the Massachusetts Institute of Technology, working with IBM and
Digital Equipment Corporation, embarked on an eight-year project
designed to integrate computers into the
university's undergraduate curriculum. The project
was called Project Athena.
began operation with nearly 50 time-sharing minicomputers: Digital
Equipment Corporation's VAX 11/750 systems running
Berkeley 4.2 Unix. Each VAX had a few terminals; when a student or
faculty member wanted to use a computer, he sat down at one of its
Within a few years, Athena began moving away from the 750s. The
project received hundreds of high-performance workstations with big
screens, fast (for the time) processors, small disks, and Ethernet
interfaces. The project's goal was to allow any user
to sit down at any computer and enjoy full access to his files and to
Of course there were problems. As soon as the workstations were
deployed, the problems of network eavesdropping became painfully
obvious; with the network accessible from all over campus, nothing
prevented students (or outside intruders) from running network spy
programs. It was nearly impossible to prevent the students from
learning the superuser password of the workstations or simply
rebooting them in single-user mode. To further complicate matters,
many of the computers on the network were IBM PC/ATs running software
that didn't have even rudimentary computer security.
Something had to be done to protect student files in the networked
environment to the same degree that they were protected in the
Athena's ultimate solution to this security problem
was Kerberos, an authentication system that uses DES cryptography to
protect sensitive information such as passwords on an open network.
When the user logs into a workstation running Kerberos, that user is
issued a ticket from
the Kerberos server. The user's ticket can be
decrypted only with the user's password; it contains
information necessary to obtain additional tickets. From that point
on, whenever the user wishes to access a network service, an
appropriate ticket for that service must be presented. As all of the
information in the Kerberos tickets is encrypted before it is sent
over the network, the information is not susceptible to eavesdropping
Windows 2000 includes a Kerberos Server, allowing Windows 2000
servers to act as authentication servers for Unix clients.
14.3.1 Kerberos Authentication
authentication is based entirely on the
knowledge of passwords that are stored on the Kerberos Server. Unlike
Unix passwords, which are encrypted with a one-way algorithm that
cannot be reversed, Kerberos passwords are stored on the server
encrypted with a conventional encryption algorithm—in most
cases, DES—so that they can be decrypted
by the server when needed. A user proves her identity to the Kerberos
Server by demonstrating knowledge of her key.
The fact that the Kerberos Server has access to the
user's decrypted password is a result of the fact
that Kerberos does not use public key cryptography. This is a
serious disadvantage of the Kerberos system. It means that the
Kerberos Server must be both physically secure and
"computationally secure." The
server must be physically secure to prevent an attacker from stealing
the Kerberos Server and learning all of the users'
passwords. The server must also be immune to login attacks: if an
attacker could log onto the server and become
root, that attacker could, once again, steal all
of the passwords.
Kerberos was designed so that the server can be stateless. (The
server actually has a lot of permanent, sensitive state—the
user passwords—but this is kept on the hard disk, rather than
in RAM, and does not need to be updated during the course of Kerberos
transactions.) The Kerberos Server simply answers requests from users
and issues tickets (when appropriate). This design makes it
relatively simple to create replicated, secondary servers that can
handle authentication requests when the primary server is down or
otherwise unavailable. Unfortunately, these secondary servers need
complete copies of the entire Kerberos database, which means that
they must also be physically and computationally secure.
22.214.171.124 Initial login
into a Unix workstation that is using Kerberos looks the same to a
user as logging into a regular Unix computer. Sitting at the
workstation, you see the traditional login: and
password: prompts. You type your username and
password, and if they are correct, you get logged in. Accessing
files, electronic mail, printers, and other resources all work as
What happens behind the scenes is far more complicated. When the
other network daemon (or, more commonly these days, the PAM library
that performs authentication for these programs) knows about
Kerberos, it uses the Kerberos system to authenticate the user. The
details differ between the two different versions of Kerberos that
are commonly available: Kerberos Version 4 and Kerberos Version 5.
With Kerberos 4, the workstation sends a message to the Kerberos
Authentication Server after
you type your username. This message contains your username and
indicates that you are trying to log in. The Kerberos Server checks
its database and, if you are a valid user, sends back a
ticket-granting ticket that is encrypted with your
password. The workstation then asks you to type in your password and
finally attempts to decrypt the encrypted ticket using the password
that you've supplied. If the decryption is
successful, the workstation then forgets your password and uses the
ticket-granting ticket exclusively. If the decryption fails, the
workstation knows that you supplied the wrong password and gives you
a chance to try again.
