From: Bill Stewart <stewarts@ix.netcom.com>
Date: Fri, 1 Mar 1996 06:00:56 +0800
To: Bruce Zambini <jlasser@rwd.goucher.edu>
Subject: Re: Simpler solutions (was Re: Stealth PGP work)
Message-ID: <199602290844.AAA03851@ix4.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


At 10:26 AM 2/28/96 -0500, Bruce Zambini wrote:
>Or, you can develop a public-key stego system...
>ie a stego system that uses bits in specific ways depending on the 
>private key of the recipient.
I assume you mean the public key of the recipient?

>Something I've been thinking about, but haven't quite figured out how to 
>do yet, except that one could use certain bits based on a PRNG and begin 
>the message with an RSA-encrypted seed (ie the first X bits will be the 
>seed, encrypted with your public PGP key).

That's equivalent to 
   stego(picture, ( RSA(sessionkey,pubkey), Symmetric(message, sessionkey)))
where Symmetric is a potentially cheaper (and weaker) algorithm than IDEA
that possibly uses transposition rather than strict block-structuring.
This approach is no more obscure than stego(stealth(PGP(message, pubkey)))
though slightly more obscure than stego(PGP(message, pubkey)), optionally
less secure,
and probably not much faster except for long messages with wimpy Symmetric.
And it still suffers from the non-stealthiness of vanilla RSA that 
Hal Finney, Adam Back, Harry Hastur, and others have discussed.
Might as well keep the stego and encryption parts separate.

You gain a certain degree of obscurity by using
   stego(picture, scramble( PGP(message, pubkey), key ))
where scramble is some cheap symmetric encryption algorithm and 
key is either the recipient's public key or keyid, and PGP is in binary mode.
This hides the message from eavesdroppers who don't know the recipient,
but not from eavesdroppers who are willing to test against the keys
of a list of usual suspects (assuming the recipient is one of them.)

#--
#				Thanks;  Bill
# Bill Stewart, stewarts@ix.netcom.com / billstewart@attmail.com +1-415-442-2215
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