What is Kerberos?
*****************

     Now this Cerberus had three heads of dogs,
     the tail of a dragon, and on his back the
     heads of all sorts of snakes.
     --- Pseudo-Apollodorus Library 2.5.12

Kerberos is a system for authenticating users and services on a network. It is built upon the assumption that the network is ``unsafe''.  For example, data sent over the network can be eavesdropped and altered, and addresses can also be faked.  Therefore they cannot be used for authentication purposes.

Kerberos is a trusted third-party service.  That means that there is a third party (the kerberos server) that is trusted by all the entities on the network (users and services, usually called principals).  All principals share a secret password (or key) with the kerberos server and this enables principals to verify that the messages from the kerberos server are authentic.  Thus trusting the kerberos server, users and services can authenticate each other.

Basic mechanism
===============

     Note This discussion is about Kerberos version 4, but version 5 works similarly.

In Kerberos, principals use tickets to prove that they are who they claim to be. In the following example, A is the initiator of the authentication exchange, usually a user, and B is the service that A wishes to use.

To obtain a ticket for a specific service, A sends a ticket request to the kerberos server. The request contains A's and B's names (along with some other fields). The kerberos server checks that both A and B are valid principals.

Having verified the validity of the principals, it creates a packet containing A's and B's names, A's network address (AADDR), the current time (TISSUE), the lifetime of the ticket (LIFE), and a secret session key (KAB). This packet is encrypted with B's secret key (KB).  The actual ticket (TAB) looks like this: ({A, B, AADDR, TISSUE, LIFE, KAB}KB).

The reply to A consists of the ticket (TAB), B's name, the current time, the lifetime of the ticket, and the session key, all encrypted in A's secret key ({B, TISSUE, LIFE, KAB, TAB}KA). A decrypts the reply and retains it for later use.

Before sending a message to B, A creates an authenticator consisting of A's name, A's address, the current time, and a ``checksum'' chosen by A, all encrypted with the secret session key ({A, AADDR, TCURRENT, CHECKSUM}KAB). This is sent together with the ticket received from the kerberos server to B.  Upon reception, B decrypts the ticket using B's secret key.  Since the ticket contains the session key that the authenticator was encrypted with, B can now also decrypt the authenticator. To verify that A really is A, B now has to compare the contents of the ticket with that of the authenticator. If everything matches, B now considers A as properly authenticated.

Different attacks
=================

Impersonating A
---------------

An impostor, C could steal the authenticator and the ticket as it is transmitted across the network, and use them to impersonate A. The address in the ticket and the authenticator was added to make it more difficult to perform this attack.  To succeed C will have to either use the same machine as A or fake the source addresses of the packets. By including the time stamp in the authenticator, C does not have much time in which to mount the attack.

Impersonating B
---------------

C can hijack B's network address, and when A sends her credentials, C just pretend to verify them. C can't be sure that she is talking to A.

Defence strategies
==================

It would be possible to add a replay cache to the server side.  The idea is to save the authenticators sent during the last few minutes, so that B can detect when someone is trying to retransmit an already used message. This is somewhat impractical (mostly regarding performance); MIT Kerberos 5 has a replay cache, while Heimdal does not.

However, most GSS-API applicatons do not need a replay cache at all.

To authenticate B, A might request that B sends something back that proves that B has access to the session key. An example of this is the checksum that A sent as part of the authenticator. One typical procedure is to add one to the checksum, encrypt it with the session key and send it back to A.  This is called mutual authentication.

The session key can also be used to add cryptographic checksums to the messages sent between A and B (known as message integrity).  Encryption can also be added (message confidentiality). This is probably the best approach in all cases.

Further reading
===============

The original paper on Kerberos from 1988 is Kerberos: An Authentication Service for Open Network Systems, by Jennifer Steiner, Clifford Neuman and Jeffrey I. Schiller.

A less technical description can be found in Designing an Authentication System: a Dialogue in Four Scenes by Bill Bryant, also from 1988.

These documents can be found on our web-page at http://www.pdc.kth.se/kth-krb/.

What is PKIX?
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PKIX is the set of Internet standards for Public Key Infrastructure (PKI), based on the ITU-T's x.509 standads.  PKI is an authentication mechanism based on public keys (the 'PK' in 'PKI').

In PKIX we have public keys "certified" by certification authorities (CAs).  A "relying party" is software that validates an entity's certificate and, if valid, trusts the certified public key to "speak for" the entity identified by the certificate.

In a PKI every entity has one (or more) certified public/private key pairs.

What is a Certification Authority (CA)?
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A Certification Authority (CA) is an entity in a PKI that issues certificates to other entities -- a CA certifies that a public key speaks for a particular, named entity.

There are two types of CAs: off-line and online.  Typically PKI hierarchies are organized such that the most security-critical private keys are only used by off-line CAs to certify the less security-critical public keys of online CAs.

Heimdal has support for off-line CAs using its Hx509 library and hxtool command.

Heimdal also has an online CA with a RESTful, HTTPS-based protocol.

What is kx509?
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kx509 is a kerberized certification authority (CA).  Heimdal implements this protocol in its KDC.  The protocol is specified by <a href="http://www.ietf.org/rfc/rfc6717.txt">RFC 6717</a>, though Heimdal has implemented a number of extensions as well.  A client is implemented by the heimtools command's kx509 sub-command.

What is kx509?
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bx509 is an online CA, like kx509, but the protocol is based on HTTPS.

Heimdal's bx509d implementation of bx509 implements two authentication bridges: a "/bx509" end-point that allows clients to trade bearer tokens (including Negotiate/Kerberos) and CSRs for certificates, and a "/bnegotiate" end-point allowing clients to trade bearer tokens (including Negotiate/Kerberos) for Negotiate tokens to HTTP servers.