TCP/IP Tutorial and Technical Overview - IBM Redbooks

After the server has validated a client, an option exists for the client to validate
the server. This prevents an intruder from impersonating the server. The
client requires then that the server sends back a message containing the time
stamp (from the client's authenticator, with one added to the time stamp
value). This message is enciphered using the session key that was passed
from the client to the server.
Let us summarize some of the central points in this scheme:
► In order for the workstation to use any end server, a ticket is required. All
tickets, other than the first ticket (also called the initial ticket), are obtained
from the TGS. The first ticket is special; it is a ticket for the TGS itself and is
obtained from the Kerberos authentication server.
► Every ticket is associated with a session key that is assigned every time a
ticket is allocated.
► Tickets are reusable. Every ticket has a lifetime, typically eight hours. After a
ticket has expired, you have to identify yourself to Kerberos again, entering
your login name and password.
► Unlike a ticket, which can be reused, a new authenticator is required every
time the client initiates a new connection with a server. The authenticator
carries a time stamp within it, and the authenticator expires a few minutes
after it is issued. (This is the reason why clocks must be synchronized
between clients and servers.)
► A server maintains a history of previous client requests for which the time
stamp in the authenticator is still valid. This way, a server can reject duplicate
requests that might arise from a stolen ticket and authenticator.
22.11.4 Kerberos database management
Kerberos needs a record of each user and service in its realm and each record
keeps only the needed information, as follows:
► Principal identifier (c,s)
► Private key for this principal (K c ,K s )
► Date of expiration for this identity
► Date of the last modification in this record
► Identity of the principal that last modified this record (c,s)
► Maximum lifetime of tickets to be given to this principal (lifetime)
► Attributes (unused)
► Implementation data (not visible externally)
The private key field is enciphered using a master key so that removing the
database will not cause any problem because the master key is not in it.
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