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Kerberos is a symmetric-key authentication scheme developed at MIT for verifying the identities of users and devices in client/server network environments. Kerberos is called a trusted third-party authentication protocol, meaning that it runs on a server that is separate from any client or server. The name comes from the three-headed dog that guarded the entrance to Hades.

The Kerberos server is called the AS (authentication server). When a client needs to access some server, the clients access the Kerberos server to obtain a ticket. Possession of the ticket defines access. This assumes that both the client and the server trust the "third-party" Kerberos authentication server.