i did not assign password for backup before, and now it ask for password for restore, how can i recover?

Well, you or someone had to elect to encrypt your backup. This can also be enforced by an Exchange account if you have such on your phone. Regardless:

If on a Mac, and you elected to save such to your Keychain, you can recover it from your Keychain. Otherwise, you can't.

While you can continue to sync/backup, if you can't remember it & want to remove it, you'll have to delete your existing backups & restore the phone as a new device. Your data will be unrecoverable.

Need password to restore your iPhone from backup! I have experienced such things.

When I need my apple iPhone 4 backup password to jailbreak! I read many suggestions from google search results, and tried a number of ways.

Finally, I obtained out of trouble with suggestion - used the "iPhone Backup Unlocker" to reset my encrypted iPhone backup password in iTunes! It is unbelievable for me personally! Just 3 clear steps! Now, my contact address, call logs, etc. are back!

Banking applications are notorious for autoencrypting backups if you have a bank app, enter the password for it in the backup encryption. hope that helps.

Hi, are you using a Mac by any chance? If you set it up to be in the Keychain app you could get it back, here's another thread that helps much whenrecovering the password on a Mac. If on a Windows, you would do well to check if your backup is encrypted because of your work Exchange account. My friend though he lost the password and it turned out to be something he used for his work accounts, be sure to try all the password you regularly use. Gluck!

That worked for me; i think it i just thought back to an original password i use to use

Very helpful - just used an old common password, even though I don't remember entering it before any backup. Thank you!