Noob Question about ENIGMA
in [Cryptography]
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From: Gerard Bok on 19 Nov 2009 11:12 On Wed, 18 Nov 2009 18:35:33 -0500, Felix Reuthner <spam(a)reuthner.net> wrote: >a question about code-breaking in WWII: >Correct me If I'm wrong, but basically it worked like that: http://www.enigmahistory.org/enigma.html -- met vriendelijke groet, Gerard Bok
From: Artie Choke on 19 Nov 2009 11:12 On Nov 19, 5:43 am, "wjhopw...(a)aol.com" <wjhopw...(a)aol.com> wrote: > 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 theEnigmawheel settings > for that message and probably all the other messages > transmitted that day.. > Something nobody has mentioned yet is the "indicator". The operator "randomly" chose a particular 3-letter message key which was supposed be different for each message (though often as not it was his girlfriend's initials or something equally non-random!) He then enciphered the key twice using the standard settings for the day, before turning the wheels to the key value and enciphering the rest of the message. The fact that the message key was enciphered _twice_ introduced a pattern which leaked some information about the daily settings. This was used in some of the early Polish cryptanalysis. Lots more here: http://en.wikipedia.org/wiki/Cryptanalysis_of_the_Enigma
From: Rich Rostrom on 19 Nov 2009 12:05 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)... These were called the "bombes". Some were at Bletchley Park, but most were elsewhere, at sites linked to BP by teleprinters. After the U.S. entered the war, U.S. resources were added to the fight. U.S. analysts went to work at BP, and the U.S. built and operated a "fleet" of additional bombes. (Mostly to attack Kriegsmarine traffic, which was often intercepted in the U.S.) The U.S. bombes were based on the British design, but incorporated some improvements of their own. > 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. This leaves out a lot of details. For one thing, the Germans had separate Enigma keys for each service. The settings used at any given moment by the Luftwaffe, Heer, and Kriegsmarine were different. As the war continued, additional separate keys were established for branches of service and theaters of operation. For instance, the Kriegsmarine keys included HYDRA - general navy operations TETIS - U-boat training command TRITON - U-boat operations SUD - Mediterranean operations MEDUSA - Mediterranean operations NEPTUN - battleships, pocket battleships, and cruisers and several others I can't think of right now. Thus, breaking an Enigma key would allow BP to read all traffic on that key for that day - but only that key. (And late in the war, some keys were changed up to three times a day!) The other important point was that the bombes were not available at all until mid-1941. By 1942, there were a few dozen working, but far more were needed - the "fleet" ultimately numbered in the hundreds, including bombes built and operated by the U.S. Thus, for the first half of the war, the codebreakers had to use other methods. The original method, which had been developed by the Poles, attacked a weakness in the German message format. The first six letters of the cipher text in a message were a three letter group that was repeated. The Poles had also deduced the wiring of the scrambler wheels of the German Enigma. They had further realized that it was not always possible for an Enigma to cipher a given letter to the same cipher value three positions later in the same message. If, in the ciphertext of that six-letter preamble, a letter occurred in positions 1 and 4, or 2 and 5, or 3 and 6, that would rule out some Enigma settings. Twenty or so messages with such repeats could pin down the exact setting. The Allies (Britain, France, and Poles in exile) used this method to break into Enigma in early 1940; they read several thousand Enigma message. Then in May, the Germans changed their format, dropping that three letter repetition. The Allies (after June 1940, the British) kept going through other tricks, all based on German sloppiness: Sillies, the Herivel Tip, and Parkerismus. > 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? Yes. The entire system of breaking Enigma through bombes depended on having some knowledge of what the text of some message on a key was. Such guessed or known message texts were called cribs. For instance, Enigma could never cipher a letter to itself. If the analyst thought a particular text occurred somewere in a message, he could line up the crib text with the ciphertext and see if any letters matched, shifting the crib text till there were no matches. There were other analytical tricks used. The procedure involved setting up a bombe with a cleartext, a ciphertext, and a starting position of the scrambler wheels. The bombe would tick through the possible combinations, stopping if at any