With Kerberos 5, the workstation waits until after you have typed
your password before contacting the server. It then sends the
Kerberos Authentication Server a message consisting of your username
and the current time encrypted with your password. The Authentication
Server looks up your username, determines your password, and attempts
to decrypt the encrypted time. If the server can decrypt the current
time (and the value is indeed current), it then creates a
ticket-granting ticket, encrypts it with your password, and sends to
Figure 14-3 shows a schematic of the initial
Figure 14-3. Initial Kerberos authentication
What is this ticket-granting ticket? It is a block of data that
contains two pieces of information:
The user's workstation can now contact the Kerberos
Ticket Granting Service to obtain tickets for any principal within
the Kerberos realm—that is, the set of
servers and users that are known to the Kerberos Server. Note that:
Passwords are stored on the Kerberos Server, not on the individual
The user's password is never transmitted on the
network—encrypted or otherwise.
The Kerberos Authentication Server can authenticate the
user's identity because the user knows the
The user can authenticate the Kerberos Server's
identity because the Kerberos Authentication Server knows the
An eavesdropper who intercepts the ticket sent to you from the
Kerberos Server will not benefit from the message because it is
encrypted using a key (your password) that the eavesdropper
doesn't know. Likewise, an eavesdropper who
intercepts the ticket sent from the Kerberos Server to the Ticket
Granting Service will not be able to make use of the ticket because
it is encrypted with the Ticket Granting Service's
126.96.36.199 Using the ticket-granting ticket
Once you have obtained a ticket-granting ticket, you will likely want
to do something that requires the use of an authenticated service.
For example, you will probably want to read the files in your home
Under Sun Microsystems' regular version of NFS, once
a file server exports its filesystem to a workstation, the server
implicitly trusts whatever the workstation wants to do. If
george is logged into the workstation, the server lets
george access the files in his home directory.
But if george becomes the superuser on his
workstation, changes his UID to that of bill, and
starts accessing bill's files, the
vanilla NFS server has no mechanism to detect this trickery or to
take evasive action.
The scenario is very different when NFS has been modified to use
When the user first tries to access his files from a Kerberos
workstation, system software on the workstation contacts the Ticket
Granting Service and asks for a ticket for the File Server Service.
The Ticket Granting Service sends the user back a ticket for the File
Server Service. This ticket contains another ticket, encrypted with
the File Server Service's password, that the
user's workstation can present to the File Server
Service to request files. The contained ticket includes the
user's authenticated name, the expiration time, and
the Internet address of the user's workstation. The
user's workstation then presents this ticket to the
File Server Service. The File Server Service decrypts the ticket
using its own password, then builds a mapping between the (UID, IP
address) of the user's workstation and a UID on the
file server. Figure 14-4 shows these operations.
Figure 14-4. Workstation/file server/Ticket Granting Service communication
As before, all of the requests and tickets exchanged between the
workstation and the Ticket Granting Service are encrypted, protecting
them from eavesdroppers.
The Ticket Granting Service was able to establish the
user's identity when the user asked for a ticket for
the File Server Service because:
The user's File Server Service Ticket request was
encrypted using the session key.
The only way the user could have learned the session key was by
decrypting the original Ticket Granting Ticket that the user received
from the Kerberos Authentication Server.
To decrypt that original ticket, the user's
workstation had to know the user's password. (Note
again that this password was never transmitted over the network.)
The File Server Service was able to establish the
user's identity because:
The ticket that it receives requesting service from the user is
encrypted with the File Server Service's own key.
Inside that ticket is the IP address and username of the user.
The only way for that information to have gotten inside the ticket
was for the Ticket Granting Service to have put it there.
Therefore, the Ticket Granting Service is sure of the
And that's good enough for the File Server Service.
After authentication takes place, the workstation uses the network
service as usual. Other Kerberized network services operate in a
Kerberos puts the time of day in the request to prevent an
eavesdropper from intercepting the Request For Service request and
retransmitting it from the same host at a later time. This sort of
attack is called a playback or
188.8.131.52 Authentication, data integrity, and secrecy
is a general-purpose system for sharing secret keys between
principals on the network. Normally, Kerberos is used solely for
authentication. However, the ability to exchange keys can also be
used to ensure data integrity and secrecy.
