@c $Id$

@node What is Kerberos?, Building and Installing, Introduction, Top
@chapter What is Kerberos?

@quotation
@flushleft
        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
@end flushleft
@end quotation

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.
@cindex authentication

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 @dfn{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.

@section Basic mechanism

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@quotation
@strong{Note} This discussion is about Kerberos version 4, but version
5 works similarly.
@end quotation

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

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

Having verified the validity of the principals, it creates a packet
containing @var{A}'s and @var{B}'s names, @var{A}'s network address
(@var{A@sub{addr}}), the current time (@var{t@sub{issue}}), the lifetime
of the ticket (@var{life}), and a secret @dfn{session key}
@cindex session key
(@var{K@sub{AB}}). This packet is encrypted with @var{B}'s secret key
(@var{K@sub{B}}).  The actual ticket (@var{T@sub{AB}}) looks like this:
(@{@var{A}, @var{B}, @var{A@sub{addr}}, @var{t@sub{issue}}, @var{life},
@var{K@sub{AB}}@}@var{K@sub{B}}).

The reply to @var{A} consists of the ticket (@var{T@sub{AB}}), @var{B}'s
name, the current time, the lifetime of the ticket, and the session key, all
encrypted in @var{A}'s secret key (@{@var{B}, @var{t@sub{issue}},
@var{life}, @var{K@sub{AB}}, @var{T@sub{AB}}@}@var{K@sub{A}}). @var{A}
decrypts the reply and retains it for later use.

@sp 1

Before sending a message to @var{B}, @var{A} creates an authenticator
consisting of @var{A}'s name, @var{A}'s address, the current time, and a
``checksum'' chosen by @var{A}, all encrypted with the secret session
key (@{@var{A}, @var{A@sub{addr}}, @var{t@sub{current}},
@var{checksum}@}@var{K@sub{AB}}). This is sent together with the ticket
received from the kerberos server to @var{B}.  Upon reception, @var{B}
decrypts the ticket using @var{B}'s secret key.  Since the ticket
contains the session key that the authenticator was encrypted with,
@var{B} can now also decrypt the authenticator. To verify that @var{A}
really is @var{A}, @var{B} now has to compare the contents of the ticket
with that of the authenticator. If everything matches, @var{B} now
considers @var{A} as properly authenticated.

@c (here we should have some more explanations)

@section Different attacks

@subheading Impersonating A

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

@subheading Impersonating B

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

@section Defence strategies

It would be possible to add a @dfn{replay cache}
@cindex replay cache
to the server side.  The idea is to save the authenticators sent during
the last few minutes, so that @var{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 @var{B}, @var{A} might request that @var{B} sends
something back that proves that @var{B} has access to the session
key. An example of this is the checksum that @var{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 @var{A}.  This is
called @dfn{mutual authentication}.

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

@section Further reading

The original paper on Kerberos from 1988 is @cite{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 @cite{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
@url{http://www.pdc.kth.se/kth-krb/}.
