I am running NC with server side encryption. I backup my database and the data directory on a daily basis. I know that in case my server dies horribly I can restore the entire instance using my backup on another machine.
I am wondering: is there also a (convenient) way how you can decrypt the data of a user without setting up a nextcloud instance?
Not officially, so there is no tool. If you are into coding, you might find a way and then you would be welcome to share your experience.
If you don’t use external storage, there is little benefit to server-side-encryption and it makes life so much harder in many cases such as to restore a single file for a user from backup. If encryption is important, I’d rather recommend to use client-side encryption (currently through 3rd-party software), there are plans to integrate this into the NC client for NC 13. I haven’t seen any alpha version of the client yet, and NC 13 is expected still in 2017. So I wouldn’t count on a reliable solution, give it some time for testing (and help testing of course).