Kerberos protocol
Encyclopedia
Kerberos 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 attack
s.
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. 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.
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., Google
, Microsoft
, Centrify Corporation and TeamF1 Inc.
, 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 X
, Red Hat Enterprise Linux
4, Oracle
's Solaris, IBM's AIX, HP's OpenVMS
, and others, include software for Kerberos authentication of users or services. Embedded implementation of the Kerberos V authentication protocol for client agents and network services running on embedded platforms is also available from companies such as TeamF1, Inc.
, the IETF
Kerberos working group is updating the specifications. Recent updates include:
, 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.
When the client needs to communicate with another node ("principal" in Kerberos parlance) it sends the TGT to the Ticket Granting Service (TGS), which shares the same host as the TGT. After verifying the TGT is valid and the user is permitted to access the requested service, the TGS issues a Ticket and session keys, which are returned to the client.
The client then sends the Ticket and keys to the service (SS).
Here is another description.
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.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
protocol
Cryptographic protocol
A security protocol is an abstract or concrete protocol that performs a security-related function and applies cryptographic methods.A protocol describes how the algorithms should be used...
which works on the basis of "tickets" to allow nodes
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
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
Mutual authentication
Mutual authentication or two-way authentication refers to two parties authenticating each other suitably. In technology terms, it refers to a client or user authenticating themselves to a server and that server authenticating itself to the user in such a way that both parties are assured of the...
— both the user and the server verify each other's identity. Kerberos protocol messages are protected against eavesdropping and replay attack
Replay attack
A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack by IP packet...
s.
Kerberos builds on symmetric key cryptography and requires a trusted third party
Trusted third party
In cryptography, a trusted third party is an entity which facilitates interactions between two parties who both trust the third party; The Third Party reviews all critical transaction communications between the parties, based on the ease of creating fraudulent digital content. In TTP models, the...
, and optionally may use public-key cryptography
Public-key cryptography
Public-key cryptography refers to a cryptographic system requiring two separate keys, one to lock or encrypt the plaintext, and one to unlock or decrypt the cyphertext. Neither key will do both functions. One of these keys is published or public and the other is kept private...
by utilizing asymmetric key cryptography during certain phases of authentication. Kerberos uses port 88 by default.
"Kerberos" also refers to a suite of free software
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
published by Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
(MIT) that implements the Kerberos protocol.
History and development
MITMassachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
developed Kerberos to protect network services provided by Project Athena
Project Athena
Project Athena was a joint project of MIT, Digital Equipment Corporation, and IBM to produce a campus-wide distributed computing environment for educational use. It was launched in 1983, and research and development ran until June 30, 1991, eight years after it began...
. The protocol was named after the Greek mythological
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
character Kerberos
Cerberus
Cerberus , or Kerberos, in Greek and Roman mythology, is a multi-headed hound which guards the gates of the Underworld, to prevent those who have crossed the river Styx from ever escaping...
(or Cerberus), known in Greek mythology as being the monstrous three-headed guard dog of Hades
Hades
Hades , Hadēs, originally , Haidēs or , Aidēs , meaning "the unseen") was the ancient Greek god of the underworld. The genitive , Haidou, was an elision to denote locality: "[the house/dominion] of Hades". Eventually, the nominative came to designate the abode of the dead.In Greek mythology, 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
Project Athena
Project Athena was a joint project of MIT, Digital Equipment Corporation, and IBM to produce a campus-wide distributed computing environment for educational use. It was launched in 1983, and research and development ran until June 30, 1991, eight years after it began...
.
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
BSD licenses
BSD licenses are a family of permissive free software licenses. The original license was used for the Berkeley Software Distribution , a Unix-like operating system after which it is named....
. In 2007, MIT formed the Kerberos Consortium to foster continued development. Founding sponsors include vendors such as Oracle
Oracle Corporation
Oracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...
, Apple Inc., Google
Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...
, Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
, Centrify Corporation and TeamF1 Inc.
TeamF1
TeamF1, Inc., is an embedded software developer that licenses production-ready networking and security software packages for the embedded systems market. It is a privately held company headquartered in Fremont, California with a research and development center in Hyderabad, India and local sales...
