From MIT's Kerberos page: Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key cryptography. A free implementation of this protocol is available from the Massachusetts Institute of Technology. Kerberos is available in many commercial products as well.
The site goes on to say Kerberos provides strong cryptography and is freely available from MIT under a BSD-like license. This information can be found on the aforementioned page.