An implementation of this is currently in the dev branch. It solves this “properly”, so it also encrypts metadata such as file names + sizes + file system structure, but of course de-duplication between clients isn’t possible anymore. Pieces such as being able to mount the encrypted backups on the server are currently missing plus a lot of other things.
Focus is currently on finalizing 2.5.y. So if you want to help, testing that would speed up that, so focus can be switched to dev.