Dimensionless Loading

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:

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!!}