Kerberos is a protocol of communication to guarantee a part of the security of your information infrastructure. Among the key concepts of security of the CIA triad: Confidentiality, Integrity, and Availability. Kerberos allows setting up communications with confidentiality, integrity and non-repudiation, it is an Authentication and Authorization System (AAS), although the step of authorization is very limited, it is essentially an authentication system.

This article is an introduction to Kerberos ecosystem. It can be interesting for a manager to understand key principles of Kerberos, a non-technical person to have a overview of this technology, or for technical person particularly with DevOps skills to increase action field and go further if necessary.

What is Kerberos?

The goal of Kerberos is to secure a communication channel over UDP and TCP by proving mutually the identity of each entity: transmitter and receiver. Kerberos can also encrypted the communication by exchanging session key securely.

Kerberos allows to secure communications on untrusted networks but where each node is trusted

Massachusetts Institute of Technology (MIT)

Kerberos is built by design from an architecture based on the symmetric key cryptography to authenticate, and to encrypt communication. It requires a trusted third party to guarantee a mutual authentication with a system of secrets and renewable tickets.

Kerberos’ name comes from Greek mythology, the three-headed guard dog of Hades.This technology is invented at MIT in the 1980s, became an IETF Standard in 1993. United States governement classified Kerberos protocol as “Auxiliary Military Equipment” and banned its export until 2000. It continues being used largely today. The important Requests for Comments (RFC) to understand Kerberos are rfc4120 and rfc6649, see all Kerberos RFCs.

The MIT Kerberos Consortium ensures development and lasting quality of the Kerberos project since 2007 with past and present sponsors of Apple, Google, Oracle, Stanford University, Microsoft … but less active now.

Kerberos is a very used in the Hadoop ecosystem to secure a environment Big Data for Spark, Hadoop YARN, Hadoop Mapreduce, or Hadoop Distributed File System… It is used by lot of companies as a Single Sign On solution at large to access to web UIs/APIs, databases, services, servers with SSH, or your local system login of your workstation and not only web HTTP applications, for any operating system. But it is also famous in the Microsoft world by the Active Directory cornerstone of your clusters of Windows servers.

The current widespread implementations are MIT Kerberos, Heimdal, and Active Directory. These are native Kerberos implementations respectively for Linux, macOS, and Windows. But they are built also for other operating systems than their native one. For most usages, these implementations are compatible between them client-server or server-server for Kerberos communication protocol, but the ticket credentials and keytabs are not standardized by the RFCs. Some implementations are diverged on some edge aspects, or propose extra features.

The historical implementation is MIT Kerberos, because it has been written by the founders of Kerberos Steve Miller and Clifford Neuman.The first implementation release was in 1987. New releases are pushblished until present with continuous improvements. The latest MIT Kerberos version is Kerberos V5 written in Language C under MIT License. You can see all available releases. The project is open source project published on a git mirror repository on GitHub by GitHub Organization krb5. There are a online documentation and short guide pdf.

MIT Kerberos

Another interesting implementation of Kerberos is in Java: Apache Kerby. The Java ecosystem integrates very well Kerberos, it is in the standard library. A lot of Java projects use Kerberos.

See Frequently Asked Questions (FAQ).

Pros and cons

You can read this paper: Why is Kerberos a credible security solution?

Pro’s

• There is better performance when using of Kerberos. Because it uses symmetric key operations, which are orders of magnitude faster than public key operations used by SSL.
• User management is simpler. For example, revoking a user can be done by simply deleting the user from the centrally managed Kerberos KDC (key distribution center). Whereas in SSL, a new certificate revocation list has to be generated and propagated to all servers
• Thanks to its ticket system, Kerberos allows temporary access if a key session is stolen.
• The passwords is never sent through network (even ciphered, robust against Man In the Middle attack).
• Kerberos allows a mutual authentication, when a client is connected to a service. The client knows which is the service and vice versa unlike SSL certificate where the server does not know a priori the client.

Con’s

• A brute force attack on Key Distribution Service (KDC) could compromise the entire network, since it contains all the passwords (in an encrypted version, but still). So the security of the KDC is essential.
• Kerberos assumes that the host it is running on is safe, so if an attacker can get hold of a client it can steal tickets and during the lifetime of the ticket do the same things a normal user could. On multi-user systems the above point even becomes more important. Meaning that the different users should not be able to access each others files and memory.
• Since anybody can request a (TGT) of any user, and that ticket is encrypted with the user’s secret key (password), it is simple to perform an offline attack on this ticket by trying to decrypt it with different passwords. Kerberos 5 introduced to solve this problem.
• Kerberos is very dependent on the Domain Name Service (DNS), a service principal should have both: DNS lookup and reverse DNS lookup working. If your DNS Server crashed your authentication system is out.
• The set up of Kerberos can be more constraining than another authentication system.

