BUUCTF: [BJDCTF2020] One leaf is blind

Title address: https://buuoj.cn/challenges#[BJDCTF2020]%E4%B8%80%E5%8F%B6%E9%9A%9C%E7%9B%AE

The title picture looks like this

010 EditorAn CRC不匹配error occurs when using open

The picture can be opened normally, but it appears CRC匹配错误that the width and height are generally modified

I don’t know what the original width and height are here. I tested and modified it manually, and it didn’t take long.

When the width is changed to:, the 01 E2height is:07 77

Save, open the picture, and be careful not to

enlarge it. The enlargement becomes like this hahahaha

After I did it, I searched it online and saw a 修复CRC错误script tqltql written by a big guy

#coding=utf-8
import zlib
import struct
#读文件
file = '1.png'  #注意，1.png图片要和脚本在同一个文件夹下哦~
fr = open(file,'rb').read()
data = bytearray(fr[12:29])
crc32key = eval(str(fr[29:33]).replace('\\x','').replace("b'",'0x').replace("'",''))
#crc32key = 0xCBD6DF8A #补上0x，copy hex value
#data = bytearray(b'\x49\x48\x44\x52\x00\x00\x01\xF4\x00\x00\x01\xF1\x08\x06\x00\x00\x00')  #hex下copy grep hex
n = 4095 #理论上0xffffffff,但考虑到屏幕实际，0x0fff就差不多了
for w in range(n):#高和宽一起爆破
    width = bytearray(struct.pack('>i', w))#q为8字节，i为4字节，h为2字节
    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]
            #print(data)
        crc32result = zlib.crc32(data)
        if crc32result == crc32key:
            print(width,height)
            #写文件
            newpic = bytearray(fr)
            for x in range(4):
                newpic[x+16] = width[x]
                newpic[x+20] = height[x]
            fw = open(file+'.png','wb')#保存副本
            fw.write(newpic)
            fw.close

flag{
    
    66666}