def set_preshared_key(self, new_key):
        """
        Set the preshared key for this VPN. A pre-shared key is only
        present when the tunnel type is 'VPN' or the encryption mode
        is 'transport'.
        
        :return: None
        """
        if self.data.get('preshared_key'):
            self.update(preshared_key=new_key)