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In modern zip utilities like 7zip, you can specify a password when creating a .ZIP file. But, how secure is this? What encryption algorithm(s) are used to secure password-protected zip files?

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up vote 26 down vote accepted

7-zip uses AES-256 encrpytion for 7z/zip archives.

http://7-zip.org/7z.html says:

7-Zip also supports encryption with AES-256 algorithm. This algorithm uses cipher key with length of 256 bits. To create that key 7-Zip uses derivation function based on SHA-256 hash algorithm. A key derivation function produces a derived key from text password defined by user. For increasing the cost of exhaustive search for passwords 7-Zip uses big number of iterations to produce cipher key from text password.

The help files of 7-zip says this about the security of a passworded 7z file:

7z

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Brute force attacks are a waste of time. I won't go into details why, I will instead direct you to Jeff Atwood's blog, he has an excellent post.

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Brute force attacks aren't always a waste of time, most users don't put an awful lot of thought into passwords, and a dictionary attack against the password they use can be a very effective way to decrypt a file. A brute force attack is only a waste of time (at present it becomes easier by the year) against a secure key. –  Mike1980 Apr 10 '10 at 17:21
    
Indeed, I once, (just out of curiosity!) took the whole-school list of user accounts from a very poorly CC'd email and wrote a quick python script to dictionary-attack each of them. Got an average of 3 hours/account, and that's over the internet. Had I had a local method of doing this, it would have been much much faster - dictionary words are not safe. –  Phoshi Apr 10 '10 at 19:55
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A brute force attack is a waste of time. Dictionary attacks are not brute force, and are a LOT faster than trying every possible combination of a set of characters up to a specific length. –  Mircea Chirea Apr 10 '10 at 21:10
    
+1 For posting a link to a blog with a link to a blog with that discusses an email that pertains to the original question. –  Oorang Jul 25 '10 at 0:36
    
@Oorang, I did that on purpose :P –  Mircea Chirea Jul 26 '10 at 7:26
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