, 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
Data Encryption Standard
The Data Encryption Standard is a block cipher that uses shared secret encryption. It was selected by the National Bureau of Standards as an official Federal Information Processing Standard for the United States in 1976 and which has subsequently enjoyed widespread use internationally. It is...
encryption algorithm (with 56-bit keys). A non-US Kerberos 4 implementation, KTH-KRB developed at the Royal Institute of Technology
Royal Institute of Technology
The Royal Institute of Technology is a university in Stockholm, Sweden. KTH was founded in 1827 as Sweden's first polytechnic and is one of Scandinavia's largest institutions of higher education in technology. KTH accounts for one-third of Sweden’s technical research and engineering education...
in Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, made the system available outside the US before the US changed its cryptography export
Export of cryptography
The export of cryptography in the United States is the transfer from the United States to another country of devices and technology related to cryptography....
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
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
and later use Kerberos as their default authentication method. Some Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
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
RC4
In cryptography, RC4 is the most widely used software stream cipher and is used in popular protocols such as Secure Sockets Layer and WEP...
cipher. While Microsoft uses the Kerberos protocol, it does not use the MIT software.
Many UNIX and UNIX-like operating systems, including FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
, Apple's Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, Red Hat Enterprise Linux
Red Hat Enterprise Linux
Red Hat Enterprise Linux is a Linux-based operating system developed by Red Hat and targeted toward the commercial market. Red Hat Enterprise Linux is released in server versions for x86, x86-64, Itanium, PowerPC and IBM System z, and desktop versions for x86 and x86-64...
4, Oracle
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
's Solaris, IBM's AIX, HP's OpenVMS
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
, and others, include software for Kerberos authentication of users or services. Embedded implementation of the Kerberos V authentication protocol for client agents and network services running on embedded platforms is also available from companies such as TeamF1, Inc.
, the IETF
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
Kerberos working group is updating the specifications. Recent updates include:
- Encryption and Checksum Specifications" (RFC 3961).
- Advanced Encryption StandardAdvanced Encryption StandardAdvanced Encryption Standard is a specification for the encryption of electronic data. It has been adopted by the U.S. government and is now used worldwide. It supersedes DES...
(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-APIGeneric Security Services Application Program InterfaceThe Generic Security Services Application Program Interface is an application programming interface for programs to access security services....
specification "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2." (RFC 4121).
Theory
Kerberos uses as its basis the symmetric Needham-Schroeder protocol. It makes use of a trusted third partyTrusted third party
In cryptography, a trusted third party is an entity which facilitates interactions between two parties who both trust the third party; The Third Party reviews all critical transaction communications between the parties, based on the ease of creating fraudulent digital content. In TTP models, the...
, termed a key distribution center
Key distribution center
In cryptography, a key distribution center is part of a cryptosystem intended to reduce the risks inherent in exchanging keys. KDCs often operate in systems within which some users may have permission to use certain services at some times and not at others.-Security overview:For instance, an...
(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
Session key
A session key is a single-use symmetric key used for encrypting all messages in one communication session. A closely related term is traffic encryption key or TEK, which refers to any key used to encrypt messages, as opposed to other uses, like encrypting other keys .Session keys can introduce...
which communicating parties use to encrypt their transmissions.
The security
Security
Security is the degree of protection against danger, damage, loss, and crime. Security as a form of protection are structures and processes that provide or improve security as a condition. The Institute for Security and Open Methodologies in the OSSTMM 3 defines security as "a form of protection...
of the protocol relies heavily on short-lived assertions of authenticity called Kerberos tickets.
Description
The client authenticates itself to the AS which forwards the username to a Key Distribution Center (KDC). The KDC issues a Ticket Granting Ticket (TGT), which is time stamped, encrypts it using the user's password and returns the encrypted result to the user's workstation. If successful, this gives the user desktop access.When the client needs to communicate with another node ("principal" in Kerberos parlance) it sends the TGT to the Ticket Granting Service (TGS), which shares the same host as the TGT. After verifying the TGT is valid and the user is permitted to access the requested service, the TGS issues a Ticket and session keys, which are returned to the client.
