The client authenticates itself to the Authentication Server (AS) which forwards the username to a key distribution center (KDC).
The KDC issues a ticket-granting ticket (TGT), which is time stamped, encrypts it using the user’s password and returns the encrypted result to the user’s workstation.
This is done infrequently, typically at user logon; the TGT expires at some point, though may be transparently renewed by the user’s session manager while they are logged in.
When the client needs to communicate with another node (“principal” in Kerberos parlance) the client sends the TGT to the ticket-granting service (TGS),
which usually shares the same host as the KDC. After verifying the TGT is valid and the user is permitted to access the requested service, the TGS issues a ticket and session keys,
which are returned to the client. The client then sends the ticket to the service server (SS) along with its service request.
The protocol is described in detail below.
User Client-based Logon
A user enters a username and password on the client machines. Other credential mechanisms like pkinit (RFC4556) allow for the use of public keys in place of a password.
The client transforms the password into the key of a symmetric cipher. This either uses the built in key scheduling or a one-way hash depending on the cipher-suite used.
The client sends a cleartext message of the user ID to the AS (Authentication Server) requesting services on behalf of the user.
(Note: Neither the secret key nor the password is sent to the AS.) The AS generates the secret key by hashing the password of the user found at the database
(e.g., Active Directory in Windows Server).
The AS checks to see if the client is in its database. If it is, the AS sends back the following two messages to the client:
Message A: Client/TGS Session Key encrypted using the secret key of the client/user.
Message B: Ticket-Granting-Ticket (TGT,which includes the client ID, client network address, ticket validity period, and the client/TGS session key)
encrypted using the secret key of the TGS.
Once the client receives messages A and B, it attempts to decrypt message A with the secret key generated from the password entered by the user.
If the user entered password does not match the password in the AS database, the client’s secret key will be different and thus unable to decrypt message A.
With a valid password and secret key the client decrypts message A to obtain the Client/TGS Session Key.
This session key is used for further communications with the TGS. (Note: The client cannot decrypt Message B, as it is encrypted using TGS’s secret key.)
At this point, the client has enough information to authenticate itself to the TGS.
Client Service Authorization
When requesting services, the client sends the following messages to the TGS:
Message C: Composed of the TGT from message B and the ID of the requested service.
Message D: Authenticator (which is composed of the client ID and the timestamp), encrypted using the Client/TGS Session Key.
Upon receiving messages C and D, the TGS retrieves message B out of message C. It decrypts message B using the TGS secret key.
This gives it the “client/TGS session key”. Using this key, the TGS decrypts message D (Authenticator) and sends the following two messages to the client:
Message E: Client-to-server ticket (which includes the client ID, client network address, validity period and Client/Server Session Key) encrypted using
the service’s secret key.
Message F: Client/Server Session Key encrypted with the Client/TGS Session Key.
Client Service Request
Upon receiving messages E and F from TGS, the client has enough information to authenticate itself to the SS.
The client connects to the SS and sends the following two messages:
Message E from the previous step (the client-to-server ticket, encrypted using service’s secret key).
Message G: a new Authenticator, which includes the client ID, timestamp and is encrypted using Client/Server Session Key.
The SS decrypts the ticket using its own secret key to retrieve the Client/Server Session Key. Using the sessions key,
SS decrypts the Authenticator and sends the following message to the client to confirm its true identity and willingness to serve the client:
Message H: the time stamp found in client’s Authenticator , encrypted using the Client/Server Session Key.
The client decrypts the confirmation using the Client/Server Session Key and checks whether the times tamp is correct. If so, then the client can trust the server
and can start issuing service requests to the server.
The server provides the requested services to the client.