From: Jim choate <ravage@wixer.bga.com>
Date: Tue, 30 Nov 93 12:07:43 PST
To: tcmay@netcom.com (Timothy C. May)
Subject: Re: Statistics of Low-Order Bits in Images
In-Reply-To: <199311301914.LAA27327@mail.netcom.com>
Message-ID: <9311301953.AA26009@wixer>
MIME-Version: 1.0
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Some other factors one needs to consider when analyzing images are:

* The lsb is going to be random if the image comes from any kind of a/d
  process. This is because all convertors have a error of +/- 1 bit.

* If an image is processed by software then the last significant bit will be
  on or off consistantly for a given color.

* By the use of exlusive-or operators it is possible to mask the data in a
  suitable manner. This does require that both sender and reciever have a
  copy or method of calculating the correct key.

The process as I understand it now to use a graphical concealment cypher is:

1. Both parties agree a priori on at least a way to generate suitable keys
   for masking.

2. You take your message and x-or it with a suitable bit stream.

3. The resultant x-or bit stream is placed in the image as the lsb.

4. Upon receipt, you strip out the x-or bit stream.

5. Generate the appropriate key and use exclusive-or to return the data
   stream.

The real question is how do you generate keys?

I see a method using the data bit stream itself as part of the answer. The
algorithm would look at some parameter like character statistics or entropy
of a text. It would look at the data stream and calculate its value. Then the
hard part is to try various bit streams and measure their x-or value. The
process is similar to Newtons Iterative Method for finding roots of
polynomials.