Enigma, but a different, more complex cypher used by the German high-command.
Check out Turing's biography by Hodges. There's a pretty good description of both how the legitimate (ie; nazi) and unauthorized encoding and decoding of enigma messages worked.
