From: "Logan Shaw" <[EMAIL PROTECTED]>
On Mon, 31 Jul 2006, jdow wrote:
Break the image into pieces. If too many pieces match on MD5 sum then
you score it higher than if lots of the image is different. But that
can get tedious to say the least.
And there's also an easy way around it. Simply add noise to
the image. There are a number of techniques, but an obvious
one to use with GIF is to assign two palette entries to
two nearly (but not quite) identical colors. For example,
put 0xffffff and 0xfffeff in your palette. Then, for every
white pixel in the original image, choose at random whether it
gets represented by a 0xffffff or 0xfffeff pixel. There will
be virtually no discernable difference to the eye, but the
files will completely different, especially since GIF uses
LZW compression on the pixel data.
There are similar methods for other formats: with JPEG, you
can just change the quality settings, causing the JPEG decoder
itself to add noise to your image. (And perfectly legit noise,
too, since the quality parameters vary on legit images.)
And of course you can just add noise to the least significant
bit in any generic format as well.
Yup, steganography with random data. Of course, you could feed them
to the FBI and say you suspect this is steganographic terrorist
planning or something. I betcha they or the CIA can find the source
of the spam if they buy into that idea....
{^_-}
