TCP/IP Tutorial and Technical Overview - IBM Redbooks

The authentication process consists of exchanging five messages (see
Figure 22-52 on page 867):
1. Client → KAS
The client sends a message {c, tgs, n} to the KAS, containing its identity (c), a
nonce (a time stamp or other means to identify this request), and requests for
a ticket for use with the ticket-granting server (TGS).
2. KAS → client
The authentication server looks up the client name (c) and the service name
(the ticket-granting server, tgs) in the Kerberos database and obtains an
encryption key for each (Kc and Ktgs ).
The KAS then forms a response to send back to the client. This response
contains an initial ticket T c,tgs, which grants the client access to the requested
server (the ticket-granting server). T c,tgs contains K c,tgs, c, tgs, nonce,
lifetime, and some other information. The KAS also generates a random
encryption key K c,tgs, called the session key. It then encrypts this ticket using
the encryption key of the ticket-granting server (K tgs). This produces what is
called a sealed ticket {T c,tgs}K tgs. A message is then formed consisting of the
sealed ticket and the TGS session key K c,tgs.
Note: In Kerberos Version 4, the message is:
{Kc,tgs,n,{Tc,tgs}Ktgs}Kc
While in Kerberos Version 5, the message is of a simpler form:
{Kc,tgs, n}Kc, {Tc,tgs}Ktgs
This simplifies the (unnecessary) double encryption of the ticket.
3. Client → TGS
Upon receiving the message, the client decrypts it using its secret key Kc, which is only known to it and the KAS. It checks to see if the nonce (n)
matches the specific request, and then caches the session key Kc,tgs for
future communications with the TGS.
The client then sends a message to the TGS. This message contains the
initial ticket {Tc,tgs}Ktgs, the server name (s), a nonce, and a new authenticator
Ac containing a time stamp. Ac is {c, nonce}. The message is:
{A c }K c,tgs , {T c,tgs }K tgs , s, n
868 TCP/IP Tutorial and Technical Overview