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Kerberos (protocol)

Kerberos (play /ˈkɛərbərəs/) is a computer network authentication protocol which works on the basis of "tickets" to allow nodes communicating over a non-secure network to prove their identity to one another in a secure manner. Its designers aimed primarily at a client–server model, and it provides mutual authentication — both the user and the server verify each other's identity. Kerberos protocol messages are protected against eavesdropping and replay attacks. Kerberos builds on symmetric key cryptography and requires a trusted third party, and optionally may use public-key cryptography by utilizing asymmetric key cryptography during certain phases of authentication.[1] Kerberos uses port 88 by default.

"Kerberos" also refers to a suite of free software published by Massachusetts Institute of Technology (MIT) that implements the Kerberos protocol.



[edit]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. In 2007, MIT formed the Kerberos Consortium to foster continued development. Founding sponsors include vendors such as Oracle,Apple Inc.GoogleMicrosoft and Centrify Corporation, and academic institutions such as KTH-Royal Institute of Technology, Stanford University, MIT and vendors such as CyberSafe offering commercially supported versions.

Authorities in the United States classified Kerberos as auxiliary military technology and banned its export because it used the DES encryption algorithm (with 56-bit keys). A non-US Kerberos 4 implementation, KTH-KRB developed at the Royal Institute of Technology 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 limited 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.

Windows 2000 and later use 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".RFC 4757 documents Microsoft's use of the RC4 cipher. While Microsoft uses the Kerberos protocol, it does not use the MIT software.

Many UNIX and UNIX-like operating systems, including FreeBSD, Apple's Mac OS XRed Hat Enterprise Linux 4, Sun's Solaris, IBM's AIX, HP's OpenVMS, and others, include software for Kerberos authentication of users or services.

As of 2005, the IETF Kerberos working group is updating the specifications. Recent updates include:

  • Encryption and Checksum Specifications" (RFC 3961).
  • Advanced Encryption Standard (AES) Encryption for Kerberos 5 (RFC 3962).
  • A new edition of the Kerberos V5 specification "The Kerberos Network Authentication Service (V5)" (RFC 4120). This version obsoletes RFC 1510, clarifies aspects of the protocol and intended use in a more detailed and clearer explanation.
  • A new edition of the GSS-API specification "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2." (RFC 4121).


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Kerberos uses as its basis the symmetric 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).

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 purposes the KDC generates a session key which communicating parties use to encrypt their transmissions.

The security of the protocol relies heavily on short-lived assertions of authenticity called Kerberos tickets.


Here is an intuitive description. The client authenticates itself to the AS and receives a ticket with a timestamp. The client then contacts the TGS, and using the ticket to prove its identity it asks for a service. If the client is eligible for the service, the TGS sends another ticket to the client. The client then contacts the server providing the service (SS) and uses the latter ticket to prove that it has approval to receive the service.

Here is another description.

Kerberos sch en
Kerberos negotiations

The client authenticates to the AS once using a long-term shared secret (e.g. a password) and receives a Ticket Granting Ticket (TGT) from the AS. Later, when the client wants to contact some SS, it can (re)use this ticket to get additional tickets from TGS, for SS, without resorting to using the shared secret. The latter tickets can be used to prove authentication to the SS.

The phases are detailed below.

[edit]User Client-based Logon

  1. A user enters a username and password on the client machine.
  2. The client performs a one-way function (hash usually) on the entered password, and this becomes the secret key of the client/user.

[edit]Client Authentication

Kerberos Protocol
  1. The client sends a cleartext message of the user ID to the AS requesting services on behalf of the user. (Note: Neither the secret key nor the password is sent to the AS.) The AS generates the secret key by hashing the password of the user found at the database (e.g. Active Directory in Windows Server).
  2. 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:
    • Message A: Client/TGS Session Key encrypted using the secret key of the client/user.
    • Message B: Ticket-Granting-Ticket (which includes the client ID, client network address, ticket validity period, and the client/TGS session key) encrypted using the secret key of the TGS.
  3. Once the client receives messages A and B, it attempts to decrypt message A with the secret key generated from the password entered by the user. If the user entered password does not match the password in the AS database, the client's secret key will be different and thus unable to decrypt message A. With a valid password and secret key the client decrypts message A to obtain the Client/TGS Session Key. This session key is used for further communications with the TGS. (Note: The client cannot decrypt 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.

[edit]Client Service Authorization

  1. When requesting services, the client sends the following two messages to the TGS:
    • Message C: Composed of the TGT from message B and the ID of the requested service.
    • Message D: Authenticator (which is composed of the client ID and the timestamp), encrypted using the Client/TGS Session Key.
  2. Upon receiving messages C and D, the TGS retrieves message B out of message C. It decrypts message B using the TGS secret key. This gives it the "client/TGS session key". Using this key, the TGS decrypts message D (Authenticator) and sends the following two messages to the client:
    • Message E: Client-to-server ticket (which includes the client ID, client network address, validity period and Client/Server Session Key) encrypted using the service's secret key.
    • Message F: Client/server session key encrypted with the Client/TGS Session Key.

[edit]Client Service Request

  1. 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:
    • Message E from the previous step (the client-to-server ticket, encrypted using service's secret key).
    • Message G: a new Authenticator, which includes the client ID, timestamp and is encrypted using client/server session key.
  2. The SS decrypts the ticket using its own secret key to retrieve the Client/Server Session Key. Using the sessions key, SS decrypts the Authenticator and sends the following message to the client to confirm its true identity and willingness to serve the client:
    • Message H: the timestamp found in client's Authenticator plus 1, encrypted using the Client/Server Session Key.
  3. 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.
  4. The server provides the requested services to the client.

[edit]Drawbacks and Limitations

  • Single point of failure: It requires continuous availability of a central server. When the Kerberos server is down, no one can log in. This can be mitigated by using multiple Kerberos servers and fallback authentication mechanisms.
  • Kerberos has strict time requirements, which means the clocks of the involved hosts must be synchronized within configured limits. The tickets have a time availability period and if the host clock is not synchronized with the Kerberos server clock, the authentication will fail. The default configuration per MIT requires that clock times are no more than five minutes apart. In practice Network Time Protocol daemons are usually used to keep the host clocks synchronized.
  • The administration protocol is not standardized and differs between server implementations. Password changes are described in RFC 3244.
  • Since all authentication is controlled by a centralized KDC, compromise of this authentication infrastructure will allow an attacker to impersonate any user.

[edit]See also