From: Bill Stewart <stewarts@ix.netcom.com>
Date: Sun, 10 Mar 1996 21:55:41 +0800
To: cypherpunks@toad.com
Subject: Re: Steganography idea: CU-SeeMe
Message-ID: <199603070739.XAA23010@ix2.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


At 08:28 AM 3/6/96 +0000, Ed Carp <erc@dal1820.computek.net> wrote:
>On Wed, 6 Mar 1996, Nelson Minar wrote:
>> So here's one idea I've had as a place to hide a channel: network
>> video, in particular CU-SeeMe video streams. CU-SeeMe is a lowtech
>
>I think it likely that people will be sending GIFs and JPEGs to each 
>other far more often than video.  Video is far more an "uncommon 
>communications channel" than is a uuencoded picture.

The Quickcam videocameras are $100; they're the next toy to buy after
you've got the Soundblaster, CDRom, and 100MB Zip drive.  The Macintosh
folks will buy them first (:-), but pretty soon they'll be very common,
especially for business LAN users to have desktop video.

The problem is that, unlike GIFs, CU-SeeMe and other low-bit-rate video 
compression schemes are _very_ lossy.  You can't just put the stego bits
in the raw image before running the compression, because they'll get lost,
and if you try to fit them in to the compressed image, you've got to do it
very carefully or they'll really drastically affect the image.

I don't know CU-SeeMe's compression algorithm, but imagine putting bits
into a run-length-encoded file: if you put them in the run-length,
you'll change the lengths of the runs a lot.  If you put them in the color,
you'll get lots of streaks of random-looking noise.  Either way,
it's quite noticeable.  So you've got to work very carefully with the algorithm.

The other problem with CU-SeeMe is dropped frames, which someone mentioned.
That's easier; any email system, especially a stego type, has to deal
with lost message.  So try to fit your messages into single CUSM frames,
and build some tracking or ack messages or maybe message numbering,
so you can detect lossage and request retransmits.  It can be as complex
as X.25-over-stego-over-CU-SeeMe, or as simple as "Message 14 of 32, RSVP".
(If you _do_ implement X.25-over-stego-over-CU-SeeMe, you've got a fine
April 1 RFC :-)
#--
#			Thanks;  Bill
# Bill Stewart, stewarts@ix.netcom.com, +1-415-442-2215 pager 408-787-1281
#