WinZip and PKZip archives use a proprietary encryption algorithm which has a long keylength (making it impossible to use key recovery on a file).  Whilst decryption of all archives is not feasible, there are some weaknesses in the algorithm that can be exploited.

If one of the files in the archive is available unencrypted, it is possible to decrypt the archive without finding the password - this is called a plaintext attack.

In addition, if a WinZip archive contains more than 5 files, it can also be unlocked without the password using a collision attack.

The usual dictionary and brute force approaches can sometimes be effective, as they typically run very quickly for archive files and can therefore cover a larger number of passwords in a given time.

Recommended software

We recommend Elcomsoft's Advanced Archive Password Recovery product.  This is an extension of the Advanced Zip Password Recovery package.  It can unlock WinZip, PKZip, RAR and ARJ files, and offers several alternative approaches to opening your file.

In addition to the usual brute force and dictionary attacks (this package is the fastest in the world, and can try 15 million passwords per second on a 1 GHz Pentium III), it also supports plaintext attacks.  This software also offers guaranteed decryption of WinZip files with 5 or more files.  In our real world testing, we have found this feature offers the best chance of recovering most archives without needing to find the password.  It is a valuable feature that is not found on competing products.

Advanced Archive Password Recovery has the usual refined Elcomsoft interface and has outstanding product support with lifetime upgrades to registered customers.  We recommend this as a complete archive recovery solution.

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