: Open the file in a tool like 010 Editor or Hxd . Look for the filename length at 0x1A . If the actual filename is just a few characters (like flag.txt ) but the length is 9001 , manually change it to the correct length (e.g., 0x08 ).

: Once lengths and CRC values are consistent, standard tools like 7z or unzip will be able to process the file correctly. CTFtime.org / PlaidCTF 2017 / zipper / Writeup

: The "Extra Field" length and content are often manipulated to hide data or throw off automated parsers.

: The filename length field in the local file header is set to an impossibly large value (e.g., 9001 or 0x2329 ), causing extraction tools to fail or truncate the filename.

: Offset 0x1A . This is the value often tampered with in CTF challenges like "zipper". Solving Steps (Long Write-up Style)

: Located at offset 0x0E . This is the checksum of the uncompressed data. Compressed Size (4 bytes) : Offset 0x12 . Uncompressed Size (4 bytes) : Offset 0x16 .