If you forgot the password to encrypt iPhone backup in iTunes, this video will help you retrieve the password to decrypt the iPhone backup.
What you need is a Windows-based computer with iTunes installed and internet access.
Here are the steps to decrypt iPhone backup in iTunes.
1) Make sure the encrypted iPhone backup is stored on the computer.
2) Get ”iTunes Password Refixer” from the web, then install and run it on the computer. http://www.isumsoft.com/itunes-password-refixer/
3) Import the iTunes backup file of your iPhone device to the software, by clicking the ”Open” button.
4) Select one attack type and from the drop-down menu set relevant password properties such as Range, Length, etc.
5) Click ”Start” to start discovering iTunes backup password.
6) After password is recovered, use the password to decrypt your iPhone backup in iTunes.
In this video, I used an iPad to make the presentation. I hope you won’t mind.
Resource from: http://www.isumsoft.com/apple/decrypt-iphone-backup-with-or-without-password.html