[rc5] Enigma and Purple (was Re: One Time Pad)

Eric Gindrup gindrup at okway.okstate.edu
Tue Oct 28 15:10:29 EST 1997


     Sure.  Where's your address?
     
        Enigma was broken initially through exhaustive (mostly manual) 
     search and then "human factors" were used to continue decoding.  In 
     particular, individual operators had habits in selecting "key phrases" 
     that could be used to (with very high accuracy) predict a new 
     encryption key.
        The exhaustive search was aided by the knowledge that the wheels 
     were "linear" in that each character was mapped to and from one and 
     only one character.  The other aid was that the set of wheels was 
     mostly fixed (except late in the war and on the U-Boats).
        Whenever the encryption key was changed to something unguessable, a 
     large brute force effort was (re-) started to extract the key.
        The Enigma code was more like a simple substitution code with a 
     highly alterable substitution rule.  This is why brute force attacks 
     were conceivable.  It's trouble was that an entire sequence of 
     (similarly encoded) messages made a good statistical sample from which 
     to apply letter frequency analysis and similar standard tools.
            -- Eric Gindrup ! gindrup at Okway.okstate.edu


______________________________ Reply Separator _________________________________
Subject: [rc5] Enigma and Purple (was Re: One Time Pad) 
Author:  <rc5 at llamas.net > at SMTP
Date:    1997/10/27 15:23


[snip]
     
This reminds me a lot of the German Enigma and Japanese Purple 
encryption schemes.  They used wheeels (4 or 5) which translated what 
one letter should be into another.  Then after each letter was encrypted 
they would move the code wheel to the next notch which would switch the 
translation tables for the characters.  This sounds like it would 
duplicate the otp if the code wheel combinations were longer than the 
message and if new code wheels were issued once they were used up
right?  Does anyone know the particulars on how those two systems worked 
and how we managed to crack them encryption scheme?  From what I 
remember we managed to crack  one of the systems before we actually got 
a hold of the code wheels.  Is this because in actuality the code wheel 
combinations weren't longer than the message or that the wheels weren't 
changed?
     
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