The user should be able to set a passphrase for Cloud sync #4
Description
At the moment the Cloud is not entirely secure. Even if the cloud access is private (and no one but the user can access it), the SECRET_KEY to crypt and decrypt is the same to all users.
To have a better end-to-end crypt, the user should be able to generate his passphrase, and connect new devices using that.
N.B.: Retrocompatibility is mandatory.