DefCamp CTF Finals - Security CCTV (misc) - NorthernCoalition

cctv.py

import cv2
import numpy as np
import os
import subprocess

# Download the current frame
os.system("curl https://security-cctv.dctf-f1nals-2017.def.camp/img/streamframe.png > out.png")

# Find the homography for the QR code displayed on the phone to a 200x200 image
# These are the coordinates of the corners of the QR code on the image
from_points = [
    (2325, 1626), # top left
    (2244, 1710), # top right
    (2121, 1655), # bottom right
    (2204, 1572)  # bottom left
]

to_points = [
    (0, 0),
    (200, 0),
    (200, 200),
    (0, 200)
]

h, _ = cv2.findHomography(np.array(from_points), np.array(to_points))

print h

base_image = cv2.imread('out.png')

warped = cv2.warpPerspective(base_image, h, (200, 200))

# Find the homography for the QR code reflection to a 200x200 image
from_points = [
    (2073, 1617),
    (2123, 1607),
    (2158, 1705),
    (2133, 1710)
]

# These points have been approximated such that the resulting image is somewhat straight
to_points = [
    (0, 0),
    (0, 110),
    (200, 55),
    (200, 0)
]

h, _ = cv2.findHomography(np.array(from_points), np.array(to_points))

print h

corner = cv2.warpPerspective(base_image, h, (200, 200))

# Fill in cyan the part of the phone QR code that is overlayed by a tissue
cv2.fillConvexPoly(warped, np.array([(0, 0), (150, 0), (55, 100), (0, 80)]), (255, 255, 0))

cv2.imwrite('back.png', warped)
cv2.imwrite('corner.png', corner)

# Make the cyan transparent to overlay the images
os.system("convert back.png -transparent cyan back_t.png")

# The reflection is a bit darker so the brightness/contrast is increased a bit
os.system("mogrify -brightness-contrast 20x50 corner.png")

# Compose both images and apply a slight shift (+2 in x, +1 in y)
os.system("composite corner.png back_t.png -geometry +2+1 -compose dstover result.png")

# Add the missing "tracking box" otherwise the QR code can't be perperly decoded
os.system("composite tracker.png result.png -gravity center -compose over result2.png")

# Add a white margin around the resulting QR code
os.system("composite result2.png white.png -gravity center -compose over result3.png")

# Increase the brightness/contrast of the overall image to make zbarimg happier
os.system("mogrify -brightness-contrast 10,20 result3.png")

# Read the QR code
token = subprocess.check_output(['zbarimg', '-q', 'result3.png']).split(':')[1].strip()

print token

# Submit it, get dat flag
os.system("curl 'https://security-cctv.dctf-f1nals-2017.def.camp/index.php' --data 'token=%s'" % token)