If you want to unlock an iPhone backup password that you don't remember or you think you never set, just try the most commonly used passwords first, such as iPhone enter password, iCloud password, computer Administrator password, etc. If those passwords do not work. You have no choice to ask for help from a third party software - Tenorshare iPhone Backup Unlocker.
iPhone Backup Unlocker is a professional software designed to help people who lost and forgot backup file password for iPhone (including iPhone 7/7 Plus/6s/6s Plus/6/6 Plus/5S), iPad, iPod on Windows computer.
Steps to unlock password to restore iPhone backup with iPhone backup unlocker
To get started, you need to download and install iPhone Backup Unlocker in your Windows computer where has iPhone backup files you want to unlock.
Launch iPhone Backup Unlocker, click "Open" to load the password protected iPhone backup file you want to restore. You can also transfer the backup file from other computer (Win or Mac) to this computer.
Select an attack type: Brute-force Attack, Brute-force with Mask Attack and Directory Attack.
Click on "Start" to find your iTunes backup password. After several minutes, your password for iPhone backup file will be listed in the File Opening Password Box.
Now go back to the iTunes and try to restore iPhone backup again. while "Enter the password to unlock your iPhone backup" show up, just enter the password were found by iPhone Backup Unlocker.