How it works?

Imagine that a client wishes to access to a service through a Kerberos Authentication. The Kerberos authentication is divided into 3 main steps:

1. Getting a Ticket-Granting Ticket (TGT) from Authentication Server (AS)
2. Getting a Ticket-Granting Service (TGS) from Ticket-Granting Server (TGS)
3. Accessing to the Service

Here client (identified as user) and service are already registered in the Kerberos database in the same Realm (understanding same Kerberos perimeter), respectively as User Principal Name (UPN) and Service Principal Name (SPN).

Kerberos authentication architecture

You can use Lightweight Directory Access Protocol (LDAP) as principal database to store users and groups of your organization. Now you have a nice Authentication, Authorization and Accounting System (AAAS) :

• Authentication is managed by the KDC to guarantee the identity of an entity.
• Authorization is controlled by the Service directly to authorize access (you can also use Access Control List (ACL) for other permissions)
• Accounting is allowed by LDAP server.

Kerberos authentication workflow with LDAP

How to test?

I suppose that you are comfortable with docker containers and command-line interface under Linux Operating System as Ubuntu. You can test directly Kerberos ecosystem in few commands.

To do that, just consult this Open Source project kerberos-docker.

Kerberos/Docker icon

This servers cluster is deployed automatically with only one command.

Kerberos architecture on docker for Kerberos/Docker project

Thanks to GSS-API implementation in C, OpenSSH use one method based on Kerberos authentication: gssapi-with-mic. That is a Message Authentication Code (MAC) methods renamed in Message Integrated Code (MIC) for disambiguation MAC address. Thus the suffix with-mic is used. But SSH continue to manage authorization (using Linux-PAM or/and AllowsUser, Allowsgroup,PermitRootLogin in /etc/ssh/sshd_config), and accounting is managed by the Operating System.

The layer of access authorization provided by Kerberos is really minimal and consists in a ~/.k5login file and a few configurations. See details here. By default Kerberos applies a default authorization rule: if host H is in realm R, the Kerberos principal u@R is allowed access to the account u@H. You change the configuration of SSH server with KerberosAuthentication yes to connect directly the service without doing kinit. And you can use also another way to use the PAM module of Kerberos pam_krb5.so thanks to the configuration UsePAM yes.

For developing your own kerberized application, you can read Best Practices for Integrating Kerberos into Your Application.

Go further

To go further, you should know that with Kerberos you can do Delegation of authentication, Servers’ replication and redundancy, Cross Realm Authentication. And it is important to distinguish Kerberos with SSL/HTTPS, SSH or JWT, Radius and Diameter.

Annexes

Key Concepts

Authentication: Action of checking that an entity is really this entity without impersonating another entity.

Authorization: Action of verifying that an entity have sufficient rights to access a system.

Accounting: Management of defined groups and users.

General concepts

Mutual authentication: It is also called two-way authentication, is a process or technology in which both entities in a communications link authenticate each other. In a network environment, the client authenticates the server and vice-versa.

AAS: Authentication and Authorization System.

AAAS: Authentication, Authorization, Accounting/Auditing System.

Client: An entity that can obtain a ticket. This entity is usually either a user or a host.

Host: A computer that can be accessed over a network.

DNS: The Domain Name System is a hierarchical decentralized naming system for computers.

Service: Any program or computer you access over a network. Examples of services include “host” (a host, e.g., when you use telnet and rsh), “ftp” (FTP), “krbtgt” (authentication; cf. ticket-granting ticket), and “pop” (email).

RFC: A Request for Comments (RFC) is a type of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), the principal technical development and standards-setting bodies for the Internet.

LDAP: Lightweight Directory Access Protocol is a directory service protocol that runs on a layer above the TCP/IP stack. It provides a mechanism used to connect to, search, and modify Internet directories. It is essentially a database that contains information about users and groups in a tree structure

HDFS: Hadoop Distributed File System is a Java-based file system that provides scalable and reliable data storage, and it was designed to span large clusters of commodity servers.

SSO: Single Sign On is a method allowing to an user to access to several applications thanks to one unique authentication.

