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From: davids@cit-vax.Caltech.Edu (David Schweizer)
Newsgroups: net.crypt
Subject: Re: Code Breaking
Message-ID: <447@cit-vax.Caltech.Edu>
Date: Wed, 7-May-86 04:50:16 EDT
Article-I.D.: cit-vax.447
Posted: Wed May  7 04:50:16 1986
Date-Received: Sat, 10-May-86 12:29:35 EDT
References: <113@radha.UUCP> <13436@ucbvax.BERKELEY.EDU>
Reply-To: davids@cit-vax.UUCP (David Schweizer)
Organization: California Institute of Technology
Lines: 44

Organization : California Institute of Technology
Keywords: 

In article <13436@ucbvax.BERKELEY.EDU> desj@brahms.UUCP (David desJardins)
writes:
>In article <113@radha.UUCP> sanand@radha.UUCP (Sanand Patel) writes:
>>[naive mode]
>>
>>plaintext--> Some very important text that must be kept secret
>>My Key ----> ThisIsMySecretKeyThisIsMySecretKeyThisIsMySecretK
>>Coded-------> Exclusive-Or of S+T, o+h, m+i etc.
>>
>>Now, not withstanding how the key is passed, is the above scheme breakable,
>>especially with very large keys (say 100 or 200 letters) ?
>
>   Yes.  This is *extremely* easy to break, with only a very moderate amount
>of ciphertext, and *no* plaintext.  
>[more crypanalytic remarks]
>   No offense intended, but cryptography is a real field.  Why would anyone
>bother with complicated encryption algorithms if trivial things like this
>would do?  I suggest an elementary book on cryptography and cryptanalysis;
>I don't happen to know which ones are good, but maybe someone can suggest one?
>
>   -- David desJardins

Start with Kahn, of course, as other people have suggested.  Then, before
going on to recent Computer Science oriented texts on cryptography, you
might want to learn to break such ciphers *by hand*.  My favorite
introductory texts on cryptanalysis are:

Abraham Sinkov
Elementary Cryptanalysis   A Mathematical Approach
Mathematical Association of America, New Mathematical Library #22  (1966)

Helen Fouche Gaines
Cryptanalysis   A Study of Ciphers and Their Solution
Dover  (1956)

Both discuss polyalphabetic substitution.  The original copyright on 
Gaines' book is 1939, well before modern electronic aids.
-- 
                               --David Schweizer
ARPA:  davids@csvax.caltech.edu
UUCP:  ...!seismo!cit-vax!davids