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several new kerberos related libraries and applications to FreeBSD:
o kgetcred(1) allows one to manually get a ticket for a particular service.
o kf(1) securily forwards ticket to another host through an authenticated
and encrypted stream.
o kcc(1) is an umbrella program around klist(1), kswitch(1), kgetcred(1)
and other user kerberos operations. klist and kswitch are just symlinks
to kcc(1) now.
o kswitch(1) allows you to easily switch between kerberos credentials if
you're running KCM.
o hxtool(1) is a certificate management tool to use with PKINIT.
o string2key(1) maps a password into key.
o kdigest(8) is a userland tool to access the KDC's digest interface.
o kimpersonate(8) creates a "fake" ticket for a service.
We also now install manpages for some lirbaries that were not installed
before, libheimntlm and libhx509.
- The new HEIMDAL version no longer supports Kerberos 4. All users are
recommended to switch to Kerberos 5.
- Weak ciphers are now disabled by default. To enable DES support (used
by telnet(8)), use "allow_weak_crypto" option in krb5.conf.
- libtelnet, pam_ksu and pam_krb5 are now compiled with error on warnings
disabled due to the function they use (krb5_get_err_text(3)) being
deprecated. I plan to work on this next.
- Heimdal's KDC now require sqlite to operate. We use the bundled version
and install it as libheimsqlite. If some other FreeBSD components will
require it in the future we can rename it to libbsdsqlite and use for these
components as well.
- This is not a latest Heimdal version, the new one was released while I was
working on the update. I will update it to 1.5.2 soon, as it fixes some
important bugs and security issues.
162 lines
6.0 KiB
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162 lines
6.0 KiB
Plaintext
@c $Id$
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@node What is Kerberos?, Building and Installing, Introduction, Top
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@chapter What is Kerberos?
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@quotation
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@flushleft
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Now this Cerberus had three heads of dogs,
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the tail of a dragon, and on his back the
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heads of all sorts of snakes.
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--- Pseudo-Apollodorus Library 2.5.12
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@end flushleft
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@end quotation
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Kerberos is a system for authenticating users and services on a network.
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It is built upon the assumption that the network is ``unsafe''. For
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example, data sent over the network can be eavesdropped and altered, and
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addresses can also be faked. Therefore they cannot be used for
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authentication purposes.
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@cindex authentication
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Kerberos is a trusted third-party service. That means that there is a
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third party (the kerberos server) that is trusted by all the entities on
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the network (users and services, usually called @dfn{principals}). All
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principals share a secret password (or key) with the kerberos server and
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this enables principals to verify that the messages from the kerberos
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server are authentic. Thus trusting the kerberos server, users and
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services can authenticate each other.
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@section Basic mechanism
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@ifinfo
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@macro sub{arg}
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<\arg\>
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@end macro
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@end ifinfo
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@tex
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@def@xsub#1{$_{#1}$}
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@global@let@sub=@xsub
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@end tex
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@ifhtml
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@macro sub{arg}
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@html
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<sub>\arg\</sub>
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@end html
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@end macro
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@end ifhtml
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@c ifdocbook
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@c macro sub{arg}
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@c docbook
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@c <subscript>\arg\</subscript>
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@c end docbook
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@c end macro
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@c end ifdocbook
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@quotation
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@strong{Note} This discussion is about Kerberos version 4, but version
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5 works similarly.
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@end quotation
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In Kerberos, principals use @dfn{tickets} to prove that they are who
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they claim to be. In the following example, @var{A} is the initiator of
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the authentication exchange, usually a user, and @var{B} is the service
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that @var{A} wishes to use.
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To obtain a ticket for a specific service, @var{A} sends a ticket
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request to the kerberos server. The request contains @var{A}'s and
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@var{B}'s names (along with some other fields). The kerberos server
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checks that both @var{A} and @var{B} are valid principals.
