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Originally Posted by GoldenRivet
What a handy device this machine would be in a military situation.... Had those brits never captured the code books allied intelligence units could have spent several life times trying to figure this thing out. Insane the amount of prep work that goes into a single message.
without the daily codes you would literally be reading gibberish
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For the most part, the British *DIDN'T* have the code books. When they did have them, though, it allowed them to read traffic, either back traffic or current.
That gave them the advantage of knowing what to look for. Military messages tend to have strict formats, and they also tend to be stereotyped. If you have a good guess as to what the plaintext might be, you can take a stab at decoding it even if you don't know the key.
The British exploited this by decoding messages in less secure ciphers (such as the 'dockyard' cipher, used by units not important enough to have an Enigma machine) and searching for the plaintext in Enigma messages. Often, they used a technique called 'gardening', where they would sow a minefield of a port on purpose, just to generate warning traffic in both Dockyard and Enigma. They could then search for the text of the Dockyard message in the Enigma messages.
Since in an Engima type machine with a reflector no letter can ever be enciphered as itself, that can help you by making certain combinations of possible rotors and positions impossible for a given plaintext. What you do is take your probably plaintext and slide it past the message, and everywhere there is a match between a plaintext letter and an enciphered one, that is an incorrect orientation. That cuts the 'key space' you have to search considerably.
If you have the wiring of the rotors, you can work out the solution. Even the four rotor Enigma was reasonably easy to defeat given the technology of the day, if you threw enough resources at the problem. There was a blackout of naval Enigma for most of 1942, after the Kriegsmarine switched to the 4 rotor Enigma, but from 1943 onward the Allies were pretty much reading it on a current basis right up until the end of the War.
Now, on the other hand, the US SIGABA and British TYPEX machines weren't broken. They didn't use a reflector, so a letter could be enciphered as itself. In the case of the SIGABA they had more encipering rotors than any Enigma, and they also had a set of rotors that controlled the movement of the enciphering rotors, making it more random instead of the regular movement of the Enigma (which is also a major weakness).
So, while the Enigma has a certain mystique that is undeniable, it was only a mediocre enciphering machine.
Now, there is a technology that was invented around 1921 that could have given the Kriegsmarine complete protection for their communications: The One Time Pad. The OTP consists of a set of random numbers or letters used to encipher a message. Each particular page is used once, then destroyed. If the U-Bootwaffe had used the OTP for it's communications, all the British would have had left is traffic analysis and direction finding. Traffic analysis can develop good intel, but it is tenuous at best, and can't really give you specifics. DF is also good, but it only tells you where a unit is right then, not where they will be in the future so that you can intercept them.
I've often wondered how the Battle of the Atlantic would have turned out if the Germans had decided to implement the OTP instead of Enigma.