GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: Basic Level

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So I pick things up pretty easily, especially on things like GIMP. However, I really want to do steganography on this sense I find it so fascinating and also it's for a project. May I get basic instructions on how to do it? It would help me so much.

What do you want to hide? An image? Some text?

There is the hide / unhide plugin see: viewtopic.php?f=9&t=3659&p=45631#p45631
for some reason, this is included in the (linux) development Gimp 2.9.1

But that is not the only way, I was once very interested in encryption including its history. Nothing is new, Steganography is just about concealing a message rather than encrypting it. In ancient Greece, a messengers head was shaved, message tattoo-ed on, hair grows back, messenger sent off, head shaved at other end, message read.


An old method uses indexed images, and modifies the colormap

What is the hidden word here:


This is a really easy example, the clues are there.

@Ofnuts I know it is, I did say really easy.

Who else then?

This is another very simple technique ..very easy to un-hide (when you know how! ).

This looks like just yet-another-copy of that well-known Standard Test Image, the Mandrill (available here: http://sipi.usc.edu/database/database.php). But it has been slightly altered ..the message lies in the difference.

In this one the secret is not in the difference...

For First Time I got it straight away, harder, maybe

You are indeed fast at finding tricks

I got the color map and first mandrill examples (still working on second mandrill, but if For First Time means what I think it does, I'm going to have to find a new tool). I can't extract anything from answer.png, however, it is indexed but the color map doesn't appear to have and encoded information. Am I missing something or did the Up/Download process damage answer.png?

No, answer.png is fine (checked on a re-downloaded version). It is only a slightly harder variant of what it answers to.

The colormap editing method, is rarely so trivial. A more usual case is a 256 colormap with very similar colours.
Try this one. http://www.eclectic-penguin.org.uk/cach ... ection.gif

One way to proceed with these (apart from trial and error) is to dump the colormap as a hex table and either search or sort to find matches.

I thought jontait2 diff method produced a really good outcome, although it is best with a 'busy' image. For Jazz2 assembling a project, worth an expanded mention.

Any other methods? edit: tacking a header or a bit on the end of an image does not count

That sounds like the hard way of doing it!
You can usually expose any colourmap redundancy (which is what this method is using) by simply swapping the colourmap (ie. Colours> Map> Set colourmap...) for something completely different.

For images with a small colourmap (up to 16 colours) such as the images posted by Rich and Ofnuts above, simply setting the colourmap to the standard "EGA" should be enough to show you what's there (if anything). Of course, the colours will all be wrong, but that doesn't really matter.

For images with larger colourmaps (up to 256 colours), try "EGA" first, then also try the standard "Gold" colourmap - that will usually expose anything not shown by "EGA" because the "Gold" colourmap is in fact just a very gradual gradient from pale-yellow to near-black, and since the colourmap actually used in indexed images is usually nothing like a very gradual gradient, the redunancy will stand out.
You can also make your own completely-different-yet-again colourmap, eg. using the "plasma" texture generator:
File> New> 400x400
Filters> Render> Clouds> Plasma... (Random seed = 0)
Image> Translate into palette... (save as "Plasma")
This will generate a new palette (which you can then use as a colourmap) with lots of bright colours but in a completely random order, quite different from the standard colourmaps (such as "EGA" and "Gold").
So, if "EGA" and "Gold" don't expose anything, "Plasma" probably will.

(For a more comprehensive technique, rather than manually searching a hex dump of the colourmap, I would suggest writing a Python script to highlight any redundancy.)

Don't have to get too serious, and there might be times when you want to keep the original image as-is as well as the message.

Certainly, and the quick technique I outlined above will show you if and where there is any redundancy which might be a hidden message/image ..if you want to expose the hidden content but preserve the rest of the image, use the colour-picker tool to sample the hidden stuff (the info window will show you the colourmap index number of the hidden stuff) and then just change that/those values in the colourmap of a fresh copy of the original image (using the Colourmap dialog).

One could think out of the box to create hidden messages.