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From: smb@ulysses.UUCP (Steven Bellovin)
Newsgroups: net.crypt
Subject: Re: Code Breaking
Message-ID: <1239@ulysses.UUCP>
Date: Mon, 28-Apr-86 22:02:13 EDT
Article-I.D.: ulysses.1239
Posted: Mon Apr 28 22:02:13 1986
Date-Received: Thu, 1-May-86 03:47:27 EDT
References: <113@radha.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 20

> plaintext--> Some very important text that must be kept secret
> My Key ----> ThisIsMySecretKeyThisIsMySecretKeyThisIsMySecretK
> 
> Now, not withstanding how the key is passed, is the above scheme breakable,
> especially with very large keys (say 100 or 200 letters) ?

As has been pointed out, this scheme is very breakable for keys as *short*
as 200 characters...  The first general solution was derived circa 1860,
by a Prussian named Kasicki; William Friedman devised a better solution
around 1920.  For security, the key must be comparable in length to the
message itself, and without any pattern.  In the case you cite, for example,
A cryptanalyst who had made a partial entry into the key could look for
key letters that made that phrase legal English.

By far the best introduction to cryptology is David Kahn's "The Codebreakers".
If you want technical details, be sure to get the hardcover edition; the
paperback is seriously abridged.  Another good introduction (though of course
much less comprehensive) is the December 1979 issue of ACM Computer Surveys.
For a detailed mathematical treatment, see Beker and Piper's book (whose title
escapes me at the moment).