C32zip -

To solve these "C32" related zip challenges, one must understand the ZIP file format : : Starts with the signature 50 4B 03 04 .

: 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

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: C32zip

: 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 . To solve these "C32" related zip challenges, one

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

: The name "C32" typically refers to the CRC32 checksum found at offset 0x0E of the local file header. In many "Zip CRC" challenges, the flag is small enough that it can be "cracked" by brute-forcing strings until their CRC32 matches the one stored in the header, without ever needing the password or the full file content. Technical Breakdown: ZIP Structure Upon inspection with a hex editor, several issues

: 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 ).