XSECURITY(1) MachTen Programmer’s Manual XSECURITY(1)

NAME
X security - X display access control

SYNOPSIS
X provides mechanism for implementing many access control
systems. Release 6 includes five mechanisms:
Host Access Simple host-based access control.
MIT-MAGIC-COOKIE-1 Shared plain-text "cookies".
XDM-AUTHORIZATION-1 Secure DES based private-keys.
SUN-DES-1 Based on Sun’s secure rpc system.
MIT-KERBEROS-5 Kerberos Version 5 user-to-user.

ACCESS SYSTEM DESCRIPTIONS
Host Access
Any client on a host in the host access control
list is allowed access to the X server. This sys-
tem can work reasonably well in an environment
where everyone trusts everyone, or when only a sin-
gle person can log in to a given machine, and is
easy to use when the list of hosts used is small.
This system does not work well when multiple people
can log in to a single machine and mutual trust
does not exist. The list of allowed hosts is
stored in the X server and can be changed with the
xhost command. When using the more secure mecha-
nisms listed below, the host list is normally con-
figured to be the empty list, so that only autho-
rized programs can connect to the display.

MIT-MAGIC-COOKIE-1
When using MIT-MAGIC-COOKIE-1, the client sends a
128 bit "cookie" along with the connection setup
information. If the cookie presented by the client
matches one that the X server has, the connection
is allowed access. The cookie is chosen so that it
is hard to guess; xdm generates such cookies auto-
matically when this form of access control is used.
The user’s copy of the cookie is usually stored in
the .Xauthority file in the home directory,
although the environment variable XAUTHORITY can be
used to specify an alternate location. Xdm auto-
matically passes a cookie to the server for each
new login session, and stores the cookie in the
user file at login.

The cookie is transmitted on the network without
encryption, so there is nothing to prevent a net-
work snooper from obtaining the data and using it
to gain access to the X server. This system is
useful in an environment where many users are run-
ning applications on the same machine and want to
avoid interference from each other, with the caveat
that this control is only as good as the access
control to the physical network. In environments
where network-level snooping is difficult, this
system can work reasonably well.

XDM-AUTHORIZATION-1
Sites in the United States can use a DES-based
access control mechanism called XDM-
AUTHORIZATION-1. It is similar in usage to MIT-
MAGIC-COOKIE-1 in that a key is stored in the .Xau-
thority file and is shared with the X server. How-
ever, this key consists of two parts - a 56 bit DES
encryption key and 64 bits of random data used as
the authenticator.

When connecting to the X server, the application
generates 192 bits of data by combining the current
time in seconds (since 00:00 1/1/1970 GMT) along
with 48 bits of "identifier". For TCP/IP connec-
tions, the identifier is the address plus port num-
ber; for local connections it is the process ID and
32 bits to form a unique id (in case multiple con-
nections to the same server are made from a single
process). This 192 bit packet is then encrypted
using the DES key and sent to the X server, which
is able to verify if the requestor is authorized to
connect by decrypting with the same DES key and
validating the authenticator and additional data.
This system is useful in many environments where
host-based access control is inappropriate and
where network security cannot be ensured.

SUN-DES-1
Recent versions of SunOS (and some other systems)
have included a secure public key remote procedure
call system. This system is based on the notion of
a network principal; a user name and NIS domain
pair. Using this system, the X server can securely
discover the actual user name of the requesting
process. It involves encrypting data with the X
server’s public key, and so the identity of the
user who started the X server is needed for this;
this identity is stored in the .Xauthority file.
By extending the semantics of "host address" to
include this notion of network principal, this form
of access control is very easy to use.

To allow access by a new user, use xhost. For
example,
xhost keith@ ruth@mit.edu
adds "keith" from the NIS domain of the local
machine, and "ruth" in the "mit.edu" NIS domain.
For keith or ruth to successfully connect to the
display, they must add the principal who started
the server to their .Xauthority file. For example:
xauth add expo.lcs.mit.edu:0 SUN-DES-1 unix.expo.lcs.mit.edu@our.domain.edu
This system only works on machines which support
Secure RPC, and only for users which have set up
the appropriate public/private key pairs on their
system. See the Secure RPC documentation for
details. To access the display from a remote host,
you may have to do a keylogin on the remote host
first.

