A computer-network authentication protocol that works on the basis of tickets.

Understanding Kerberos

Kerberos ensures a high level of security to network resources. It is compromised by 3 different parties. The Key Distribution Center (KDC), the client user and the server with the desired service to access. The KDC is installed as part of the domain controller and performs two service functions: the Authentication Service (AS) and the Ticket-Granting Service (TGS).

AS (Authentication Service) Exchange

When initially login on to a network, users must negotiate access by providing a log-in name and password in order to be verified by the AS portion of a KDC within their domain. The KDC has access to Active Directory user account information. Once successfully authenticated, the user is granted a Ticket Granting Ticket (TGT) that is valid for the local domain. The TGT has a default lifetime of 10 hours and may be renewed throughout the user's log-on session without requiring the user to re-enter his password. The TGT is cached on the local machine in volatile memory space and used to request sessions with services throughout the network.

The AS request identifies the client to the KDC in plain text. If preauthentication is enabled, a time stamp will be encrypted using the user's password hash as an encryption key. If the KDC reads a valid time when using the user's password hash (stored in Active Directory) to decrypt the time stamp, the KDC knows that request isn't a replay of a previous request. The preauthentication feature mat be disabled for a specific user in order to support some applications that don't support the security feature. You can UN-check the "Do not require Kerberos" option in AD.

If the KDC approves the client's request for a TGT, the reply (referred to as the AS reply) will include two sections: a TGT encrypted with a key that only the KDC (TGS) can decrypt and a session key encrypted with the user's password hash to handle future communications with the KDC. Because the client system cannot read the TGT contents, it must blindly present the ticket to the TGS for service tickets. The TGT includes time to live parameters, authorization data, a session key to use when communicating with the client and the client's name..

TGS Exchange

The user presents the TGT to the TGS portion of the KDC when desiring access to a server service. The TGS on the KDC authenticates the user's TGT and creates a ticket and session key for both the client and the remote server. This information, known as the service ticket, is then cached locally on the client machine.

The TGS receives the client's TGT and reads it using its own key. If the TGS approves of the client's request, a service ticket is generated for both the client and the target server. The client reads its portion using the TGS session key retrieved earlier from the AS reply. The client presents the server portion if the TGS reply to the target server in the client/server exchange.

Client/Server Exchange Detail

The client blindly passes the server portion of the service ticket to the server in the client/server request to establish a client/server session. If mutual authentication is enabled, the target server runs a time stamp encrypted using the service ticket session key. If the time stamps decrypts correctly, not only has the client authenticated himself to the server, but the server also has authenticated itself to the client. The target server never has to directly communicate with the KDC. This reduces downtime and pressure on the KDC.

A TGT and a service ticket are needed to access services on remote computers, but they also required to successfully log on to a local system. When the log-on window appears, password encryption using a one-way hash algorithm occurs immediately and negotiations commence with the KDC for a valid TGT and service ticket. The process is the same as accessing a remote service. An access token is created for the user containing all security groups to which they belong. This access token is attached to the user's log-on session and is subsequently inherited by any process or application the user starts.

Referral Tickets

The AS and TGS functions are separate within the KDC. This permits the user to use the TGT obtained from an AS in his domain to obtain service tickets from a TGS in other domains. This is accomplished through referral tickets.

Once a trust has been established between two domains, referral tickets can be granted to clients requesting authorization for services in other domains. When there is a trust established between the two domains, an inter-domain key based on the trust password becomes available for authenticating KDC functions. This can best be explained by example of a user/client seeking services in another domain. User client in request authority for a server in User utilizes referral tickets.

  1. The client contacts it's domain KDC TGS using a TGT. The KDC recognizes a request for a session with a foreign domain server and responds by returning a referral ticket for the KDC in the foreign domain.

  2. The client contacts the KDC of the foreign domain with the referral ticket. This ticket is encrypted with the inter-domain key. Given that the decryption works, the TGS service for the foreign domain returns a service ticket for the server service.

  3. The client performs the client/server exchange with the server and begins the user session with the service.

This is only a very small understanding on what is going on with Kerberos.


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