Exam - Kerberos

Exam - Kerberos

1. principals & their roles

   • A Kerberos principal is a unique identity to which Kerberos can assign tickets. Principals can have an arbitrary number of components: the primary, the instance and the realm.
     • format typically: primary / instance @ realm

2. applications

   • Campus network where access to various resources (printing, file storage (SMB), computing time, proxy authentication, authorisation) needs to be controlled for a population of users, but where the servers do not necessariyl know about (or trust) the users.

3. weaknesses

   • availability
     • Kerberos has a single point of failure at the authentication server or the ticket granting server.
   • scalability
     • Kerberos systems can only scale to support as much as the central authentication and/ or TGT servers can handle.
   • revocation
     • Ticket granting tickets are good for 10 hours. If a ticket is compromised, there is no mechanism to revoke the ticket.
   • time synchronization reliance
     • Clocks on the network cannot be more than 5 minutes out of sync for Kerberos to work.
   • TGT lifetime
     • The relatively long life and the fixed structure of the TGT opens the door for offline attacks to figure out the encryption key. In Kerberos version 4, the encrpytion algorithm was DES which can be compromised today.

4. entities

   • authentication server
   • ticekt granting server
   • client
   • server

5. ***Authentication and Key Exchange***

   • HIGH LEVEL! For exam detail see shared notes!!
   • User authenticates to authentication server (AS).
     • AS sends a ticket granting ticket to the ticket granting server (TGS).
       • User forwards ticket granting ticket to TGS with a request to access a different network server.
         • TGS decrypts ticket granting ticket with key shared between AS and TGS.
           • TGS checks ticket for validity and correct timeframe.
             • TGS sends a service granting ticket to user.
               • User forwards service granting ticket to network server.
                 • Network server decrypts service granting ticket with key shared between TGS and network server.
                   • If service granting ticket is valid and within the correct timeframe, user is allowed access to server.