KERBEROS
Kerberos is a secret key cryptography system using a KDC -- Key distribution center which keep authorized people and their master keys. A more precise statement is that the client has knowledge of an encryption key that is known by only the user and the authentication server. Similarly, each application server shares an encryption key with the authentication server, i.e., the server key. The Kerberos Authentication System uses a series of encrypted messages to prove to a verifier that a client is running on behalf of a particular user.
The Kerberos protocol uses strong cryptography so that a client can prove its identity to a server (and vice versa) across an insecure network connection. After a client and server has used Kerberos to prove their identity, they can also encrypt all of their communications to assure privacy and data integrity as they go about their business.