How safe are encrypted zip files? What if we have the unzipped form of an encrypted zipped file?

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I know that if we use a complicated password for zip files that has more than 8 characters, it's quite safe and can't be cracked easily...
For example, for an 8-digits complicated password (containing ABC,abc,123,!@#), it takes 83.5 days for a supercomputer to test all combinations, and 22 years for a 9-digit!!!!)
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But in an example, we have an encrypted zip file that contains multiple files, and we have the extracted form of one of those files

In this case and with having the original and zipped-encrypted file, can we write a program that cracks the password easily...? Not by taking months or even years to crack?

It it's possible, there will be problems; for example, a zip file may contain ""thumbs.db"" or ""desktop.ini"" files, that use almost the same pattern everywhere... and more common: every file format has its own hex pattern too!!! (For example, EF BB BF for UTF-8 text files)... and situations that we know the contents of a part of a text file... etc.

Posted 23-Jun-14 11:50am

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Sergey Alexandrovich Kryukov · Accepted Answer · 2014-06-23T12:23:00

Nothing prevents you from packing other ZIP files in a bigger ZIP file, all encrypted with different passwords. I cannot say how much sense would it make, but it addresses your concern.
More serious problem is another one: how would you deliver the passwords themselves? One solution is the public-key cryptography which is not used by ZIP, but can be separately used just for key distribution, and then, using the secret message based on the public-key cryptography, you can transmit the ZIP passwords.

—SA