1@c $Id: whatis.texi,v 1.5 2001/01/28 22:11:23 assar Exp $ 2 3@node What is Kerberos?, Building and Installing, Introduction, Top 4@chapter What is Kerberos? 5 6@quotation 7@flushleft 8 Now this Cerberus had three heads of dogs, 9 the tail of a dragon, and on his back the 10 heads of all sorts of snakes. 11 --- Pseudo-Apollodorus Library 2.5.12 12@end flushleft 13@end quotation 14 15Kerberos is a system for authenticating users and services on a network. 16It is built upon the assumption that the network is ``unsafe''. For 17example, data sent over the network can be eavesdropped and altered, and 18addresses can also be faked. Therefore they cannot be used for 19authentication purposes. 20@cindex authentication 21 22Kerberos is a trusted third-party service. That means that there is a 23third party (the kerberos server) that is trusted by all the entities on 24the network (users and services, usually called @dfn{principals}). All 25principals share a secret password (or key) with the kerberos server and 26this enables principals to verify that the messages from the kerberos 27server are authentic. Thus trusting the kerberos server, users and 28services can authenticate each other. 29 30@section Basic mechanism 31 32@ifinfo 33@macro sub{arg} 34<\arg\> 35@end macro 36@end ifinfo 37 38@tex 39@def@xsub#1{$_{#1}$} 40@global@let@sub=@xsub 41@end tex 42 43@ifhtml 44@macro sub{arg} 45<\arg\> 46@end macro 47@end ifhtml 48 49@quotation 50@strong{Note:} This discussion is about Kerberos version 4, but version 515 works similarly. 52@end quotation 53 54In Kerberos, principals use @dfn{tickets} to prove that they are who 55they claim to be. In the following example, @var{A} is the initiator of 56the authentication exchange, usually a user, and @var{B} is the service 57that @var{A} wishes to use. 58 59To obtain a ticket for a specific service, @var{A} sends a ticket 60request to the kerberos server. The request contains @var{A}'s and 61@var{B}'s names (along with some other fields). The kerberos server 62checks that both @var{A} and @var{B} are valid principals. 63 64Having verified the validity of the principals, it creates a packet 65containing @var{A}'s and @var{B}'s names, @var{A}'s network address 66(@var{A@sub{addr}}), the current time (@var{t@sub{issue}}), the lifetime 67of the ticket (@var{life}), and a secret @dfn{session key} 68@cindex session key 69(@var{K@sub{AB}}). This packet is encrypted with @var{B}'s secret key 70(@var{K@sub{B}}). The actual ticket (@var{T@sub{AB}}) looks like this: 71(@{@var{A}, @var{B}, @var{A@sub{addr}}, @var{t@sub{issue}}, @var{life}, 72@var{K@sub{AB}}@}@var{K@sub{B}}). 73 74The reply to @var{A} consists of the ticket (@var{T@sub{AB}}), @var{B}'s 75name, the current time, the lifetime of the ticket, and the session key, all 76encrypted in @var{A}'s secret key (@{@var{B}, @var{t@sub{issue}}, 77@var{life}, @var{K@sub{AB}}, @var{T@sub{AB}}@}@var{K@sub{A}}). @var{A} 78decrypts the reply and retains it for later use. 79 80@sp 1 81 82Before sending a message to @var{B}, @var{A} creates an authenticator 83consisting of @var{A}'s name, @var{A}'s address, the current time, and a 84``checksum'' chosen by @var{A}, all encrypted with the secret session 85key (@{@var{A}, @var{A@sub{addr}}, @var{t@sub{current}}, 86@var{checksum}@}@var{K@sub{AB}}). This is sent together with the ticket 87received from the kerberos server to @var{B}. Upon reception, @var{B} 88decrypts the ticket using @var{B}'s secret key. Since the ticket 89contains the session key that the authenticator was encrypted with, 90@var{B} can now also decrypt the authenticator. To verify that @var{A} 91really is @var{A}, @var{B} now has to compare the contents of the ticket 92with that of the authenticator. If everything matches, @var{B} now 93considers @var{A} as properly authenticated. 94 95@c (here we should have some more explanations) 96 97@section Different attacks 98 99@subheading Impersonating A 100 101An impostor, @var{C} could steal the authenticator and the ticket as it 102is transmitted across the network, and use them to impersonate 103@var{A}. The address in the ticket and the authenticator was added to 104make it more difficult to perform this attack. To succeed @var{C} will 105have to either use the same machine as @var{A} or fake the source 106addresses of the packets. By including the time stamp in the 107authenticator, @var{C} does not have much time in which to mount the 108attack. 109 110@subheading Impersonating B 111 112@var{C} can hijack @var{B}'s network address, and when @var{A} sends 113her credentials, @var{C} just pretend to verify them. @var{C} can't 114be sure that she is talking to @var{A}. 115 116@section Defense strategies 117 118It would be possible to add a @dfn{replay cache} 119@cindex replay cache 120to the server side. The idea is to save the authenticators sent during 121the last few minutes, so that @var{B} can detect when someone is trying 122to retransmit an already used message. This is somewhat impractical 123(mostly regarding efficiency), and is not part of Kerberos 4; MIT 124Kerberos 5 contains it. 125 126To authenticate @var{B}, @var{A} might request that @var{B} sends 127something back that proves that @var{B} has access to the session 128key. An example of this is the checksum that @var{A} sent as part of the 129authenticator. One typical procedure is to add one to the checksum, 130encrypt it with the session key and send it back to @var{A}. This is 131called @dfn{mutual authentication}. 132 133The session key can also be used to add cryptographic checksums to the 134messages sent between @var{A} and @var{B} (known as @dfn{message 135integrity}). Encryption can also be added (@dfn{message 136confidentiality}). This is probably the best approach in all cases. 137@cindex integrity 138@cindex confidentiality 139 140@section Further reading 141 142The original paper on Kerberos from 1988 is @cite{Kerberos: An 143Authentication Service for Open Network Systems}, by Jennifer Steiner, 144Clifford Neuman and Jeffrey I. Schiller. 145 146A less technical description can be found in @cite{Designing an 147Authentication System: a Dialogue in Four Scenes} by Bill Bryant, also 148from 1988. 149 150These documents can be found on our web-page at 151@url{http://www.pdc.kth.se/kth-krb/}. 152