Ok, just my guess, not sure if that's what they do. They can store the encrypted key on their server and send the encrypted key to the client whenever the clie

Ok, just my guess, not sure if that's what they do. They can store the encrypted key on their server and send the encrypted key to the client whenever the client requests it. The client decrypts the key locally and use the decrypted key to decrypt the data. With this scheme, the user can use a client on any machine to download his encrypted key and use it locally.