Smiling ASCII - CTFlearn

Points: 40 Solves: 1081 Level: Medium

Find the flag on the smiling face.

Let’s go

First of all, let’s check what file has inside:

strings smiling.png | less

On the end of file we see: RGlkIHlvdSBrbm93IHRoYXQgcGl4ZWxzIGFyZSwgbGlrZSB0aGUgYXNjaWkgdGFibGUsIG51bWJlcmVkIGZyb20gMCB0byAyNTU/Cg== with is a base64 message:

$ echo RGlkIHlvdSBrbm93IHRoYXQgcGl4ZWxzIGFyZSwgbGlrZSB0aGUgYXNjaWkgdGFibGUsIG51bWJlcmVkIGZyb20gMCB0byAyNTU/Cg== | base
64 -d

> Did you know that pixels are, like the ascii table, numbered from 0 to 255?

So we need dig deeper but first let’s take a look of image. After zooming in we see various shades of gray, but in top left corner we have something like this:

Ending

We know that we should pay attention to the image and it is all about encoded characters. So let’s write a quick program in python that will read us these colors and convert them to ascii.

from PIL import Image

image = Image.open("smiling.png")
width, height = image.size
for y in range(height):
    for x in range(1):
        pixel = image.getpixel((x,y))
        if pixel == (255, 255, 255, 255): break
        print(chr(pixel[3]), end="")

And we have the flag