I’d like to give my users some private network storage (private from me, ie. something encrypted at rest with keys that root cannot obtain).

Do you have any recommendations?

Ideally, it should be something where files are only decrypted on the client, but server-side decryption would be acceptable too as long as the server doesn’t save the decryption keys to disk.

Before someone suggests that, I know I could just put lucks-encrypted disk images on the NAS, but I’d like the whole thing to have decent performance (the idea is to allow people to store their photos/videos, so some may have several GB of files).