From: lwloen@rchland.vnet.ibm.com (Larry Loen)
Subject: Re: Attack Methods
Date: Thu, 19 Nov 1992 17:31:02 GMT
Reply-To: lwloen@vnet.ibm.com
Disclaimer: This posting represents the poster's views, not necessarily those of IBM
Organization: IBM Rochester

In article <1992Nov18.224350.11512@cis.uab.edu> Kenneth Sloan writes:
 [long series of observations showing he understood my previous posting
 to his posting about transpositions]

>My other point was - the padding scheme relies on the receiver being
>able to recognize the intended message.  You point out that this annoys
>people.  To get around this, I proposed that the padding be chosen so
>that no fragment of the padding formed a word in the lexicon - so that
>an automatic dictionary lookup could extract the words in the message
>from the jumble - this eliminates all whitespace, and eliminates the
>need for a human to look at the full padded text.  I thought that an
>advantage of this scheme might be that message text would be more evenly
>distributed in the (untransposed) test.  I asked if this would be
>counterbalanced by the "common lexicon" providing too much of a lever
>for the opponent.  

>I gather from your one-line reply to this point that the restricted
>lexicon simply makes multiple anagramming easier.  Is this still true if
>the padding has the same statistics as the message - and even
>intentional misleading matches?   It appears so - even matches in the
>padding tend to reveal bits of the transposition...oh well.

First, my appologies for getting your name wrong in my prior post.  I
don't know how I did it, but I appologise.  

On to the analysis.

Well, there is an aliasing kind of problem.  Just as in real text,
there are multiple occurances of "the".  However, if you have three or
four or five messages of the same length, the other fragments won't
fall on nice boundaries and this can actually help out.  It is harder,
to be sure, but you do eventually win.  We are probably not even 
talking about double the effort, however.  I've done a little of this
in my time and find even two or three texts work out pretty well once
you get that first correct fragment put together.

In short, the padding may slow you down.  But, not enough to notice.  It
_is_ nice to make it algorithmic and so not burden the legitimate users.
And, a restricted lexicon is indeed easier because there are more "the"
type words to find.

>But...distributing the message differently within the padded blocks
>makes it less likely that "common 3 letter combinations" will fall in
>the same place in 2 different messages.  Does this matter?

No, it doesn't really affect the probabilities much.  Think about it.
Take any random fragment of this note and take a random fragment of
another.  Whether you start at the beginning, the middle, cut and
paste a few paragraphs first, it will merely change "which" common
three letter groups line up.  Besides, you can actually start with
two letter groups if you wish.  That's a different set of trade-offs,
but you can't optimize against how large a group the attacker chooses.

>===================
>
>Finally (I promise, I'll go away after this) - is multiple anagramming
>bothered by schemes which re-write the plain text in shorthand-style
>alphabets designed so that the second-order statistics are flattened?
>Perhaps this is too much of a tangent from the original point - if so,
>sorry.

If I understand the point, we are departing from pure transposition and
beginning to introduce a little substitution.  If the shorthand is
a fixed, user-driven function, then all that has to be known is how the
shorthand works.  If it isn't part of the "key" (that is, does not vary),
it is no different in practice than learning that English rather than
French is used.  On the other hand, if the shorthand is somehow varied
by some sort of key, we have left transposition ciphers behind, albeit
slightly.

Remember, too, that sometimes a partial break is sufficient, so only a
little bit of shorthand may make no practical difference.
 

>Thanks for the reply - I learned from it.

You are quite welcome.

-- 
   Larry W. Loen        |  My Opinions are decidedly my own, so please
                        |  do not attribute them to my employer
Newsgroups: sci.crypt