Down Bad

Description

by Hiumee

The flag is right there!

down_bad.png

Solution

# Let's try opening png
└─$ display ./down_bad.png # Fails To Open
                                        
└─$ pngcheck ./down_bad.png 
./down_bad.png  CRC error in chunk IHDR (computed 1d9c52c0, expected a9d5455b)
ERROR: ./down_bad.png

Step 1: Fix CRC

https://www.wikiwand.com/en/PNG#Examples

down-bad-2

down-bad-3

Where flag? :(

The challenge name down_bad, word "down" and the woman downvoting us made me thing it's like Tunnel Vision from PicoCTF. Image height bytes could have been manipulated to not see some chunks. So let's try to extend the image!

down-bad-4

down-bad-5

Flag: TFCCTF{28ae25c96850245ffdd70a880158f9f3}

TIL: Surprisingly Windows doesn't care about CRC as much as Linux, so if you were on Windows you could open the file, and after you changed height without touching CRC it's still openable.