Heimdal is a free implementation of Kerberos 5. Kerberos is a system for authenticating users and services on a network. It is built upon the assumption that the network is "unsafe". Kerberos is a trusted third-party service. That means that there is a third party (the Kerberos server) that is trusted by all the entities on the network (users and services, usually called "principals"). All principals share a secret password (or key) with the Kerberos server and this enables principals to verify that the messages from the Kerberos server are authentic. Thus trusting the Kerberos server, users and services can authenticate each other.