From: Karl Lui Barrus <klbarrus@owlnet.rice.edu>
Date: Fri, 22 Oct 93 16:03:00 PDT
To: cypherpunks@toad.com
Subject: Re: Subliminal Channels
In-Reply-To: <9310222041.AA00922@anon.penet.fi>
Message-ID: <9310222300.AA17989@flammulated.owlnet.rice.edu>
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wonderer wrote:

>Is there any published work on hiding subliminal messages
>in pictures? It seems like that would be really easy.

Someone did write a program which embeds information in a JPEG, but I
don't have any other info (like where to get it, who wrote it, etc.
:-)

Actually though, embedding a message in a picture leans toward
steganography.

Subliminal channels, covert channels, and steganography are related to
one another, and seem to overlap.  I don't have precise definitions
(does anybody else?).  From reading, here is what I think these three
methods are:

Covert channel: you and I agree that if I mail you a letter with the
stamp affixed sideways, that messageA is communicated.  Or, we agree
that a post sent to alt.test with the subject "el mariachi" conveys
messageA.  Or that my running a process and killing before 9:30 p.m.
conveys messageA. 

Subliminal channel: you and I share a secret and convey our message in
the digital signature of innocent message we pass to one another.

Steganography: I write an innocent message, and I change the spacing
between words, to convey messageA.  Or, I embed the message in the low
order bits of a picture or a sound file.

So, it looks like that in subliminal channels and steganography, the
actual message is passed, while covert channels don't (it's like
winking at a friend - no message but lots of information).

Also, it seems that steganography alters the innocent message itself,
while a subliminal channel doesn't.

Obviously, there is great room for hair-splitting.  These are the
differences as I understand them!  Any other input?

-- 
Karl L. Barrus: klbarrus@owlnet.rice.edu         
keyID: 5AD633 hash: D1 59 9D 48 72 E9 19 D5  3D F3 93 7E 81 B5 CC 32 

"One man's mnemonic is another man's cryptography" 
  - my compilers prof discussing file naming in public directories