decomphose-325-points.md

Decomphose - 325 points

Image arithmetic is super neat until there's more than two images involved.

file 1

file 2

file 3

file 4

Solution

To begin, I decompressed file 1 and noticed that it contained images with seemingly random pixel noise:

Upon closer inspection, some pixels seem to be surrounded on all sides by black pixels:

The other three folders contained similar images.

Perhaps if I could isolate those bordered pixels from each image and overlay them, I could find some clue.

I opened the first photo in GIMP (a free, open-source photo editing application).

After a bit of mucking around trying to find a way to isolate those pixels, I figured out a method:

1. Set black pixels transparent

Do Layers > Transparency > Color to Alpha... and set the color selection to #000000:

I obtained this:

2. Fill noisy pixels

Select the paint-bucket tool and set the threshold to 200 and the color to #000000.

Then click on any of the noisy pixels and they will all turn black. (the paint bucket won't cross alpha borders)

3. Remove noisy pixels

Now we can simply repeat step 1 again to remove the noisy pixels. (There are a few artifacts but it won't matter)

Combine all the images

In order to combine all the images, I simply opened the images as layers in GIMP and repeated the process above for each layer.

Note: a handy shortcut is CMD-F or CTRL-F to repeat the last filter (eg. Color to Alpha)

My final image looked like this:

And zooming in on the flag, we obtain:

easyctf{wh4t_a_5weet_fFLag_2b04e1}

Writeup by Harrison Green

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