Kerberos V5 is an authentication system developed at MIT.  
WWW: http://web.mit.edu/kerberos/www/

Abridged from the User Guide:
       Under Kerberos, a client sends a request for a ticket to the 
   Key Distribution Center (KDC). The KDC creates a ticket-granting 
   ticket (TGT) for the client, encrypts it using the client's 
   password as the key, and sends the encrypted TGT back to the 
   client. The client then attempts to decrypt the TGT, using
   its password. If the client successfully decrypts the TGT, it 
   keeps the decrypted TGT, which indicates proof of the client's 
   identity. The TGT permits the client to obtain additional tickets, 
   which give permission for specific services.
       Since Kerberos negotiates authenticated, and optionally encrypted,   
   communications between two points anywhere on the internet, it
   provides a layer of security that is not dependent on which side of a
   firewall either client is on.
       The Kerberos V5 package is designed to be easy to use. Most of the  
   commands are nearly identical to UNIX network programs you are already
   used to. Kerberos V5 is a single-sign-on system, which means that you 
   have to type your password only once per session, and Kerberos does   
   the authenticating and encrypting transparently.  

Jacques Vidrine <n@nectar.com>