XSECURITY(7x)															     XSECURITY(7x)

NAME

       Xsecurity - X display access control

SYNOPSIS

       X provides mechanism for implementing many access control systems.  The sample implementation 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 system can work reasonably well in an
	      environment where everyone trusts everyone, or when only a single 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 mechanisms listed below, the host list is normally configured to be the empty list, so that only authorized programs can con-
	      nect 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  pre-
	      sented  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 automatically when this form of access control is used.  The user's copy of the cookie is  usu-
	      ally  stored  in	the .Xauthority file in the home directory, although the environment variable XAUTHORITY can be used to specify an
	      alternate location.  Xdm automatically 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 network 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 running 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 .Xauthority file and is shared with the X server.  However, 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 connections, the identifier is the address plus port number; for local
	      connections it is the process ID and 32 bits to form a unique id (in case multiple connections 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, Kerberos.  Each principal has a secret key known only to it and Ker-
	      beros.  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 authentication thus uses the user-to-user scheme of Kerberos 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 protocol, 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 correct authorization infor-
       mation 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 connection 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	connections.   Each  entry
       additionally  contains  the authorization name and whatever private authorization data is needed by that authorization type to generate the
       correct information 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 understand format.  It also understands that SUN-DES-1 and MIT-KERBEROS-5 use string values for the autho-
       rization data, and displays them appropriately.

       The X server (when running on a workstation) reads authorization information from a file name passed on the command  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
       authorization 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 use-
       ful 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 generator, 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 encryption 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 princi-
	      pal for the machine on which it is running (unix.hostname@domain, e.g., "unix.expire.lcs.mit.edu@our.domain.edu").  Putting the cor-
	      rect  principal  name in the .Xauthority file causes Xlib to generate the appropriate authorization 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 .Xauthor-
	      ity file.  An entry with no data must still exist to tell clients that MIT-KERBEROS-5 is available.

	      Unlike  the .Xauthority file for clients, the authority file passed by xdm to a local X server (with ``-auth filename'', see xdm(1))
	      does contain the name of the credentials cache, since the X server will not have the KRB5CCNAME environment variable set.  The  data
	      of  the  MIT-KERBEROS-5 entry is the credentials cache name and has the form ``UU:FILE:filename'', where filename is the name of the
	      credentials cache file created by xdm.  Note again that this form is not used by clients.

FILES

       .Xauthority

SEE ALSO

       X(7x), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)

X Version 11							    Release 6.6 						     XSECURITY(7x)