If you are on a shared Nextcloud instance and you don’t know if you can trust the administrators, you have two options:
- Don’t upload your personal/private files
- Or encrypt them externally with e.g.
cryptmount
(user-space, nosudo
required) and external key on USB stick or any other similar program (e.g. VeraCrypt/TrueCrypt).
Then upload that encrypted container to the instance. Sure you want should keep it small so only put very critical files in it.