Weekly CTF 2013 Write-Up: 9/4 - 9/11

This is a write-up for the CSG CTF problem on: 9/4 - 9/11 - 10 points.

For starters, our hint is: A mysterious logo appears. </static/csg_logo7.png>

Visiting the page:

Since I have an image and the problem is only worth 10 points, I’m not expecting anything crazy, so I’ll run it through strings first. If you strings the file (Wikipedia, Strings (Unix cmd)), you see:
Strings output, csg_logo7.png

This isn’t much data, but it does have some Adobe ‘filler’, meaning this image was made or edited in an Adobe program (xmp:CreatorTool="Adobe Photoshop CS5 Windows" - to be exact). To be safe and not waste time, I ran it through an online ELA tool (Wikipedia, Error analysis), Online ELA Tool.

After playing with the “Error Scale” slider we get an artifact in the logo:
Online ELA, csg_logo7.png

You get the flag, key{YOU_FOUND_ME!}

Since the key was added by changing the Gamma values, this solution was also available by using a paint bucket tool on it, such as Microsoft Paint or Gimp: