Kerberos is a protocol for authenticating service requests between trusted hosts across an untrusted network, such as the internet. Kerberos is built in to all major operating systems, including Microsoft Windows, Apple OS X, FreeBSD and Linux.
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Since Windows 2000, Microsoft has incorporated the Kerberos protocol as the default authentication method in Windows, and it is an integral component of the Windows Active Directory service. Broadband service providers also use Kerberos to authenticate DOCSIS cable modems and set-top boxes accessing their networks.
Kerberos was originally developed for Project Athena at the Massachusetts Institute of Technology (MIT). The name Kerberos was taken from Greek mythology; Kerberos (Cerberus) was a three-headed dog who guarded the gates of Hades. The three heads of the Kerberos protocol represent a client, a server and a Key Distribution Center (KDC), which acts as Kerberos' trusted third-party authentication service.
Users, machines and services using Kerberos need only trust the KDC, which runs as a single process and provides two services: an authentication service and a ticket granting service. KDC "tickets" provide mutual authentication, allowing nodes to prove their identity to one another in a secure manner. Kerberos authentication uses conventional shared secret cryptography to prevent packets traveling across the network from being read or changed and to protect messages from eavesdropping and replay attacks.
Kerberos protocol overview
A simplified description of how Kerberos works follows; the actual process is more complicated and may vary from one implementation to another. For the purposes of this discussion, the initiating client in the scenario below is a corporate laptop running Windows, and an end user is trying to log into the corporate network.
To start the Kerberos authentication process, the initiating client sends a request to an authentication server for access to a service. The initial request is sent as plaintext because no sensitive information is included in the request.
The authentication server retrieves the initiating client's private key, assuming the initiating client's username is in the KDC database. If the initiating client's username cannot be found in the KDC database, the client cannot be authenticated and the authentication process stops. If the client's username can be found in the KDC database, the authentication server generates a session key and a ticket granting ticket. The ticket granting ticket is timestamped and encrypted by the authentication server with the initiating client's password.
The initiating client is then prompted for a password; if what is entered matches the password in the KDC database, the encrypted ticket granting ticket sent from the authentication server is decrypted and used to request a credential from the ticket granting server for the desired service. The client sends the ticket granting ticket to the ticket granting server, which may be physically running on the same hardware as the authentication server, but performing a different role.
The ticket granting service carries out an authentication check similar to that performed by the authentication server, but this time sends credentials and a ticket to access the requested service. This transmission is encrypted with a session key specific to the user and service being accessed. This proof of identity can be used to access the requested "kerberized" service, which, once having validated the original request, will confirm its identity to the requesting system.
The timestamped ticket sent by the ticket granting service allows the requesting system to access the service using a single ticket for a specific time period without having to be re-authenticated. Making the ticket valid for a limited time period makes it less likely that someone else will be able to use it later; it is also possible to set the maximum lifetime to 0, in which case service tickets will not expire. Microsoft recommends a maximum lifetime of 600 minutes for service tickets; this is the default value in Windows Server implementations of Kerberos.
The MIT Kerberos Consortium was founded in September 2007 to further the development of Kerberos. In 2013, the consortium was expanded and renamed the MIT Kerberos and Internet Trust Consortium.
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