Kerberos () is a computer-network authentication protocol that 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 it 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 during certain phases of authentication. Kerberos uses UDP port 88 by default. The protocol was named after the character Cerberus, also spelled Kerberos, from Greek mythology, the ferocious three-headed guard dog of Hades.
History and development The Massachusetts Institute of Technology (MIT) developed Kerberos in 1988 to protect network services provided by Project Athena. Its first version was primarily designed by Steve Miller and Clifford Neuman based on the earlier Needham–Schroeder symmetric-key protocol. Kerberos versions 1 through 3 were experimental and not released outside of MIT. Kerberos version 4, the first public version, was released on January 24, 1989. Since Kerberos 4 was developed in the United States, and since it used the Data Encryption Standard (DES) encryption algorithm, U.S. export control restrictions prevented it from being exported to other countries. MIT created an exportable version of Kerberos 4 with all encryption code removed, called "Bones". Eric Young of Australia's Bond University reimplemented DES into Bones, in a version called "eBones", which could be freely used in any country. Sweden's Royal Institute of Technology released another reimplementation called KTH-KRB. Neuman and John Kohl published version 5 in 1993 with the intention of overcoming existing limitations and security problems. Version 5 appeared as RFC 1510, which was then made obsolete by RFC 4120 in 2005. In 2005, the Internet Engineering Task Force (IETF) Kerberos working group updated specifications. Updates included:
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 Generic Security Services Application Program Interface (GSS-API) specification "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2" (RFC 4121). 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., Google, Microsoft, Centrify Corporation and TeamF1 Inc., and academic institutions such as the Royal Institute of Technology in Sweden, Stanford University, MIT, and vendors such as CyberSafe offering commercially supported versions.
Protocol
Description The client authenticates itself to the Authentication Server (AS) which is part of the key distribution center (KDC). The KDC issues a ticket-granting ticket (TGT), which is time stamped and encrypts it using the ticket-granting service's (TGS) secret key and returns the encrypted result to the user's workstation. This is done infrequently, typically at user logon; the TGT expires at some point although it may be transparently renewed by the user's session manager while they are logged in. When the client needs to communicate with a service on another node (a "principal", in Kerberos parlance), the client sends the TGT to the TGS, which is another component of the KDC and usually shares the same host as the authentication server. The service must have already been registered with the TGS with a Service Principal Name (SPN). The client uses the SPN to request access to this service. After verifying that the TGT is valid and that the user is permitted to access the requested service, the TGS issues a service ticket (ST) and session keys to the client. The client then sends the ticket to the service server (SS) along with its service request.
The protocol is described in detail below.
User Login and Pre-authentication A user enters a username and password on the client machine. Other credential mechanisms like PKINIT (RFC 4556) allow for the use of public keys or smart cards in place of a password. The client transforms the password into a symmetric key using a one-way hash or standard key derivation function, depending on the cipher suite used. In modern Kerberos (v5), the client often uses this key to encrypt a current timestamp. This encrypted timestamp is sent to the AS along with the username as "pre-authentication" to prove the user knows the password before the AS issues any tickets, preventing offline brute-force attacks.
Client Authentication The client sends a plaintext message of the user ID to the AS (Authentication Server) requesting services on behalf of the user. (Note: Neither the secret key nor the password is sent to the AS.) The AS checks to see whether the client is in its database. If it is, the AS generates the secret key by hashing the password of the user found at the database (e.g., Active Directory in Windows Server) and 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 (TGT, 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. 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.
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