PKZip utilizes a proprietary stream cipher which is vulnerable to a known plaintext attack, as described by Eli Biham and Paul C. Kocher in A Known Plaintext Attack on the PKZIP Stream Cipher.
In ZIP Attacks with Reduced Known-Plaintext, Michael Stay describes an improved known plaintext attack which works against files created by non-PKZip zip programs, such as InfoZip, WinZip, and NetZip. The Stay attack requires the Zip archive to contain a minimum of five files.
In addition to these two attacks, it is also possible to recover a Zip password utilizing a brute force attack or a dictionary attack.
Software to Recover a Zip Password
Several programs, with widely varying capabilities, are available to help you recover a lost Zip password.
Zip Key, by Passware, is a commercial program which implements the Biham-Kocher and Stay attacks, as well as dictionary and brute force attacks.
Ultimate Zip Cracker, by VDG Software
, is a commercial program which implements the Biham-Kocher attack, a dictionary attack, and a brute-force attack.
The Password Recovery Toolkit, by AccessData, is a commercial program which implements the Stay attack or a dictionary attack.
Advanced Zip Password Recovery, by Elcomsoft, is a commercial program which implements the Biham-Kocher attack, a dictionary attack, and a brute-force attack.
Advanced Archive Password Recovery, by Elcomsoft, is a commercial program which implements the Stay attack, in addition to the capabilities of their Advanced Zip Password Recovery program.
Visual Zip Password Recovery Processor, by ZipCure, is a commercial program which implements dictionary and brute force attacks.
PkCrack is a free program which implements the Biham-Kocher attack. To use PkCrack, you need another ZIP-archive, containing at least one of the files from the encrypted archive in unencrypted form. This one has to be compressed with the same compression method used for the encrypted file.
Security Enhancements in WinZip 9
WinZip 9 introduced the use of 128 and 256-bit key AES encryption. Unless there is a flaw in the WinZip implementation of AES, this should make WinZip 9 passwords significantly more difficult to recover.
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