Cryptomator creates a vault folder that encrypts each file before Dropbox, Drive, or OneDrive syncs it. The cloud sees scrambled filenames and ciphertext. The desktop apps are open source and free. The phone apps are a paid unlock. A vault mounts like a normal drive while it is open.
The cloud cannot preview or search inside the vault. That is what you bought. Do not leave the vault mounted on a shared computer. The passphrase is the only recovery. Cryptomator does not replace full-disk encryption for a stolen laptop that was unlocked. It replaces trust in the storage company.
Use it when you already pay for a big cloud and you do not want that company to read one folder. cryptomator.org
Create a small Cryptomator vault, copy a file in, close it, and confirm the cloud folder shows only ciphertext. Then open it on the phone after you pay for the mobile license, or decide the phone does not get that vault. Do not point a vault at a folder that other apps are also syncing in the clear. One passphrase per vault is fine. Write it down. Cloud provider versioning can keep old ciphertext, which is good, and can also restore a deleted vault you meant to destroy.
