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Kerberos builds on Symmetric Key Cryptography and requires a Trusted Third Party . Extensions to Kerberos can provide for the use of Public Key Cryptography during certain phases of the authentication protocol. HISTORY AND DEVELOPMENT MIT developed Kerberos to protect network services provided by Project Athena . The protocol was named after the Greek Mythological character '' Kerberos '' (or ''Cerberus''), known in Greek mythology as being the ''monstrous three-headed guard dog of Hades ''. Several versions of the protocol exist; versions 1–3 occurred only internally at MIT. Steve Miller and Clifford Neuman, the primary designers of Kerberos version 4, published that version in the late 1980s, although they had targeted it primarily for Project Athena. Version 5, designed by John Kohl and Clifford Neuman, appeared as RFC 1510 in 1993 (made obsolete by RFC 4120 in 2005), with the intention of overcoming the limitations and security problems of version 4. MIT makes an implementation of Kerberos freely available, under copyright permissions similar to those used for BSD . Authorities in the in Sweden , made the system available outside the US before the US changed its Cryptography Export regulations (''circa'' 2000). The Swedish implementation was based on a version called eBones. eBones was based on the exported MIT Bones release (stripped of both the encryption functions and the calls to them) based on version Kerberos 4 patch-level 9. This somewhat limited Kerberos was called the eBones release. A Kerberos version 5 implementation, Heimdal , was released by basically the same group of people releasing KTH-KRB. Windows 2000 , Windows XP and Windows Server 2003 use a variant of Kerberos as their default authentication method. Some Microsoft additions to the Kerberos suite of protocols are documented in RFC 3244 "Microsoft Windows 2000 Kerberos Change Password and Set Password Protocols". Apple's Mac OS X also uses Kerberos in both its client and server versions. As Of 2005 , the IETF Kerberos workgroup is updating the specifications {Link without Title} . Recent updates include:
DESCRIPTION Kerberos uses as its basis the Needham-Schroeder protocol. It makes use of a Trusted Third Party , termed a ''Key Distribution Center'' (KDC), which consists of two logically separate parts: an Authentication Server (AS) and a Ticket Granting Server (TGS). Kerberos works on the basis of "tickets" which serve to prove the identity of users. The KDC maintains a database of secret keys; each entity on the network — whether a client or a server — shares a secret key known only to itself and to the KDC. Knowledge of this key serves to prove an entity's identity. For communication between two entities, the KDC generates a Session Key which they can use to secure their interactions. USES The following software is able to use Kerberos for authentication:
PROTOCOL One can specify the protocol as follows in Security Protocol Notation , where ''Alice'' (''A'') authenticates herself to ''Bob'' (''B'') using a server ''S''. Here,
:A asks S to initiate communication with B :S generates a fresh KAB, and sends it to A together with a timestamp and the same data encrypted for B. :A passes on the message to B, obtains a new TA and passes it under the new session key :B confirms receipt of the session key by returning a modified version of the timestamp to A We see here that the Security of the protocol relies heavily on timestamps T and lifespans L as reliable indicators of the ''freshness'' of a communication (see the BAN Logic ). In relation to the following Kerberos operation, it is helpful to note that the server S here stands for both authentication service (AS), and ticket granting service (TGS). In , is the client to server ticket, is the authenticator, and confirms B's true identity and its recognition of A. This is required for mutual authentication. KERBEROS OPERATION What follows is a simplified description of the protocol. The following abbreviations will be used:
In one sentence: the client authenticates itself to AS, then demonstrates to the TGS that it's authorized to receive a ticket for a service (and receives it), then demonstrates to the SS that it has been approved to receive the service. In more detail: # A user enters a username and password on the Client . # The client performs a One-way Hash on the entered password, and this becomes the secret key of the client. # The client sends a Clear-text Message to the AS requesting services on behalf of the user. Sample Message: "User XYZ would like to request services". Note: Neither the secret key nor the password is sent to the AS. # The AS checks to see if the client is in its database. If it is, the AS sends back the following two messages to the client:
# Once the client receives messages A and B, it decrypts message A to obtain the ''client/TGS session key''. This session key is used for further communications with TGS. (Note: The client cannot decrypt the Message B, as it is encrypted using TGS's secret key.) At this point, the client has enough information to authenticate itself to the TGS. # When requesting services, the client sends the following two messages to the TGS:
# Upon receiving messages C and D, the TGS decrypts message D (Authenticator) using the ''client/TGS session key'' and sends the following two messages to the client:
# Upon receiving messages E and F from TGS, the client has enough information to authenticate itself to the SS. The client connects to the SS and sends the following two messages:
# The SS decrypts the ticket using its own secret key and sends the following message to the client to confirm its true identity and willingness to serve the client:
# The client decrypts the confirmation using the ''client/server session key'' and checks whether the timestamp is correctly updated. If so, then the client can trust the server and can start issuing service requests to the server. # The server provides the requested services to the client. KERBEROS DRAWBACKS
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