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diff --git a/crypto/heimdal/doc/whatis.texi b/crypto/heimdal/doc/whatis.texi deleted file mode 100644 index 97d4da2aa505..000000000000 --- a/crypto/heimdal/doc/whatis.texi +++ /dev/null @@ -1,149 +0,0 @@ -@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 - -@ifinfo -@macro sub{arg} -<\arg\> -@end macro -@end ifinfo - -@tex -@def@xsub#1{$_{#1}$} -@global@let@sub=@xsub -@end tex - -@ifhtml -@macro sub{arg} -<\arg\> -@end macro -@end ifhtml - -@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 Defense 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/}. |