Parade_buster-final.7z

Concatenating these extracted bytes from all files produced a final, valid image file named , which contained the flag for the challenge. CTFtime.org / TastelessCTF 2020 / 7/12 / Writeup

The file Parade_Buster-Final.7z was the concluding challenge in the series from TastelessCTF 2020 . This write-up details the process of identifying a hidden PNG file embedded across hundreds of separate archives. Challenge Overview Parade_Buster-Final.7z

: To solve this, a script was used to loop through all 376 files, extract a specific block of bytes (typically 0x11 bytes) from each, and concatenate them. Final Result Concatenating these extracted bytes from all files produced

The offset for the hidden PNG data could be calculated by comparing the reported "Packed Size" from the 7z -slt l command with the actual file structure. Challenge Overview : To solve this, a script

The challenge provided a set of 376 individual 7zip archives (e.g., part2_0.7z through part2_375.7z ). Inside these archives were multiple "junk" files (e.g., junk_0.bin ), which initially appeared to be the primary data for extraction. Analysis & Solution

The PNG file was striped and hidden in the trailing bytes of the 7z archives, just before the footer.

: Opening the first archive ( part2_0.7z ) in a hex editor revealed a PNG header ( 89 50 4E 47 ) and an IHDR chunk buried within the file structure itself, rather than in the extracted contents. The Pattern :