If eavesdropping is an ongoing concern, all information transmitted
between the workstation and the service can be encrypted using a key
that is exchanged between the two principals. Unfortunately,
encryption carries a performance penalty. At MIT's
Project Athena, encryption was used for transmitting highly sensitive
information such as passwords but was not used for most data
transfer, such as files and electronic mail.
For single-user workstations, Kerberos provides significant
additional security beyond that of regular passwords. However, if two
people are logged into the workstation at the same time, then the
workstation will be authenticated for both
users. These users can then pose as each other. This threat is so
significant that at MIT's Project Athena, remote
login services were disabled on workstations to prevent an attacker
from logging in while a legitimate user was being authenticated. It
is also possible for someone to subvert the local software to capture
the user's password as it is typed (as with other
In early 1996, graduate students with the COAST Laboratory at Purdue University discovered a long-standing weakness
in the key generation for Kerberos 4. The weakness allows an attacker
to guess session keys in a matter of seconds. A patch has been widely
distributed; be sure to install it if you are using Kerberos 4.
184.108.40.206 Kerberos 4 versus Kerberos 5
Kerberos has gone through five major
revisions during its history. Both Kerberos 4 and Kerberos 5 are now
Kerberos 4 is more efficient than Kerberos 5, but more limited. For
example, Kerberos 4 can work only over TCP/IP networks. Kerberos 4
has not been updated in many years and is currently deprecated. In
fact, some Kerberos 4 implementations are vulnerable to
buffer-overflow attacks, and no patches have been posted.
Kerberos 5 fixes minor problems with the Kerberos protocol, making it
more resistant to determined attacks over the network. Kerberos 5 is
also more flexible: it can work with different kinds of networks. It
also has provisions for working with encryption schemes other than
DES. Although algorithms such as Triple-DES have been implemented,
their use is not widespread largely because of legacy applications
that expect DES encryption.
Finally, Kerberos 5 supports delegation of authentication, ticket
expirations longer than 21 hours, renewable tickets, tickets that
will work sometime in the future, and many more options. If you are
going to use Kerberos, you should definitely use Version 5. IETF is
working to revise and clarify RFC 1510, which defines Kerberos 5, and
major protocol extensions are expected to follow.
14.3.2 Getting Kerberos
Kerberos is a complicated process that depends on the version of
Kerberos you have, the kind of computer, and the version of your
computer's operating system. It's a
difficult task that requires you to have the source code for your
computer system or the source code for replacement programs. It is
not a task to be undertaken lightly.
Fortunately, you don't have to go through this
arduous process. Kerberos or Kerberos-like security systems are now
available from several companies, and they are a standard part of
several operating systems, including Solaris, Mac OS X, and many
Linux and BSD distributions. These days, there is no reason to be
running anything but secure network services.
As an additional benefit, a version of Kerberos 5 has been included
in Microsoft Windows from the Windows 2000 release onwards. Thus,
with some effort, it is possible to make Kerberos interoperable
between all your various Unix machines and Windows
The MIT Kerberos source code is available from http://web.mit.edu/kerberos/www/. This is also the site where you can find official
updates, patches, and bug announcements. There is also a free
software implementation of Kerberos called Heimdal that is under
active development; it is largely compatible with
MIT's Kerberos, so most of what we discuss below
should apply to Heimdal as well. You can get Heimdal at http://www.pdc.kth.se/heimdal/.
As the changes required to your system's software
are substantial if you need to do them yourself, the actual
installation process will not be described here. See the
documentation provided with Kerberos for details.
14.3.3 Using Kerberos
workstation equipped with Kerberos is only slightly different from
using an ordinary workstation. In most implementations, all of the
special Kerberos housekeeping functions are performed automatically.
When the user logs in, the password typed is used to acquire a
Kerberos ticket, which in turn grants access to the services on the
network. Additional tickets are automatically requested as they are
needed. Tickets for services are automatically cached. All of a
user's tickets are automatically destroyed when the
user logs out.
Of course, the Kerberos client needs to know where to find Kerberos
servers. This can be configured manually on each client
(traditionally in the krb5.conf file), or
Kerberos servers can be advertised through DNS SRV records. IETF
Internet Draft draft-ietf-krb-wg-krb-dns-locate
describes this approach.
