From: "Chris Adams" <adamsc@io-online.com>
Date: Fri, 7 Jun 1996 19:54:55 +0800
To: "cypherpunks" <cypherpunks@toad.com>
Subject: Re: Where does your data want to go today?
Message-ID: <199606070509.WAA20975@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


On 30 May 1996 07:56:26 pdt, EALLENSMITH@ocelot.Rutgers.EDU wrote:

>From:	IN%"mclow@owl.csusm.edu"  "Marshall Clow" 29-MAY-1996 19:18:31.60
>
>>* Why encrypt before compression? If the encryption is any good, then the
>>encrypted data won't compress much at all. However, compression before
>>encryption has its own problems.
>
>	What problems does compression before encryption have? It at least
>seems to work for PGP.


The most significant would be the presence of a compression header (what
has the data 
telling how to decompress a datastream). If you leave this in (for
instance, assuming you 
were to use PKZIP (STUPID!) you would have PK as the first two bytes
EVERY TIME!) you give 
your codebreaker a HUGE advantage. You can bypass this by removing ANY
information which 
isn't message specific and standardizing on one compression method.
Alternately, you could 
use your session key to encrypt a length word and then pad the start (and
end) with a 
string of random characters.

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