How to Remove the Password from a Zip File Without Knowing the Password

8 April 2009 No Comment

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.

Steps
1. 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.
2. 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 “free zip brute force” and you will find plenty.
3. 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.
4. Once you find the password you can open the file and remove the password.

Warnings
• Copying/Downloading licensed software without paying for it, or without consent from the owner, is illegal and may result in extreme legal persecution ($250,000 fine and 5 year in jail!)
• 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 then yours to do it. Also a good program like elcomsoft ’s is able to try and crack the same file with multiple computers so it will be a lot faster. Again elcomsoft ’s software isn’t free and pirating is a crime.