point the ciphertext "came through". The bombes were operated by a small army (over 2,000 by 1945) of "Wrens" (Women's Royal Navy Service). Now, where did the analysts get their cribs? If one had been reading the traffic on a particular key on Monday, Tuesday, and Wednesday, it was usually possible to guess what would appear in messages sent on Thursday. The names and titles of message senders and recipients were often useful, and in German such phrases could be quite long ("STURMBANNFUHRER VONDEMBACHZELEWSKI", "GENERAL DER PANZERTRUPPE VONSCHWEPPENBURG"). However, these assigments could change frequently, and German operators were taught to scatter a few random Xs into such phrases. Stereotyped reports were also useful. BP became quite fond of a German officer at an observation post in the impassable Qattara Depression, who reported every day that he had nothing to report. The phrase "AN IDA BISON" was a very common crib. It represented "A1B", the designation of the staff officer for intelligence at a particular HQ. Yet another source of cribs was the retransmission of messages on different keys. Sometimes the message was sent with Enigma and also some lower-grade system; or a message sent on a broken Enigma key was resent unchanged on a different key. The Germans were very sloppy about this. However, it was not always easy for the British to use this. Sometimes, to break one key with very "hot" traffic, it was necessary first to break another key, which might have nothing of interest itself, but would provide a crib on the other key. Continuity was also extremely important; the analysts needed to read a key every day, whether there was anything valuable on it or not, to be able to read it later on if it became "hot". In one area, the British had a huge advantage. This was the traffic of the Abwehr (German espionage service). Much of the Abwehr's message traffic was reports from or orders to their agents in Britain - who were all double agents under British control. So of course the British had a wealth of cribs for that key. > IIRC, there actually were attempts to get the Germans to > send specific texts... For Abwehr Enigma, planting cribs was trivial. If the British passed on some apparently "hot" secret document, the Abwehr could not resist transmitting the original text to Berlin to show off with. > 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. The British did this regularly; the practice was called "gardening".
From: Don Phillipson on 20 Nov 2009 17:17 "Felix Reuthner" <spam(a)reuthner.net> wrote in message news:he1vt7$5tk$1(a)online.de... > . . . How did the code-breakers identify the correctly > decrypted text among the zillions of garbled attempts? Did they have the > possibility to check for sequences that usually appeared in a > transmission? Methods included: 1. Brute force (all possible combinations), facilitated by the "bombes" (programmable electro-mechanical machines.) 2. Exclusion of negative possibilities: e.g. most ciphers include (Rule A) that no letter P may be enciphered as itself = P (Rule B) that no reciprocal Q=R may occur elsewhere as R=Q These two rules function negatively in any deciphering programme: i.e. any setting which permits non-A or non-B can be skipped as a wrong setting: and brute force methods may usefully winnow out and discard such non-rule keys. 3. Enemy breaches of good cipher practice, e.g. replicating standard terminology in short routine messages (such as weather reports), e.g. using the same word or the same number of nonsense characters as filler material to pad out a message to standard length. For details see only recent books such as: Sebag-Montefiore, Enigma: the Battle for the Code (2000) Simon Singh, The Code Book (1999) Calvocoressi, Top Secret Ultra (1985) Earlier books (e.g. by Kahn, Lewin, Winterbotham) do not really answer your question (i.e. were perhaps censored.) -- Don Phillipson Carlsbad Springs (Ottawa, Canada)
From: Greg Rose on 20 Nov 2009 19:53
In article <he73s5$ees$1(a)theodyn.ncf.ca>, Don Phillipson <e925(a)SPAMBLOCK.ncf.ca> wrote: >[...] >2. Exclusion of negative possibilities: e.g. most ciphers include >(Rule A) that no letter P may be enciphered as itself = P This is certainly true of the Enigma, but is certainly *not* true of "most ciphers". In fact it would be considered a serious weakness in any modern cipher. >(Rule B) that no reciprocal Q=R may occur elsewhere as R=Q Again, the enigma actually enforces this; at a given place in the operation, if plaintext R would be enciphered to ciphertext Q, it is certainly the case that plaintext Q would be enciphered as R. In fact, that is exactly how the operators decrypted! As for your "elsewhere", I don't know quite what you mean; at other points in the encryption process, it is entirely likely that this might happen, basically by chance. Greg. -- Greg Rose 232B EC8F 44C6 C853 D68F E107 E6BF CD2F 1081 A37C |