On Sun, 16 Sep 2007 10:56:53 -0300, "Lucas Prado Melo" <[EMAIL PROTECTED]> 
> How can I identify (using gimp) if a photograph has been faked?

There are different techniques that can be applied.  Some of the focus
on detecting if the image has been modified (detecting suspicious
patterns in the pixels after loading the image) and some others focus
on analyzing the file, not the pixels.  You already got several
replies mentioning the first part (changes in the image), so let me
give you a few hints about the second part (changes in the file).

For example, if you have a JPEG image to analyze, then the first thing
you can do is to see if the image has been saved directly by a digital
camera or if it came from GIMP, Photoshop or any other image editing
software.  If the EXIF metadata tells you that the image was saved by
Photoshop, then you can already be sure that it did not come straight
out of the camera.  This does not necessarily mean that the contents
of the image have been modified, but at least you know that the image
may be different from what was taken by the camera.

And even if the JPEG file does not contain any EXIF block or if some
clever hacker has replaced the EXIF block by the original one from the
camera, then you can look at the JPEG quantization tables and check if
these match the tables that are used by some cameras, or if these are
the tables used by Photoshop or any other program.  This is a bit
harder to fake and most of those who create fake images and go through
the trouble of replacing the EXIF block ignore the fact that the
quantization tables in the JPEG file can betray them.  Note that I
recently added a feature in GIMP that allows you to preserve the
quantization tables from the original image, so that could in theory
be used to "improve" some forgeries.  However, there are other details
in the JPEG file layout can can reveal if the file came straight out
of the camera or if it was processed by some other software.  I do not
want to say too much about that because I do not want to give too many
ideas to the crooks, but let's say that the way some things are split
or ordered in the JPEG file is usually different between digital
cameras and image editing software.

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