Kerberos

Kerberos is a network authentication protocol designed to provide strong authentication for client/server applications by using secret-key cryptography. It enables nodes to prove their identity and to obtain service tickets to access resources on insecure networks. It was developed at MIT as part of Project Athena and named after the three-headed guardian of the underworld in Greek mythology.

In Kerberos, a trusted entity called the Key Distribution Center, or KDC, issues tickets. When a user logs in, the client sends credentials to the Authentication Service (AS) portion of the KDC, which issues a Ticket-Granting Ticket (TGT) after validating the user’s secret key. The TGT proves the user’s identity and is used to request service tickets from the Ticket-Granting Service (TGS). The client presents the TGT to the TGS to obtain a service ticket for a specific server. The service ticket is used to authenticate to that server; tickets and session keys are protected with keys shared between the KDC and the service. Tickets have lifetimes and the process uses synchronized time to prevent replay attacks.

Kerberos version 5 is the current standard, specified in RFC 4120, with extensions in related RFCs. It supports mutual authentication, cross-realm authentication, and single sign-on across services within a Kerberos realm or trusted realms. It is widely deployed in enterprise environments, notably Windows Active Directory and various Unix-like systems via implementations from MIT, Heimdal, and macOS.

Security and operational considerations: Kerberos relies on trusted time synchronization and secure key management. Practically deployed with keytab files for services and strong password handling. Its security model assumes the KDC is trusted; compromise of the KDC undermines the entire realm. Ticket lifetimes and renewal policies balance usability and risk.