Theoretically you can't decrypt the backup without the username and password of the person who the backup belongs to.
Practically it's probably eventually, I mean, for gods sake people hack into government secure servers, so they can get to your phone backup! Even data on your iPhone can be retrieved by Apple, they can install/modify the iOS installation on the phone to give themselves a backdoor to your data. However, they seem particularly reluctant to do this, even for the FBI.