Foreword

Sometimes CTF Miscellaneous gives a png picture, which cannot be opened under Linux (white screen), and the picture can be viewed under Windows, and the width and height may be modified

Parse IHDR

example

Selected from 2020 Anheng Monthly Race
Download the compressed package and extract a PNG image, which cannot be opened under linux, as shown in the figure

The guess is that the height or width has been changed, and CRC blasting is required to obtain the original width and height, and just modify it back.

script

import zlib
import struct

filename = 'test.png'
with open(filename, 'rb') as f:
    all_b = f.read()
    crc32key = int(all_b[29:33].hex(),16)
    data = bytearray(all_b[12:29])
    n = 4095            #Theoretically 0xffffffff, but considering the actual screen/cpu, 0x0fff is almost the same
    for w in range(n):          #High and wide blast together
        width = bytearray(struct.pack('>i', w))     #q is 8 bytes, i is 4 bytes, h is 2 bytes
        for h in range(n):
            height = bytearray(struct.pack('>i', h))
            for x in range(4):
                data[x+4] = width[x]
                data[x+8] = height[x]
            crc32result = zlib.crc32(data)
            if crc32result == crc32key:
                print("Wide is:",end="")
                print(width)
                print("High as:",end="")
                print(height)
                exit(0)

According to the results, Gao was changed

modify it back

The picture is displayed normally and the flag is obtained