Noob Question about ENIGMA
in [Cryptography]
|
From: Felix Reuthner on 18 Nov 2009 18:35 Hi, a question about code-breaking in WWII: Correct me If I'm wrong, but basically it worked like that: German radio traffic was encrypted wit ENIGMA machines and settings that were changed daily. In Bletchley Park, they had a nifty machine (or many) that could go trough all possible settings (usually within a few hours), so it could find the correct setting for the day. After that, decrypting all German radio traffic for the given day was trivial. Now the 1000$ question: How did the code-breakers identify the correctly decrypted text among the zillions of garbled attempts? Did they have the possibility to check for sequences that usually appeared in a transmission? IIRC, there actually were attempts to get the Germans to send specific texts, but my memory is a bit hazy there. Possibly that's only from a novel. For example, if five enemy bombers are observed dropping mines in a specific location, the British could guess what the German radio operator would report. But I would be surprised if that analysis could have been automated back in WW2. So, how did they do it? Felix
From: robertwessel2 on 18 Nov 2009 23:49 On Nov 18, 5:35 pm, Felix Reuthner <s...(a)reuthner.net> wrote: > Hi, > a question about code-breaking in WWII: > Correct me If I'm wrong, but basically it worked like that: German radio > traffic was encrypted wit ENIGMA machines and settings that were changed > daily. In Bletchley Park, they had a nifty machine (or many) that could > go trough all possible settings (usually within a few hours), so it > could find the correct setting for the day. After that, decrypting all > German radio traffic for the given day was trivial. > > Now the 1000$ question: How did the code-breakers identify the correctly > decrypted text among the zillions of garbled attempts? Did they have the > possibility to check for sequences that usually appeared in a > transmission? IIRC, there actually were attempts to get the Germans to > send specific texts, but my memory is a bit hazy there. Possibly that's > only from a novel. For example, if five enemy bombers are observed > dropping mines in a specific location, the British could guess what the > German radio operator would report. But I would be surprised if that > analysis could have been automated back in WW2. > So, how did they do it? In fact, most of the Enigma decrypts were some form of known plaintext attack. These texts were often called "cribs" and were collected from a variety of sources, including regular broadcasts from weather stations, assumptions about routine status messages ("nothing going on"), as well as deliberate efforts to get a known message sent (for example the deliberate mining of certain locations), and very extensive traffic analysis. In some cases the plaintext came from messages that were found to have been sent more than once - once in a low grade code (for example weather reports). They used machines ("Bombes") to step through vast numbers of combinations, combining cryptanalysis and brute force. The code breaking effort was a vast effort, and had considerable resources. The Wikipedia article is a decent summary: http://en.wikipedia.org/wiki/Cryptanalysis_of_the_Enigma http://en.wikipedia.org/wiki/Bombe
From: Gordon Burditt on 18 Nov 2009 23:50 >a question about code-breaking in WWII: >Correct me If I'm wrong, but basically it worked like that: German radio >traffic was encrypted wit ENIGMA machines and settings that were changed >daily. In Bletchley Park, they had a nifty machine (or many) that could >go trough all possible settings (usually within a few hours), so it >could find the correct setting for the day. After that, decrypting all >German radio traffic for the given day was trivial. > >Now the 1000$ question: How did the code-breakers identify the correctly >decrypted text among the zillions of garbled attempts? Did they have the >possibility to check for sequences that usually appeared in a >transmission? IIRC, there actually were attempts to get the Germans to >send specific texts, but my memory is a bit hazy there. Possibly that's >only from a novel. For example, if five enemy bombers are observed >dropping mines in a specific location, the British could guess what the >German radio operator would report. But I would be surprised if that >analysis could have been automated back in WW2. >So, how did they do it? Boilerplate in messages is one way to verify correct decryption, at least for a first pass. Anything that looked real after this could be looked at by humans. For example, if I were trying to decrypt email messages, I'd look for strings like: "\nFrom: " "\nTo: " "\nSubject: " "\nDate: " in the first few hundred bytes of the message. Further, you can guess a fair portion of the Date: header, and there will be common repetitions of sources and destinations of messages. You might also get used to the format of messages sent, for example, a message sent at 5PM from a particular station on a particular frequency might always have strings like: Subject: Daily Weather Report and another station might usually have: From: Col. Wilhelm Klink, Commandant, Stalag 13 To: General Berkhalter Subject: My Perfect No-Escape Record in the headers for about half of the messages. There might be standard headers indicating the origin of the message is the central command. Long-winded but standard ways of saying "This message is classified top secret" might also be useful bits of known plaintext. It is also possible (I'm not familiar with World War II message traffic) that every message had "Heil Hitler" after the header. That could be a big weakness. I have heard about a few attempts to get the Germans and/or Japanese to send particular messages, often to break a code-within-a-code. Things like map coordinates, bases, or unit designations might be coded this way. For example, the plaintext might refer to Airbase Delta Gamma, but you don't know which airbase that is, so you pick one, overfly it, and see which airbase is reporting spotting enemy planes. I seem to recall identification of a code for a particular Japanese base/island in the Pacific was made by doing something to the water tower on the island suspected to be the one in question, then noting that it reported a water shortage. I think I heard this on a PBS documentary.