OpenSSH: Implementation Open Source of Secure SHell (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network.

ACL: Access Control List allows to manage permissions attached to an object.

PAM: A Pluggable Authentication Module allows to standardize security layer dynamically though system of same customizable modules for all applications.

Kerberos concepts

Kerberos: In Greek mythology, the three-headed dog that guards the entrance to the underworld. In the computing world, Kerberos is a network security package that was developed at MIT.

KDC: Key Distribution Center. A machine that issues Kerberos tickets composed of Ticket-Granting Server (TGS), Authentication Server (AS) and database instances.

AS: Authentication Service gives Ticket-Granting Ticket (TGT) to authenticate a client.

AD: Active Directory is a Microsoft product which makes use of a Kerberos implementation.

Keytab: Shortcut for key table a key tab is the file containing one or more keys. A host or service uses a keytab file in much the same way as a user uses his/her password.

Realm: A REALM is in Kerberos terms equivalent to a Domain in Domain Name Server (DNS) by convention in UPPERCASE.

Principal: You can understand that as UNIX username. There 2 types of principles: Service Principal Name (SPN) and User Principal Name (UPN). It is a string that names a specific entity to which a set of credentials may be assigned. It can have an arbitrary number of components: component1/component2/…@REALM The typical format of a typical Kerberos principal is primary/instance@REALM.

primary: The first part of a Kerberos principal. In the case of a user, it is the username. In the case of a service, it is the name of the service. instance: The second part of a Kerberos principal. It gives information that qualifies the primary. The instance may be null. In the case of a user, the instance is often used to describe the intended use of the corresponding credentials. In the case of a host, the instance is the fully qualified hostname. realm: The logical network served by a single Kerberos database and a set of Key Distribution Centers. By convention, realm names are generally all uppercase letters, to differentiate the realm from the internet domain. Ticket: A temporary set of electronic credentials that verify the identity of a client for a particular service (typically 8 or 10 hours of life time).

TGT: Ticket-Granting Ticket. A special Kerberos ticket to guarantee authentication that permits the client to obtain additional Kerberos tickets within the same Kerberos realm given by Authentication Service (AS).

TGS: Ticket-Granting Service or Ticket-Granting Server (depends on context). The TGS exchange is initiated by a client when it wishes to obtain authentication credentials for a given server

GSS-API: Acronym for Generic Security Service Application Program Interface (GSSAPI, also GSS-API) is an application programming interface for programs to access security services.

Session Key: Random number generated by the KDC when it builds a ticket. It is the shared secret that the KDC, the client, and the validating server have in common.

KVNO: It is an acronym for Key version number. Kerberos can hold multiple keys for a principal. To distinguish between those keys every key has version number. The number is incremented by one when the password or encryption is changed.

KRBTGT Account: This is a special account to create Ticket Granting Tickets.

Key salt: The salt is a string concatenated to the password to make sure the string2key function creates a unique encryption key, via a one-way hash. In Kerberos 5 the salt is the complete principal name including the realm. The reason the salt is used is to make sure the outcome of the one way hash is unique across realms.

References

• ROBINSON Trevor (eztenia). Kerberos. Canonical Ltd. Ubuntu Article, November 2014. Link: https://help.ubuntu.com/community/Kerberos.
• MIGEON Jean. Protocol, Installation and Single Sign On, The MIT Kerberos Admnistrator’s how-to Guide. MIT Kerberos Consortium, July 2008. p 62.
• BARRETT Daniel, SILVERMAN Richard, BYRNES Robert. SSH, The Secure Shell: The Definitive Guide, 2nd Edition. O’Reilly Media, June 2009. p. 672. Notes: Chapter 11. ISBN-10: 0596008953, ISBN-13: 978-0596008956
• GARMAN, Jason. Kerberos: The Definitive Guide, 2nd Edition. O’Reilly Media, March 2010. p. 272. ISBN-10: 0596004036, ISBN-13: 978-0596004033.
• Phillip J. Windley. Digital Identity. O’Reilly Media, August 2005. p. 254.
• O’MALLEY Owen, ZHANG Kan, RADIA Sanjay, MARTI Ram, and HARRELL Christopher. Hadoop Security Design. Yahoo! Research Paper, October 2009. p 19. ISBN-10: 0596008783, ISBN-13: 978-0596008789
• MATTHIAS Karl, KANE Sean. Docker: Up & Running. O’Reilly Media, June 2015. p. 232. ISBN-10: 1491917571, ISBN-13: 978-1491917572.