D-CTF 2015: She said it doesn't matter (Misc) Write-up

Description:

Void. Empty. Null.

Solution:

1. Download m100.png and open it with default Image viewer, I found IHDR CRC error?

2. Check m100.png using pngcheck and result below

root@ubuntu:~# pngcheck -v m100.png 
File: m100.png (65141 bytes)
  chunk IHDR at offset 0x0000c, length 13
    666 x 519 image, 32-bit RGB+alpha, non-interlaced
  CRC error in chunk IHDR (computed 3ff4fc62, expected 35468913)

3. Try to change IHDR value from 35468913 to 3ff4fc62 in HexEd.it online hex editor.

Default value:

After change value:

Export to view

I think it may be enough, but not have flag. :(

4. Try to change Image Height value from 519 (207) to 550 (226), Decimal to Hex :)

Default value:

After change value and export to m100_fixsize.png.

5. Check m100_fixsize.png using pngcheck again and result below

root@ubuntu:~# pngcheck -v m100_fixsize.png
File: m100_fixsize.png (65141 bytes)
  chunk IHDR at offset 0x0000c, length 13
    666 x 550 image, 32-bit RGB+alpha, non-interlaced
  CRC error in chunk IHDR (computed f3042af1, expected 3ff4fc62)
ERRORS DETECTED in m100_fixsize.png

6. Try to change IHDR value from 3ff4fc62 to f3042af1 in HexEd.it online hex editor again.

After change value:

Export to view and get a flag :D

Flag: s1z3_d03s_ma773r_baby