MIT-KERBEROS-5
Kerberos is a network-based authentication scheme
developed by MIT for Project Athena. It allows
mutually suspicious principals to authenticate each
other as long as each trusts a third party, Ker-
beros. Each principal has a secret key known only
to it and Kerberos. Principals includes servers,
such as an FTP server or X server, and human users,
whose key is their password. Users gain access to
services by getting Kerberos tickets for those
services from a Kerberos server. Since the X
server has no place to store a secret key, it
shares keys with the user who logs in. X authenti-
cation thus uses the user-to-user scheme of Ker-
beros version 5.

When you log in via xdm, xdm will use your password
to obtain the initial Kerberos tickets. xdm stores
the tickets in a credentials cache file and sets
the environment variable KRB5CCNAME to point to the
file. The credentials cache is destroyed when the
session ends to reduce the chance of the tickets
being stolen before they expire.

Since Kerberos is a user-based authorization proto-
col, like the SUN-DES-1 protocol, the owner of a
display can enable and disable specific users, or
Kerberos principals. The xhost client is used to
enable or disable authorization. For example,
xhost krb5:judy krb5:gildea@x.org
adds "judy" from the Kerberos realm of the local
machine, and "gildea" from the "x.org" realm.

THE AUTHORIZATION FILE
Except for Host Access control, each of these systems uses
data stored in the .Xauthority file to generate the cor-
rect authorization information to pass along to the X
server at connection setup. MIT-MAGIC-COOKIE-1 and XDM-
AUTHORIZATION-1 store secret data in the file; so anyone
who can read the file can gain access to the X server.
SUN-DES-1 stores only the identity of the principal who
started the server (unix.hostname@domain when the server
is started by xdm), and so it is not useful to anyone not
authorized to connect to the server.

Each entry in the .Xauthority file matches a certain con-
nection family (TCP/IP, DECnet or local connections) and X
display name (hostname plus display number). This allows
multiple authorization entries for different displays to
share the same data file. A special connection family
(FamilyWild, value 65535) causes an entry to match every
display, allowing the entry to be used for all connec-
tions. Each entry additionally contains the authorization
name and whatever private authorization data is needed by
that authorization type to generate the correct informa-
tion at connection setup time.

The xauth program manipulates the .Xauthority file format.
It understands the semantics of the connection families
and address formats, displaying them in an easy to under-
stand format. It also understands that SUN-DES-1 and MIT-
KERBEROS-5 use string values for the authorization data,
and displays them appropriately.

The X server (when running on a workstation) reads autho-
rization information from a file name passed on the com-
mand line with the -auth option (see the Xserver manual
page). The authorization entries in the file are used to
control access to the server. In each of the authoriza-
tion schemes listed above, the data needed by the server
to initialize an authorization scheme is identical to the
data needed by the client to generate the appropriate
authorization information, so the same file can be used by
both processes. This is especially useful when xinit is
used.

MIT-MAGIC-COOKIE-1
This system uses 128 bits of data shared between
the user and the X server. Any collection of bits
can be used. Xdm generates these keys using a
cryptographically secure pseudo random number gen-
erator, and so the key to the next session cannot
be computed from the current session key.

XDM-AUTHORIZATION-1
This system uses two pieces of information. First,
64 bits of random data, second a 56 bit DES encryp-
tion key (again, random data) stored in 8 bytes,
the last byte of which is ignored. Xdm generates
these keys using the same random number generator
as is used for MIT-MAGIC-COOKIE-1.

SUN-DES-1
This system needs a string representation of the
principal which identifies the associated X server.
This information is used to encrypt the client’s
authority information when it is sent to the X
server. When xdm starts the X server, it uses the
root principal for the machine on which it is run-
ning (unix.hostname@domain, e.g.,
"unix.expire.lcs.mit.edu@our.domain.edu"). Putting
the correct principal name in the .Xauthority file
causes Xlib to generate the appropriate authoriza-
tion information using the secure RPC library.

MIT-KERBEROS-5
Kerberos reads tickets from the cache pointed to by
the KRB5CCNAME environment variable, so does not
use any data from the .Xauthority file. An empty
entry must still exist to tell clients that MIT-
KERBEROS-5 is available.

FILES
.Xauthority

SEE ALSO
X(3), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)

X Version 11 Release 6 4