I found a solution to your problem, as I was having the same problem for about a week. If you are using Mac, then go to Applications -> utilities -> keychain Access. Once the program opened, type iPhone. You`ll find the device and login in this list. Click on one of them, the new window will open. Click on show password, enter your Mac administrator password, then you can see your iPhone encryption password. Hope it helps you.

If the above can’t help you, I also found a solution using an external program to decrypt backup password. You can try to use the application below just download the trial version and check if the password will be detected and will be recovered. It is “iPhone Backup Password Recovery” (get it from http://www.windowspasswordsrecovery.com/product/iphone-backup-unlocker.htm )

Please let me know if this helps. Thanks