It’s not possible to remove a password from a ZIP-file unless you know the password. However there are ways to discover the password.
- Use a Dictionary attack: This type attack is similar to the brute force attack but it takes less time but has a chance of failure. The dictionary attack basically just uses a pre-defined word-list to try and crack the file. Since this has a chance of failure and doesn’t save all that much compared to a brute force attack its best to just use the brute force attack.
- Brute-force with Mask Attack: In the field of decryption, mask is a commonly used term refers to the password features you still remember. For example, the possible password length, any digits, letters, special characters like pond, dollar, or asterisk in the password; what is the fist and last characters of the password, etc. You can use this attack if you still remember part of your password. For example, if you still remember your password is 4 characters and begins with “z”, then you can define the mask as “z???”. Thus reduces the detecting scale for the program, as it will only seek for the last 3 characters instead of 4 characters of the whole.
- Brute force attack: Just like you can force a lock open you can also force a zip file open. In this way you try out every single possibility the password could be. Don ‘t worry you don ‘t have to do this manually, there are many programs out there that can brute-force the file. This technique just lets your computer go trough every single possibility. examples a program which can do this is is elcomsoft ‘s distributed password recovery. This however is licensed software so you need to pay for it. Just Google for “ZIP Password Cracker” and you will find plenty.
- Social engineering: Figure out the password by guessing. This step implies you know where the file was downloaded, or more especially who uploaded it. By this knowledge you might be able to figure out the password just by guessing. A lot of zips downloaded from certain sites have as password the URL to the site for example. This step will almost never be successful unless you have good knowledge of the history of the ZIP file.
Once you find the password you can open the file and remove Zip password.
Brute force attacks can take a lot of time depending on your processor speed. If you do want to use this approach consider asking people with a better PC than yours to do it.