From: wjhopwood on 19 Nov 2009 00:43 On Nov 18, 6:35 pm, Felix Reuthner wrote: > Hi, > a question about code-breaking in WWII: >..... How did the code-breakers identify the correctly > decrypted text among the zillions of garbled attempts? Simply put, when the text of the message made sense they knew they had broken it. > Did they have the possibility to check for sequences > that usually appeared in a transmission? In essence, yes. Successful decoding depended on finding the correct key to the Enigma wheel settings for that message and probably all the other messages transmitted that day.. Finding the key rested in large part on a certain amount of tedious trial and error as well as guesswork coupled with having the known identity of the sender and the addressee. >From this information, an intelligent assumption could follow as to what might constitute the basic theme of the message and possible words and phrases which might be used within that theme. Frequently such sequences might be found at the beginning and end of the message, the portions which would contain the name(s) of the addressee(s) and of the sender. These might be already known through signal intelligence which, from call signs, direction finding, and traffic analysis produced fairly accurate information regarding the identity of the sender as well as to whom the message was being sent. German operators sometimes got careless by using stock words and phrases in the opening and closing sentences of the message, those buffer phrases unrelated to the basic text and inserted in an attempt to decoy enemy decoders. Such opening phrases as "Heil Hitler" and the like were often found in such locations and were of considerable aid to decoders. Another decoding aid was an idiosyncracy of the Enigma machine which kept it from ever substituting the same letter for itself. (If the letter "H" was used in the real text it would never come out an "H" in the coded text and the same with all the other letters) This was of considerable assistance to the code breakers. WJH
From: Rich Rostrom on 19 Nov 2009 10:23
On Nov 18, 10:50 pm, gordonb.4t...(a)burditt.org (Gordon Burditt) wrote: > I seem to recall identification of a code for a particular Japanese > base/island in the Pacific was made by doing something to the water > tower on the island suspected to be the one in question, then noting > that it reported a water shortage. I think I heard this on a PBS > documentary. The incident you are thinking of took place before the battle of Midway. U.S. codebreakers were at that time reading much of the Japanese military cipher traffic, and the intelligence analysts believed they had uncovered plans for an attack on Midway. However, the messaged did not refer to Midway by name, but to the code name "AO". To make certain that "AO" stood for Midway, the Navy had Midway report that its water distillation plant had broken down. This report was sent by radio "in clear", and was picked up by the Japanese. Shortly afterwards, a Japanese dispatch stated that "AO" was short of fresh water. Bingo! (The message directing Midway to make this false report was sent via the telegraph cable from Hawaii to Midway, which was intact and of couse secure. BTW, it was part of the trans-Pacific telegraph cable, and the next link - from Midway to Wake Island - was also intact, even though Wake had been occupied by the Japanese. Of course this cable went unused during the war, except for an occasional obscenity or insult.) |