Tags: steganography png forensics
Rating:
# Dimensionless Loading
## Misc
We're given a PNG with the width and height fields set to 0. However, the original CRC is intact. The CRC is a unique value used to check for the integrity of the preceeding chunk, similar to a hash funciton. Because of this, we can 'crack' the original dimensions with this:
```py
from zlib import crc32
from pwn import p32
target = 0x5b8af030
header = "49 48 44 52 00 00 00 99 00 00 00 99 08 06 00 00 00".replace(' ', '').decode('hex')
def check_size(w,h, header):
w = p32(w)[::-1]
h = p32(h)[::-1] header = header.replace("\x00\x00\x00\x99" + "\x00\x00\x00\x99", w+h)
if crc32(header) == target:
print(list(w),list(h)) for x in range(2000):
for y in range(2000):
check_size(x,y,header)
```
Sizes are: 0x00 0x00 0x05 0x62 0x00 0x00 0x01 0x6B
Adding this back to the image, we get the original image, containing the flag.
[IMAGE HERE]
# `ractf{m1ss1ng_n0_1s_r34l!!}`
