In this specific challenge, players are given a ZIP file ( zipp.zip ) that appears broken. Upon inspection with a hex editor, several issues are discovered that prevent standard extraction:
: Located at offset 0x0E . This is the checksum of the uncompressed data. Compressed Size (4 bytes) : Offset 0x12 . Uncompressed Size (4 bytes) : Offset 0x16 .
: 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 ). C32zip
: Offset 0x1A . This is the value often tampered with in CTF challenges like "zipper". Solving Steps (Long Write-up Style)
: The "Extra Field" length and content are often manipulated to hide data or throw off automated parsers. In this specific challenge, players are given a
: 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.
: Use binwalk or file to confirm it is a ZIP. Try to unzip it; if it fails with "filename too long" or "offset error," the headers are tampered with. Compressed Size (4 bytes) : Offset 0x12
: If the file is encrypted or the data is missing, check the uncompressed size . If the size is very small (e.g., 4-6 bytes), you can use a script to find which alphanumeric string produces the CRC32 hash found in the header.