Cloud based storage services like Dropbox and Google Drive are a cause for concern when it comes to privacy. They store encryption keys for your files on their servers. Therefore, anyone who manage to get access to their servers, whether it be a hacker or employee, will be able to get your keys and by extension your files.
However, these services are seamless. That is, all you have to do is download, install, login, and they immediately start working. There is no key given to you to memorize or download. Yet they say that you own the key and the keys are not stored on their servers.
How is it possible that both the keys are not stored on their servers (Zero-knowledge), and you do not have to keep/memorize the keys yourself (seamless)?