Cryptomator

Cryptomator solves the specific problem of trusting cloud providers: it creates encrypted vaults inside your synced folders, encrypting each file individually (so sync stays efficient) and mounting the vault as a normal drive while unlocked. Boxcryptor’s shutdown made it the default answer, it’s open source, audited, and the desktop apps are free.

Filename and per-file encryption means some metadata (file sizes, rough structure) remains observable, and the mounted-drive experience via FUSE occasionally hiccups with apps that do unusual file locking. Mobile apps cost a one-time fee. It protects data at rest in the cloud — it does nothing about the provider seeing when and how much you sync.