Kerberos

The Salient Points:

• A "trusted third party" system using symmetric cryptography (DES)

• Allows authentication of clients and servers in an untrusted network

• Clients obtain "tickets" for services from a Ticket Granting Service

• Ticket sent by client contains a session key
  ==> encrypted channel may be established between client and server

• Comes in two major flavors: V4, V5 (which aren't very compatible)

• Widely used and extensively tested

The Kerberos FAQ