Is open source really required? We at Haystack Software make a backup app (Arq) with client-side encryption that doesn't require any server side code (it supports a number of cloud providers' APIs). We don't publish the app's source, but the data format is open/documented, the data go in your cloud account, and you can monitor network traffic from it to ensure it's not connecting to us or anyplace unwanted. Your data don't come to us at all -- they're sent to your cloud account.