The client then sends the Ticket and keys to the service (SS).
Here is another description.
| Kerberos negotiations |
The client authenticates to the AS once using a long-term shared secret
Shared secret
In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. The shared secret can be a password, a passphrase, a big number or an array of randomly chosen bytes....
(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.
User Client-based Logon
- A user enters a username and password on the client machineClient (computing)A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
. - The client performs a one-way functionOne-way functionIn computer science, a one-way function is a function that is easy to compute on every input, but hard to invert given the image of a random input. Here "easy" and "hard" are to be understood in the sense of computational complexity theory, specifically the theory of polynomial time problems...
(hash usually) on the entered password, and this becomes the secret key of the client/user.
Client Authentication
- 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).
- 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.
- 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.
Client Service Authorization
- 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.
- 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.
Client Service Request
- 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.
- 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.
- 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.
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 ProtocolNetwork Time ProtocolThe Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
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 KDCKey distribution centerIn cryptography, a key distribution center is part of a cryptosystem intended to reduce the risks inherent in exchanging keys. KDCs often operate in systems within which some users may have permission to use certain services at some times and not at others.-Security overview:For instance, an...
, compromise of this authentication infrastructure will allow an attacker to impersonate any user.
See also
- Single sign-onSingle sign-onSingle sign-on is a property of access control of multiple related, but independent software systems. With this property a user logs in once and gains access to all systems without being prompted to log in again at each of them...
- Identity managementIdentity managementIdentity management is a broad administrative area that deals with identifying individuals in a system and controlling access to the resources in that system by placing restrictions on the established identities of the individuals.Identity management is multidisciplinary and covers many...
- SPNEGOSPNEGOSPNEGO is a GSSAPI "pseudo mechanism" that is used to negotiate one of a number of possible real mechanisms....
- S/KeyS/KEYS/KEY is a one-time password system developed for authentication to Unix-like operating systems, especially from dumb terminals or untrusted public computers on which one does not want to type a long-term password. A user's real password is combined in an offline device with a short set of...
- Secure remote password protocolSecure remote password protocolThe Secure Remote Password protocol is a password-authenticated key agreement protocol.- Overview :The SRP protocol has a number of desirable properties: it allows a user to authenticate themselves to a server, it is resistant to dictionary attacks mounted by an eavesdropper, and it does not...
(SRP) - Generic Security Services Application Program InterfaceGeneric Security Services Application Program InterfaceThe Generic Security Services Application Program Interface is an application programming interface for programs to access security services....
(GSS-API) - Host Identity ProtocolHost Identity ProtocolThe Host Identity Protocol is a host identification technology for use on Internet Protocol networks, such as the Internet. The Internet has two main name spaces, IP addresses and the Domain Name System. HIP separates the end-point identifier and locator roles of IP addresses...
(HIP)
External links
- How Kerberos Authentication Works
- Kerberos page at MIT
- Kerberos Working Group at IETF
- Kerberos Consortium at MIT
- White Papers at MIT
- Vendor Documentation and Specifications at MIT
- Kerberos How-to
- The Kerberos FAQ (last modified 8/18/2000)
- Shishi, a free Kerberos implementation for the GNU system
- Designing an Authentication System: A Dialogue in Four Scenes. Humorous play concerning how the design of Kerberos evolved.
- RFC 4120: The Kerberos Network Authentication Service (V5)
- Description of Kerberos 5 in the SPORE library
- Kerberos Authentication in Windows Server 2003
- Kerberos Tutorial
- Novell Inc's Comment to the Proposed Settlement between Microsoft and the Department of Justice - Microsoft purposefully breaks Kerberos interoperability
- Kerberos in FreeBSD
- Embedded Kerberos Implementation by TeamF1TeamF1TeamF1, Inc., is an embedded software developer that licenses production-ready networking and security software packages for the embedded systems market. It is a privately held company headquartered in Fremont, California with a research and development center in Hyderabad, India and local sales...
- Heimdal, an implementation of Kerberos 5