Kerberos (protocol)
Kerberos (/ˈkɜːrbərɒs/) 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.
Developer(s)
Kerberos builds on symmetric-key cryptography and requires a trusted third party, and optionally may use public-key cryptography during certain phases of authentication.[2] Kerberos uses UDP port 88 by default.
The protocol was named after the character Kerberos (or Cerberus) from Greek mythology, the ferocious three-headed guard dog of Hades.[3]
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The Massachusetts Institute of Technology (MIT) developed Kerberos in 1988 to protect network services provided by Project Athena.[4][5] Its first version was primarily designed by Steve Miller and Clifford Neuman based on the earlier Needham–Schroeder symmetric-key protocol.[6][7] Kerberos versions 1 through 3 were experimental and not released outside of MIT.[8]
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,[8] called "Bones".[9] 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.[10]
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:
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[edit]
Description[edit]
The client authenticates itself to the Authentication Server (AS) which is part ot 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.
Support by operating systems[edit]
Microsoft Windows[edit]
Windows 2000 and later versions use Kerberos as their default authentication method.[13] 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 and extends the Kerberos protocol, it does not use the MIT software.
Kerberos is used as the preferred authentication method: in general, joining a client to a Windows domain means enabling Kerberos as the default protocol for authentications from that client to services in the Windows domain and all domains with trust relationships to that domain.[13]
In contrast, when either client or server or both are not joined to a domain (or not part of the same trusted domain environment), Windows will instead use NTLM for authentication between client and server.[13]
Internet web applications can enforce Kerberos as an authentication method for domain-joined clients by using APIs provided under SSPI.
Microsoft Windows and Windows Server include setspn, a command-line utility that can be used to read, modify, or delete the Service Principal Names (SPN) for an Active Directory service account.[14][15]
Unix and other operating systems[edit]
Many Unix-like operating systems, including FreeBSD, Apple's macOS, Red Hat Enterprise Linux, Oracle's Solaris, IBM's AIX, HP-UX and others, include software for Kerberos authentication of users or services. A variety of non-Unix like operating systems such as z/OS, IBM i and OpenVMS also feature Kerberos support. Embedded implementation of the Kerberos V authentication protocol for client agents and network services running on embedded platforms is also available from companies .
Security[edit]
The Data Encryption Standard (DES) cipher can be used in combination with Kerberos, but is no longer an Internet standard because it is weak.[16] Security vulnerabilities exist in products that implement legacy versions of Kerberos which lack support for newer encryption ciphers like AES.
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