A public notebook server needs to be secure. You should set a password and use an SSL certificate to connect to it. We need the certificate to provide secure communication over HTTPS (for more information, see https://en.wikipedia.org/wiki/Transport_Layer_Security). HTTPS adds a secure layer on top of the standard HTTP protocol widely used on the Internet. HTTPS also encrypts data sent from the client to the server and back. A certificate authority is often a commercial organization that issues certificates for websites. Web browsers have knowledge of certificate authorities and can recognize certificates. A website administrator needs to create a certificate and get it signed by a certificate authority.