LOLZZ-CTF-WriteUps

The Joker 2

Inside the zip file, the file Ryōiki Tenkai.exe greets you. Hmm, seems like something will happen when we run it…

Also Would you lose.png.

By reversing the exe file, we can get the logic:

1. Get all .txt and .png files in Desktop and Downloads folders.
2. Zip the files.
3. Generate random 4-byte key and xor within the zip file.
4. Add the header, CRC32, 8-byte extended 4-byte key, length, in front of the xor-zip file.
5. Convert No 4.’s payload into RGBA array structure, and generate png file.

Now we can finally recover the flag.

import cv2

image = cv2.imread("Would you lose.png", flags=cv2.IMREAD_UNCHANGED)
f = open("data", "wb")

for row in image:
    for col in image:
        f.write(bytes(col))

f = open("data", "rb")
f.read(4)
keys = f.read(4)
f.read(8)

data = f.read()
data = [b^keys[i % 4] for i,b in enumerate(data)]
for i in range(len(data)//4):
    x = 4*i
    data[x], data[x+2] = data[x+2], data[x]

f = open("out.zip", "wb")
f.write(bytes(data))

Inside the out.zip, there is Stand_Proud_You_Are_Strong.png.

flag is: 0xL4ugh{I_4lon3_4m_Th3_H0n0r3d_0ne_641}