When performing an iOS backup/restore through iTunes, you may be prompted for a password to "unlock your backup". Often times this password was not set, it is a default password iTunes looks for or expects. This article covers multiple possibilities as to what password iTunes may require.

What is my iTunes backup/restore password?

The following are the most common backup unlock passwords:

1. iTunes Store password (this is the most common password)
2. Device Unlock Code
3. 0000
4. Old iTunes password. To reveal, do the following:
   • macOS Users
     1. Go to Applications > Utilities and launch Keychain Access.app.
     2. Search for iPhone Backup.
     3. Double-click most recent iPhone Backup entry.
     4. Click Show password (at the bottom).
     5. When prompted, enter admin-level password for your computer.
     6. Your iTunes backup password is revealed.
     7. Copy/Paste or write down the password and attempt another iOS restore with this password.
   • Windows Users
     1. Change your iTunes password via the iTunes Store.
     2. Try both the old and new password when prompted for the backup unlock password during an iOS restore.
     3. Try your Windows password (that has admin-level rights).
     4. Try previous Windows passwords (that have admin-level rights).

If none of these passwords or methods are successful, you will need to set up your iOS device as a new device and start a new backup.

For more information, here is the source article: Binary Age Web Article "iTunes Asks for a Password to Unlock a Backup that You Never Set"