From: daleh@ix.netcom.com (Dale Harrison (AEGIS))
Date: Thu, 12 Jan 95 14:34:47 PST
To: cypherpunks@toad.com
Subject: Re: How do I know if its encrypted?
Message-ID: <199501122233.OAA02325@ix3.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


You wrote: 

>You don't always need the correct answer.  You just need the correct
>answer most of the time.  You're trying to create a presumption about
>behavior.  Ensuring that you can't read almost all of the traffic is a
>pretty good way to assure people that you don't try to make sense of
>any of it.


I'm still not making my point.  Encryption is not a data 'state' that can 
be tested for in the way that liquid/solid/vapour are states 
of matter.  Encryption is a data 'interpretation' for which there are an 
arbitrairly large number of interpretations available for any 
given dataset. There is no algorithmic test that can applied to a dataset 
that will be able to establish the existance or non-existance 
of a given interpretation.  If you're given an unkown dataset and are 
asked the question, 'Is this an image file' or 'Is this an encrypted 
file' or 'Is this an audio clip', there is no algorithmic test that can 
answer any of these questions in either the affermative or the negative. 
 This is just an alternate phrasing of Goedel's Undecidability Theorem.  
This problem lies permanantly outside the outer boundary of algorithmic 
capability.

Let me now spin a little tale as to how this affects an operator of a 
re-mailer or datahaven.  This tale has two characters, Paco the child 
pornogropher and Eric the honest RM/DH operater.  Paco begins by 
inventing the new 'Foolproof Barometric Graphic Image Format' (aka FooBar 
GIF) of which only Paco knows the internals.  A FooBar GIF has a 
statistical profile that looks remarkably like a PGP file, in fact it 
even comes with a PGP header!  Nothing illegal here.  Now Paco writes a 
FooBar GIF Viewer which he sales to child-porn types. Again, there's 
nothing illegal about the sale of such a piece of software.  Paco now 
anonymously loads Eric's DH with lots of child-porn FooBar GIF's via 
Eric's anon-RM.  These files of course sail right through Eric's filters 
with nary a scratch.  As far as Eric knows he's holding PGP encrypt 
files.  Now Paco advertises the availability of lots of 'good' picture on 
Eric's DH that can be used with the newly purchased FooBar GIF Viewers.  

Then the cops get ahold of one of Paco's FooBar GIF Viewers and downloads 
some FooBar GIFs from Eric's DH and the last words we ever hear from Eric 
are "I swear I thought they were PGP files, I swear to God I thought they 
were....." as the authorities drag his carcass off to jail.  Whose going 
to believe Eric's protests of innocence?  "After all", says the 
prosecutor to the jury, "wasn't Eric explicitly filtering out what he 
didn't want in. If fact", says the prosecutor, "his filter seems to have 
been designed specifically to allow these kiddie-porn files in and to 
reject all others."  A sad end indeed!

Dale H.