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Having verified the validity of the principals, it creates a packet
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containing @var{A}'s and @var{B}'s names, @var{A}'s network address
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(@var{A@sub{addr}}), the current time (@var{t@sub{issue}}), the lifetime
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of the ticket (@var{life}), and a secret @dfn{session key}
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@cindex session key
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(@var{K@sub{AB}}). This packet is encrypted with @var{B}'s secret key
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(@var{K@sub{B}}). The actual ticket (@var{T@sub{AB}}) looks like this:
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(@{@var{A}, @var{B}, @var{A@sub{addr}}, @var{t@sub{issue}}, @var{life},
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@var{K@sub{AB}}@}@var{K@sub{B}}).
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The reply to @var{A} consists of the ticket (@var{T@sub{AB}}), @var{B}'s
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name, the current time, the lifetime of the ticket, and the session key, all
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encrypted in @var{A}'s secret key (@{@var{B}, @var{t@sub{issue}},
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@var{life}, @var{K@sub{AB}}, @var{T@sub{AB}}@}@var{K@sub{A}}). @var{A}
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decrypts the reply and retains it for later use.
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@sp 1
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Before sending a message to @var{B}, @var{A} creates an authenticator
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consisting of @var{A}'s name, @var{A}'s address, the current time, and a
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``checksum'' chosen by @var{A}, all encrypted with the secret session
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key (@{@var{A}, @var{A@sub{addr}}, @var{t@sub{current}},
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@var{checksum}@}@var{K@sub{AB}}). This is sent together with the ticket
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received from the kerberos server to @var{B}. Upon reception, @var{B}
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decrypts the ticket using @var{B}'s secret key. Since the ticket
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contains the session key that the authenticator was encrypted with,
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@var{B} can now also decrypt the authenticator. To verify that @var{A}
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really is @var{A}, @var{B} now has to compare the contents of the ticket
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with that of the authenticator. If everything matches, @var{B} now
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considers @var{A} as properly authenticated.
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@c (here we should have some more explanations)
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@section Different attacks
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@subheading Impersonating A
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An impostor, @var{C} could steal the authenticator and the ticket as it
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is transmitted across the network, and use them to impersonate
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@var{A}. The address in the ticket and the authenticator was added to
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make it more difficult to perform this attack. To succeed @var{C} will
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have to either use the same machine as @var{A} or fake the source
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addresses of the packets. By including the time stamp in the
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authenticator, @var{C} does not have much time in which to mount the
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attack.
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@subheading Impersonating B
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@var{C} can hijack @var{B}'s network address, and when @var{A} sends
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her credentials, @var{C} just pretend to verify them. @var{C} can't
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be sure that she is talking to @var{A}.
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@section Defence strategies
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It would be possible to add a @dfn{replay cache}
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@cindex replay cache
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to the server side. The idea is to save the authenticators sent during
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the last few minutes, so that @var{B} can detect when someone is trying
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to retransmit an already used message. This is somewhat impractical
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(mostly regarding efficiency), and is not part of Kerberos 4; MIT
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Kerberos 5 contains it.
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To authenticate @var{B}, @var{A} might request that @var{B} sends
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something back that proves that @var{B} has access to the session
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key. An example of this is the checksum that @var{A} sent as part of the
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authenticator. One typical procedure is to add one to the checksum,
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encrypt it with the session key and send it back to @var{A}. This is
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called @dfn{mutual authentication}.
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The session key can also be used to add cryptographic checksums to the
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messages sent between @var{A} and @var{B} (known as @dfn{message
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integrity}). Encryption can also be added (@dfn{message
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confidentiality}). This is probably the best approach in all cases.
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@cindex integrity
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@cindex confidentiality
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@section Further reading
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The original paper on Kerberos from 1988 is @cite{Kerberos: An
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Authentication Service for Open Network Systems}, by Jennifer Steiner,
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Clifford Neuman and Jeffrey I. Schiller.
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A less technical description can be found in @cite{Designing an
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Authentication System: a Dialogue in Four Scenes} by Bill Bryant, also
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from 1988.
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These documents can be found on our web-page at
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@url{http://www.pdc.kth.se/kth-krb/}.
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