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March 9, 2007

Windows Compressed (Zipped) Folders cannot extract WinZip AES Encrypted Zips

If you create a WinZip compatible AES encrypted zip with Chilkat or with WinZip (it doesn’t matter which), you’ll find that the Windows Compressed (Zipped) Folders functionality cannot extract the .zip. It can open the .zip and view the files contained within, but it cannot extract. If you try, you’ll get a "unknown compression method" error.

To clarify what I’m talking about: open Windows Explorer and right-click on an AES encrypted zip archive. Then select "Open With –> Compressed (zipped) Folders". Next, in the Windows Explorer "File" menu, select "Extract All". Regardless of whether you set a password or not, you’ll get the "unknown compression method" error. Windows Compressed Folders is able to open Zip 2.0 Encrypted zips (i.e. password protected zips), but this is weak encryption.

The "unknown compression method" refers to the fact that in WinZip AES encrypted
zips, the encryption method field in each file’s header is set to the value 99. There are
12 extra bytes of data for each file that indicate the true compression method, the
key length, and a bit of data used to determine if you have the password correct.
The Windows Compressed Folders doesn’t recognize 99 as a valid compression method.

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