Theory

Kerberos is a network authentication protocol that enables secure communication over an insecure network. It allows clients and servers to prove their identities to each other without transmitting passwords over the network. The protocol relies on a trusted third party—called the **Kerberos Authentication Server (AS)**—to authenticate users and issue tickets that grant access to network services.

Kerberos uses symmetric key cryptography, where the same key is used to encrypt and decrypt messages. This approach ensures confidentiality and integrity during communication between clients and servers.

When a user logs in, the following steps occur:

1. Request for a Ticket
   The client sends a request to the Authentication Server (AS), including the user's identity and a timestamp, to access a specific network service.

2. Authentication Server Response
   The AS responds with a Ticket Granting Ticket (TGT) and a session key, both encrypted using the user's password. The session key is a temporary cryptographic key used to secure communication between the user and the ticket-granting system.

3. Request for a Service Ticket
   The client then sends the TGT and a request for a service ticket to the Ticket Granting Server (TGS).

4. Service Ticket Response
   The TGS validates the TGT and issues a service ticket, which contains another session key encrypted using the service’s secret key.

5. Service Request
   The client presents this service ticket to the desired service (e.g., a file server), establishing trust without revealing credentials.

6. Service Response
   The service decrypts the ticket, validates it, and initiates secure communication with the client using the shared session key.

Kerberos provides mutual authentication:

• The client authenticates to the AS with its credentials.
• The AS proves its identity by sending an encrypted TGT.
• The service trusts the client because the TGS vouches for it through the ticket.

This protocol is widely used in enterprise environments (notably Microsoft Windows domains) and is known for its security and efficiency.