I've lost the password to a "zip" file. How do I open the file?

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Summary: ZIP files are a archive format that includes optional password protection. If you don't have the zip file password you may still be able to get in.

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In a perfect world, the answer should be "you can't". I mean, you should be able to open a password protected files ONLY if you have the password right? Otherwise what's the point?

The fact that the answer isn't "you can't" should concern you.

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First, the answer: if you search Google for "zip file lost password" you'll get a page full of results for various providers of zip file password crackersrecovery tools. Some are free, some are not. While I haven't tried them, the tools are out there.

In looking at the tools, the techniques to crack a zip file password boil down to three different approaches:

• Brute Force Attack - make up and try every possible password. This sounds time consuming and it can be. However if you can provide hints, such as the approximate length of the password you used or perhaps the first character of your forgotten password, you can cut down the time dramatically.

• Dictionary Attack - by quite literally using a dictionary of words, the tool can try various combinations until something works. If you recall that your password was in fact a word or combination of words, a dictionary attack can once again reduce the amount of time it might take to discover the forgotten password.

• Known-plaintext Attack - If you have an original unencrypted copy of any file in the zip file (the "known plaintext", in cryptography-speak), that file, plus the encrypted zip file, can be used to reverse-calculate the password and thus extract all the other files.

Now, many of these techniques rely on time. Depending on your computer and the password to be discovered, you may end up turning a password crackerrecovery tool loose and have it take a few hours - or perhaps days - to recover the password. Perhaps that's too long, but perhaps not. That depends on your needs.

There are a couple of very important lessons to be learned here, however:

Password protecting a ZIP file is fairly poor security. It can be cracked, simple as that. With enough resources, and a poor or even moderately secure password, it can be cracked reasonably quickly.

"Strong" passwords are a must. If you're going to use words from the dictionary as a password, it's almost like having no password at all, cracking it is that easy. If you use a good, long and strong password then you can increase dramatically the amount of time it will take to crack or recover the zip file.

It's possible that the protection provided by a zip file is enough for you, and that's fine. Zip files are a compression format first, and an encrypting archive second. As long as you understand the previous two points, then you can at least make a reasonably informed decision as to which tool to use.

As an alternative, I would recommend something like TrueCrypt which uses much more robust algorithms to produce a virtually uncrackable encryption. If you forget the password to a TrueCrypt volume, you've lost your data. Period.

Like zip files, TrueCrypt volumes can contain many files in a single encrypted package. Unlike zip files, TrueCrypt volumes do not compress the data and you must select the size of your volume when you create it. But both of these differences are easily managed: if you want, you can compress files prior to putting them in a TrueCrypt volume, and it's easy to "grow" your volume, if needed by creating a new one and copying the contents of the old.

As with most things, choosing the right tool for the job is, perhaps, the most important decision even before you start.

Related:

• Ask Leo! - How can I keep data on my laptop secure?

• Ask Leo! - Can I password-protect a folder?

• Ask Leo! - Recommendation: 7-zip file archiving utility

• Wikipedia - Known-plaintext attack.

Article C2918 - January 31, 2007