Returns *session_dict* as a base64 encrypted json string.

def prepare(session_data={}, #pylint: disable=dangerous-default-value
                passphrase=None):
        """
        Returns *session_dict* as a base64 encrypted json string.

        """
        if passphrase is None:
            passphrase = settings.DJAODJIN_SECRET_KEY
        return encode(session_data, passphrase,
            json_encoder=crypt.JSONEncoder)