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IceCTF 2018 - Picasso [Forensics]

Forensics – 1. Picasso
Author: 5ynax and Valrkey

Worth: $150

The challenge involved a GIF that we needed to extract a message from.

So, for this challenge, we had two ways that we solved it at almost the same time. We have the long (5ynax) way and we have the fast (valrkey) way.

The Long Way

In the long way, I decided to extract each frame of the automated GIF and then import them into GIMP to layer them on top of one another. I used an online tool to get all of the frames, there was a lot of them, I used After I got the frames split up, I downloaded them and moved them to my box with GIMP ready for analysis. I later learned that I could have just imported the GIF into GIMP directly using the open as layers routine, but that's neither here nor there.

For each layer there is an Alpha Channel. In GIMP under the colors menu, you can select the Color to Alpha Routine to get this box:

This allows you to choose a color and then set the Transparency or Opacity for the specified color. In this case, the background is white, so we target that color. I went through all frames and changed the Opacity to 0 and revealed this:

I changed the color to Grey because it was easier for me to see. Obviously, the IceCTF{wow_fast} flag was right there for us to take and enter for cash money.
The process works, it's just a bit more work than what valrkey did.

The Fast Way

I know nothing of photo layering and editing, so I tried to go about it a different way.

My first attempt was to speed up the GIF to a crazy high frequency so that the parts would blur together if you squinted your eyes just right. I used the same site as 5ynax to try to increase the speed but was unable to up the frame rates enough to see the intended flag.

My second attempt was to take a long exposure photo of the GIF. I downloaded a long-exposure camera app on my phone. Being the photography n00b I am, I was ending up with a nice white square until 5k33tz suggested inverting the colors. As a negative, the black background would be drawn over by the moving white lines. After tossing my phone in a table clamp for stability, the end result came out pretty clear:

Flag = IceCTF{wow_fast}


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