ZIPs tricks

There are a handful of command-line tools for zip files that will be useful to know about.

  • unzip will often output helpful information on why a zip will not decompress.

  • zipdetails -v will provide in-depth information on the values present in the various fields of the format.

  • zipinfo lists information about the zip file's contents, without extracting it.

  • zip -F --out and zip -FF --out attempt to repair a corrupted zip file.

  • fcrackzip brute-force guesses a zip password (for passwords <7 characters or so).

Zip file format specification

One important security-related note about password-protected zip files is that they do not encrypt the filenames and original file sizes of the compressed files they contain, unlike password-protected RAR or 7z files.

Another note about zip cracking is that if you have an unencrypted/uncompressed copy of any one of the files that is compressed in the encrypted zip, you can perform a "plaintext attack" and crack the zip, as detailed here, and explained in this paper. The newer scheme for password-protecting zip files (with AES-256, rather than "ZipCrypto") does not have this weakness.


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