Kerberos isn't entirely transparent. If you are
logged into a Kerberos workstation for more than eight
hours, something odd happens: network services
suddenly stop working properly. The reason for this is that tickets
issued by Kerberos expire after eight hours, a technique designed to
prevent a replay attack. (In such an attack, somebody capturing one
of your tickets sits down at your workstation after you leave, using
the captured ticket to gain access to your files.) Thus, after eight
hours, you must run the
kinit program, and provide your username and
password a second time, to be issued a new ticket for the Kerberos
Ticket Granting Service.
Kerberos mixes well with LDAP
(discussed in the next section). Kerberos can be used to authenticate
and secure LDAP queries and updates. Conversely, the LDAP database
can store information about users that is more extensive than the
data maintained by Kerberos alone, such as the
user's home directory, shell, phone number, or other
organizational information. Together, the two services can provide
all of the functionality of NIS or NIS+, and they are increasingly
being used to do so. Jason Heiss provides a good guide to this
process on his page "Replacing NIS with Kerberos and
LDAP" at http://www.ofb.net/~jheiss/krbldap/.
LDAP is sometimes used to store Kerberos keys. The Windows
implementation of Kerberos uses Microsoft's Active
Directory Service (a flavor of LDAP) to store Kerberos keys. Heimdal
Kerberos supports this functionality. MIT Kerberos does not, out of
concern that sensitive security infrastructure should be centralized
at the Kerberos Server, rather than distributed via LDAP.
14.3.4 Kerberos Limitations
Although Kerberos is an excellent solution
to a difficult problem, it has several shortcomings:
- Every network service must be individually modified for use with Kerberos
Because of the Kerberos design, every program that uses Kerberos must
be modified. The process of performing these modifications is often
called "Kerberizing" the
application. The amount of work that this entails depends entirely on
the application program. Typically, to Kerberize an application, you
must have the application's source code.
You might recognize this problem as similar to making a service
and you'd be right. In fact, because PAM offers
several Kerberos modules, any service that can use PAM for
authentication can, through PAM, use Kerberos. This is probably the
most flexible and convenient way to Kerberize a service. See Chapter 4 for more information about PAM.
- Kerberos doesn't work well in a time-sharing environment
Kerberos is designed for an environment in which there is one user
per workstation. Because of the difficulty of sharing data between
different processes running on the same Unix computer, Kerberos keeps
tickets in the /tmp directory. If a user is
sharing the computer with several other people, it is possible that
the user's tickets can be stolen (copied by an
attacker). Stolen tickets can then be used to obtain fraudulent
- Kerberos requires a secure Kerberos Server
By design, Kerberos requires that there be a secure central server
that maintains the master password database. To ensure security, a
site should use the Kerberos Server for absolutely nothing beyond
running the Kerberos Server program. The Kerberos Server must be kept
under lock and key in a physically secure area. In some environments,
maintaining such a server is an administrative and/or financial
- Kerberos requires a continuously available Kerberos Server
If the Kerberos Server goes down and is not replicated, the Kerberos
network is unusable.
- Kerberos stores all passwords encrypted with a single key
Adding to the difficulty of running a secure server is the fact that
the Kerberos Server stores all passwords encrypted with the
server's master key, which happens to be located on
the same hard disk as the encrypted passwords. This means that, in
the event that the Kerberos Server or its backups are compromised,
all user passwords must be changed.
- Kerberos does not protect against modifications to system software (Trojan horses)
Kerberos does not have the computer authenticate itself to the
user—that is, there is no way for a user sitting at a computer
to determine whether the computer has been compromised. This failing
is easily exploited by a knowledgable attacker.
For example, an intruder can modify the
workstation's system software so every
username/password combination typed is recorded automatically or sent
electronically to another machine controlled by the attacker.
Alternatively, a malicious attacker can simply modify the
workstation's software to spuriously delete the
user's files after the user has logged in and
authenticated himself to the File Server Service. Both of these
problems are consequences of the fact that, even in a networked
environment, many workstations contain local copies of the programs
that they run.
- Kerberos may result in a cascading loss of trust
Another problem with Kerberos is that if a server password or a user
password is broken or otherwise disclosed, it is possible for an
eavesdropper to use that password to decrypt other tickets and use
this information to spoof servers and users.
Kerberos is a workable system for network security, and it is widely
used. But more importantly, the principles behind Kerberos are
increasingly available in network security systems that are available
directly from vendors.