About iTunes and Encrypted Backups

Something about iTunes and encrypted backups, reference from support.apple.com

If an iOS device is protected with a passcode, iTunes prompts you to enter the passcode when you connect to your computer. Once you successfully enter the passcode, iTunes will recognize that device as authorized and you will not need your passcode to back up or sync. When restoring from an iOS 4 (or later) backup, if the device had a passcode set, iTunes will ask if you want to set a passcode (and remind you that you had protected your device with a passcode). iTunes will not ask you to set a passcode when restoring from iOS 3.x and prior backups. 

In the iTunes Summary screen, select "Encrypt iPhone backup" if you want to encrypt the information stored on your computer when iTunes makes a backup. Encrypted backups are indicated by a padlock icon, and a password is required to restore the information to iPhone. You may want to write down the password for your backup and store it in a safe place. If you use a Mac, when you set a password you can select to store the password in the keychain. With iOS 4 and later, you can transfer the iOS keychain backup to a new device if you encrypt the backup.

Warning: If you encrypt an iPhone backup in iTunes and then forget your password, you will not be able to restore from backup and your data will be unrecoverable. If you forget the password, you can continue to back up and use the device, however you will not be able to restore the encrypted backup to any device without the password. You do not need to enter the password for your backup each time you back up or sync.

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