SHA2017 CTF Teaser round - [Crypto 200] Crypto Engine

flag

solve.py

import io
import requests
from PIL import Image

def get_image(s):
  r = requests.get('https://cryptoengine.stillhackinganyway.nl/encrypt?text=' + s)
  return Image.open(io.BytesIO(r.content))

def get_color(im, i):
  return im.getpixel((1 + 40 * i, 1))

flag = Image.open('flag')
res = ''
for i in range(12):
  im = get_image(res + 'aaa')
  c1 = [x ^ ord('a') for x in get_color(im, i)]
  c2 = get_color(flag, i)

  res += ''.join(chr(x ^ y) for x, y in zip(c1, c2))
  print(res)