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If you're using the latest version of either the PKZip or the WinZip file compression utility to create .zip files, beware: The files you make may not be accessible to other .zip software. The incompatibility is creating confusion over a format that has known remarkable stability for years.

The problem stems from the two popular programs' implementations of a long-overdue new feature, AES encryption (the password protection built in to the previous versions of the .zip format is easily hacked). PKZip publisher PKWare--the company founded by the late .zip inventor Phil Katz and the traditional keeper of the .zip format--added AES to PKZip in January; the beta of WinZip 9, with incompatible AES encryption, debuted in May.

WinZip couldn't use PKZip-compatible encryption because PKWare didn't disclose any details on its encryption implementation until after WinZip's beta appeared. And PKWare still has not released information on certificate-based encryption, so complying with it is impossible for WinZip or anyone else.

Neither PKZip nor WinZip makes encrypted archives by default, so most new .zip files will be in the old, open format. The problem will surface only with encrypted files, but they will still bear the .zip extension, which may confuse people who can't open such files with their .zip utility. (Competitors StuffIt and PentaZip put different extensions on files with secure encryption.) If you want to use the encryption in a new version of PKZip or WinZip, make sure that recipients of your files know--and that they have whichever .zip program you use.

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