First, to my friend - this is my shameless "stupid questions and crohn's" account. Please don't judge. hahaha.
So I searched the sub, and I found a really helpful post on how cracking the codes work. But it's left out a couple key points.
First, how did they intercept the messages in the first place? It seems that a bunch of messages were sent over radio, so did they just have a bunch of people, and a different person listening to every single radio channel, and hope that someone in the room got lucky? Like, I've got the whole deciphering part. I don't get the whole, how did, say, the Allies get ahold of the German's communications to thus decipher?
Second, how did they agree on the code? So, let's say the US and England are sending messages between each other. And let's say these messages are coded using a one time pad encryption key, right? Or a rotation. So, before any messages can be sent, the US and England both have to KNOW that this is how they're going to send messages. Does a guy fly to england and is like, "hey dudes, here's the encryption key and we're using a 13 rotation also"? There's no way they send it plaintext "What's up everybody, hope Hitler's not listening in, here's the code we're using from here on out." Like, okay. Different platoons used different codes, sure, but there's still the giant overarching issue of say, Churchill and Roosevelt. They coordinated Normandy, right? Legit planning. How did the generals all agree "yup this is how we'll talk"?
And how did they know when a code was cracked, and once they did, how did they decide what the new code was going to be (and how did they tell each other)?
Thanks!
Messages and signals were sent over radio, especially to naval ships (for obvious reasons). This was in the form of morse code.
The Allies had listening stations, nicknamed Y-Stations. The operators at the stations were highly skilled in finding and listening to transmissions and it is said they could even identify the person transmitting the message due to their style of using the equipment.
Once received, the messages were normally couriered to Bletchley Park for cataloging/decoding.
There is a map of Y-Stations here : https://www.google.com/maps/d/viewer?oe=UTF8&ie=UTF8&msa=0&mid=z8XJlx2bITuQ.kHAeditzMaj8 I have no idea if its complete, I do know that there were other listening stations abroad for other theatres outside of Europe.
There is a page at the IWM website with a sound clip of morse: http://archive.iwm.org.uk/upload/package/10/enigma/enigma9.htm
With regards to the encryption system I don't know much about the Allies, but enigma was actually quite complicated.
To start with the enigma used 3 (later 4) rotors. The rotor has two sets of electrical contacts, one on each side, each contact was wired to another on the other side. Each rotor wring was different (e.g. one rotor had contact 1 going to contact 22, another may have 1 going to 13 etc). electrical current would be put through one contact and it would pass to the other contact to exit the ring. Each rotor had the letters of the alphabet on the outside. So, if current entered the rotor at position A it may exit at J, this would change, according to another setting called Ringstellung (ring setting). If the ring setting changed A may now turn into P.
Initially there were 8 different rotors that could be used within the three slots, each rotor could be placed in any slot.
The choice of rotors, their position in the machine and the ring settings were all in code books that were distributed with each engima machine.
The rotors would be placed in the machine with a specific letter selected:
Also on the code books were the Kenngruppen this was a selection of three letters that gave the initial settings and were prefixed with another two random letters of the operators choice. The Kenngruppen was used by the receiver to set the rotor positions.
Next was a message key, this was another collection of three letters, which would be repeated twice and were encoded with the days settings. These three letters were picked by the operator. So, if the operator selected FDR as the key this may be encoded to REWTRC. The receiver of the message would find this would decrypt to FDRFDR and would set his rotors to F D R accordingly. The rest of the message could now be decrypted.
This was actually a flaw of the enigma - knowing that no letter would ever be encrypted to itself (e.g. F would never encrypt to F) and that the first and fourth letters were related, (as were the 2nd and 5th and 3rd and 6th) given enough samples for that day you could start to work out the settings. for example given REWTRC we know that the first letter is neither R or T, the second is neither E or R and the last its neither W or C. Given enough data you could work this out!
The Germans acknowledged the flaw and changed this to stop repeating letters, instead a random key would be selected by the operator (e.g. FDR) and then a second set of letters - GUL. The operator set his rotors to FDR, then encoded GUL, which might become RYI, he then sets GUL on the rotors and encrypts the message. The message is then sent with FDRRYI at the start. The receiver would set his rotors to FDR and decrypt RYI, giving him GUL, which he then used as the rotor settings to actually decrypt the message.
This is a quick(!) explanation of how the engima worked and how both parties agreed on the encryption settings. Please note that different parts of the German military had different operating procedures and these did change during the war as well.
I can happily recommend Hugh Sebag-Montefiore's "Enigma - The Battle For The Code". It obviously does cover the code breaking efforts at GCHQ but also has lots of technical detail of how the engima worked, the various different versions and operating procedures.
In addition there are resources on the web and even emulators of enigma machines. E.g. http://users.telenet.be/d.rijmenants/en/enigmamenu.htm http://practicalcryptography.com/ciphers/enigma-cipher/ http://www.madlab.org/guides/enigma.html http://www.enigmaworldcodegroup.com/