The force is with those who read the source.

Internetwache CTF 2016: Quick Run (misc 60)

2016-02-25
writeup
Internetwache CTF/misc

Description

Someone sent me a file with white and black rectangles. I don’t know how to read it. Can you help me?
Attachment: misc60.zip

Exploit

Since the = padding in the README.txt, I try to use base64 decode and get something like

██████████████████████████████████████████████
██              ██  ██  ██  ██              ██
██  ██████████  ██████████  ██  ██████████  ██
██  ██      ██  ██  ████    ██  ██      ██  ██
██  ██      ██  ██████████  ██  ██      ██  ██
██  ██      ██  ██  ████  ████  ██      ██  ██
██  ██████████  ████    ██  ██  ██████████  ██
██              ██  ██  ██  ██              ██
████████████████████████    ██████████████████
████  ████  ██  ████    ██    ████  ██  ██  ██
██            ██    ████████  ██  ████████  ██
██  ██  ██████            ████████  ██  ██  ██
██  ████  ██  ██  ██  ██  ██  ██  ██  ██  ████
████  ██    ██  ██  ██  ██  ██  ██  ██  ██  ██
████████████████████  ██  ██  ██  ██  ██  ████
██              ██████  ██  ██  ██  ██  ██  ██
██  ██████████  ████  ██  ██  ██  ██  ██  ████
██  ██      ██  ██  ██  ██  ██  ██  ██  ██  ██
██  ██      ██  ████  ██  ██  ██  ██  ██  ████
██  ██      ██  ██████  ██  ██  ██  ██  ██  ██
██  ██████████  ██    ██    ████  ██  ██  ████
██              ██████    ████████  ██      ██
██████████████████████████████████████████████

It is clear that they are QR code. However, I can’t scan the QR code in the text format even I adjust the line hight and change the color into black and white. Therefore, I write a script to get the message. I use PIL to create the QR code image and scan it by zbarlight. (Actually, during the competition, I simply show the QR code and scan by my mobile.) The message is Flagis:IW{QR_C0DES_RUL3}.

decode.pydownload
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
from base64 import b64decode
from PIL import Image
from zbarlight import scan_codes

def create_qrcode(qr):
    img = Image.new('1', (23, 23), 'white')
    qr = qr.split('\n')

    for i in range(1, 22):
        for j in range(2, 44, 2):
            if qr[i][j] == ' ':
                img.putpixel((i, j/2), 0) # add a black block

    return img


with open('README.txt') as f:
    lines = f.read().split('\n')[:-1]

b64code = ''
message = ''
for l in lines:
    b64code += l
    if len(l) < 76:
        qr_text = b64decode(b64code).decode('utf8')
        qrcode = create_qrcode(qr_text)
        message += scan_codes('qrcode', qrcode)[0]
        b64code = ''

        #qrcode.resize((500, 500)).show()
        #raw_input('')

print message

Flag: IW{QR_C0DES_RUL3}

Blog comments powered by Disqus