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17.2. Kerberos Terminology

Kerberos has its own terminology to define various aspects of the service. Before learning how kerberos works, it is important to learn the following terms.

ciphertext

Encrypted data.

client

An entity on the network (a user, a host, or an application) that can get a ticket from Kerberos.

credential cache or ticket file

A file which contains the keys for encrypting communications between a user and various network services. Kerberos 5 supports a framework for using other cache types, such as shared memory, but files are more thoroughly supported.

crypt hash

A one way hash used to authenticate users. While more secure than plain text, it is fairly easy to decrypt for an experienced cracker.

GSS-API

The Generic Security Service Application Program Interface [RFC-2743] is a set of functions which provide security services which clients can use to authenticate to servers and which servers can use to authenticate to clients without specific knowledge of the underlying mechanism. If a network service (such as IMAP) uses GSS-API, it can authenticate using Kerberos.

key

Data used when encrypting or decrypting other data. Encrypted data cannot be decrypted without the proper key or extremely good guessing.

Key Distribution Center (KDC)

A service that issues Kerberos tickets, usually run on the same host as the Ticket Granting Server

key table or keytab

A file that includes an unencrypted list of principals and their keys. Servers retrieve the keys they need from keytab files instead of using kinit. The default keytab file is /etc/krb5.keytab. The KDC administration server, /usr/kerberos/sbin/kadmind, is the only service that uses any other file (it uses /var/kerberos/krb5kdc/kadm5.keytab).

kinit

The kinit command allows a principal who has already logged in to obtain and cache the initial Ticket Granting Ticket (TGT). For more on using the kinit command, see its man page.

principal

The principal is the unique name of a user or service that can authenticates using Kerberos. A principal's name is in the form root[/instance]@REALM. For a typical user, the root is the same as their login ID. The instance is optional. If the principal has an instance, it is separated from the root with a forward slash ("/"). An empty string ("") is considered a valid instance (which differs from the default NULL instance), but using it can be confusing. All principals in a realm have their own key, which for users is derived from a password or is randomly set for services.

realm

A network that uses Kerberos, composed of one or more servers called KDCs and a potentially large number of clients.

service

A program accessed over the network.

ticket

A temporary set of electronic credentials that verify the identity of a client for a particular service.

Ticket Granting Service (TGS)

A server that issues tickets for a desired service which are in turn given to users for access to the service. The TGS usually runs on the same host as the KDC

Ticket Granting Ticket (TGT)

A special ticket that allows the client to obtain additional tickets without applying for them from the KDC.

unencrypted password

A plain text, human-readable password.

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