How to unlock your backup fille in itunes if you forget the password

If you don't remember deliberately choosing to use backup encryption then perhaps you changed the setting accidentally and responded with your default password at the time. Things to try are an empty password, 1234, any of your past Apple ID, computer, or Wi-Fi passwords, or your current or previous device PINs.

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If you cannot recover the password there is no way to get access to the backup. That is what the password is for.