Bill is a security professional. He is in a meeting with co-workers and describes a system that will make web sessions more secure. He says when a user connects to the web server and starts a secure session, the server sends a certificate to the user. The certificate includes a public key. The user can encrypt data with the public key and send it to the server. Because the server holds the private key, it can decrypt the data. Because no other entity has the private key, no one else can decrypt the data. What is Bill describing