Kerberos

Open source (non-proprietary) and public domain (free) user authentication scheme typically used as network security system in a client-server environment. Kerberos verifies whether a system user is legitimate both at the time of user log-in, and every time he or she requests a service. It uses private or secret keys (as opposed to public keys) called 'tickets' to encrypt data. Developed at Massachusetts Institute of Technology (MIT), Kerberos is based on symmetric key cryptography and data encryption standard algorithms, and is named after the fierce three-headed guard dog of the Greek mythology.