(heimdal.info.gz) What is Kerberos?
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2 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.
2.1 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 (A<ADDR>), the
current time (T<ISSUE>), the lifetime of the ticket (LIFE), and a
secret "session key" (K<AB>). This packet is encrypted with B's secret
key (K<B>). The actual ticket (T<AB>) looks like this: ({A, B,
A<ADDR>, T<ISSUE>, LIFE, K<AB>}K<B>).
The reply to A consists of the ticket (T<AB>), B's name, the current
time, the lifetime of the ticket, and the session key, all encrypted in
A's secret key ({B, T<ISSUE>, LIFE, K<AB>, T<AB>}K<A>). 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, A<ADDR>, T<CURRENT>,
CHECKSUM}K<AB>). 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.
2.2 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.
2.3 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 efficiency),
and is not part of Kerberos 4; MIT Kerberos 5 contains it.
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.
2.4 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/'.
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