Nostego

Writeup by: GoProSlowYo

Team: OnlyFeet

Writeup URL: GitHub

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It cannot be stego because the source is attached.

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Initial Research

We are given an image that looks like “noise” or “snow” on a TV screen and a python script that seems to have generated said noisy image. We can probably reverse the python script to output the original image.

from PIL import Image
import sys

if len(sys.argv) != 3:
    print("Usage: %s [infile] [outfile]" % sys.argv[0])
    sys.exit(1)

image = Image.open(sys.argv[1]).convert("F")
width, height = image.size
result = Image.new("F", (width, height))

ROUNDS = 32

for i in range(width):
    for j in range(height):
        value = 0
        di, dj = 1337, 42
        for k in range(ROUNDS):
            di, dj = (di * di + dj) % width, (dj * dj + di) % height
            value += image.getpixel(((i + di) % width, (j + dj + (i + di)//width) % height))
        result.putpixel((i, j), value / ROUNDS)

result = result.convert("RGB")
result.save(sys.argv[2])

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Reversing

code.

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Victory

Submit the flag and claim the points:

ALLES!{flag-goes-here}