Home > CTF, Stegano > DEFKTHON CTF Misc 200 Writeup

DEFKTHON CTF Misc 200 Writeup

If there is a frustrating side in CTF, then I would rather vote for Steganography. It is interesting too see, how message can embedded inside covert objects, but it is always a tedious job to analyze the patterns and extract them back ๐Ÿ˜€ :P. I found it always difficult ๐Ÿ˜ฆ . I have seen challenges with convert objects, either as an image file or as an audio file, but the challenge was designed differently.ย It was quite weird though.ย The challenge was to construct an image from a given RGB values. You can download the file here. Actually I didn’t solved it alone. I was able to get it done with my team mates and my friend. Initially my thought was to build an image with those values with an assumed width and height.

import os,sys
import Image
im = Image.open("black.jpg")
pix = im.load()
h,w = im.size
m = open("flag.txt","r").readlines()
o = -1
for i in range(255):
   for j in range(255):
       if (o < 61366 ):
           steg = eval("("+m[o].strip()+")")
           pix[i,j]= steg

When I ran the code, the ramifications came as a contrary to my expectations. The method didn’t work though.


But later my friend directed me with a hint of doing it in a right way. Start with a black image and assign the RGB values to it. Way simpler it is, but I think I complicated it very much with the above snippet.. ๐Ÿ˜€ ( credit goes to you guyz (vivek and ajith! ๐Ÿ˜‰ )

import Image,csv
img = Image.new( 'RGB', (503,122), "black")
pixels = img.load()
reader = csv.reader(f, delimiter=',')
mycsv = list(reader)
for i in range(img.size[0]): # for every pixel:
 for j in range(img.size[1]):
 pixels[i,j] = (int(mycsv[k][0]), int(mycsv[k][1]), int(mycsv[k][2]))

Now, I should be getting it ๐Ÿ™‚


The flag is youc@n’tseeme.ย 

  1. February 15, 2015 at 12:16 pm

    Thanks for the write up , but i have a stupid question ^^”
    Where exactly did you put the code ??

  2. February 15, 2015 at 12:28 pm

    Thanks For the write-up , But i have a stupid question ^^”

    Where exactly did you put your code ??

  3. February 24, 2015 at 4:50 am

    Find it just above the 2nd image

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