C32zip May 2026

: 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

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

: Offset 0x1A . This is the value often tampered with in CTF challenges like "zipper". Solving Steps (Long Write-up Style) C32zip

: 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.

: 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. : Once lengths and CRC values are consistent,

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:

: 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 Compressed Size (4 bytes) : Offset 0x12

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