( What is Kerberos?

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 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
         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,
 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.
 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.
 Defense 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.
 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
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