Shell Programming and Scripting

Protocol

The security of the protocol relies heavily on participants maintaining loosely synchronized time and on short-lived assertions of authenticity called Kerberos tickets.

What follows is a simplified description of the protocol. The following abbreviations will be used:

* AS = Authentication Server
* SS = Service Server
* TGS = Ticket-Granting Server
* TGT = Ticket-Granting Ticket

The client authenticates to the AS once using a long-term shared secret (e.g. a password) and receives a TGT from the AS. Later, when the client wants to contact some SS, it can (re)use this ticket to get additional tickets for SS without resorting to using the shared secret. These tickets can be used to prove authentication to SS.

The phases are detailed below.

User Client-based Logon

1. A user enters a username and password on the client machine.
2. The client performs a one-way function (hash mostly) on the entered password, and this becomes the secret key of the client/user.

Client Authentication

1. The client sends a cleartext message to the AS requesting services on behalf of the user. Sample message: "User XYZ would like to request services". Note: Neither the secret key nor the password is sent to the AS. However, even though the secret key is not sent to the AS, the AS is still able to generate the same secret key by hashing the password for the client/user from its own security database (e.g. Active Directory in Windows Server

2. 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 (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.

3. Once the client receives messages A and B, it 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

1. When requesting services, the client sends the following two 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.

2. 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

1. 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.

2. 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 timestamp found in client's Authenticator plus 1, encrypted using the Client/Server Session Key.

3. The client decrypts the confirmation using the Client/Server Session Key and checks whether the timestamp is correctly updated. If so, then the client can trust the server and can start issuing service requests to the server

4. The server provides the requested services to the client.