John A wrote:
On Sun, 23 Sep 2012 22:37:01 +0200, Wolfgang Weisselberg
Eric Stevens wrote:

matter. The question is, can anyone alter the image in such a way that
the fact that it has been altered is indetectable?

Of course someone can.

Change a single pixel in such a way that it's still within it's
typical statistic value range for the true image. Don't do
anything except exchanging the single JPEG block that contains
the pixel (write your own program to do it or do it by hand).

Presto: even having a second photo (with pixel-exact registration)
of the same scene and access to the same scene you cannot detect
the manipulation.

(Of course, the manipulation will not be relevant.)

You can embed a message that way, using the least-significant bits to
encode it.

http://en.wikipedia.org/wiki/Steganography

Yep --- and if there's then any sort of pattern in the LSB, your
hiding there has failed. Note: Often an encryption or compression
output contains something like a header ...

-Wolfgang