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XSecurity(1X)							XSecurity(1X)
X11R6									X11R6


  XSecurity, Xsecurity - X display access control


  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.


  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 environ-
      ment 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 connect to the display.

      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 automatically 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 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.

      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	authentica-

      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 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 requester is authorized to	con-
      nect by decrypting with the same DES key and validating the authentica-
      tor and additional data. This system is useful in	many environments
      where host-based access control is inappropriate and where network
      security cannot be ensured.

      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@	ruthATmit.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.eduATour.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.

      Kerberos is a network-based authentication scheme	developed by MIT for
      Project Athena.  It allows mutually suspicious principals	to authenti-
      cate each	other as long as each trusts a third party, Kerberos.  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 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	ini-
      tial 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:gildeaATx.org

      adds "judy" from the Kerberos realm of the local machine,	and "gildea"
      from the "x.org" realm.


  Except for Host Access control, each of these	systems	uses data stored in
  the .Xauthority file to generate the correct 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 iden-
  tity 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 (Fami-
  lyWild, 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 authorization data, and
  displays them	appropriately.

  The X	server (when running on	a workstation) reads authorization informa-
  tion from a file name	passed on the command line with	the -auth option (see
  the Xserver(1X) 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

      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

      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-

      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 running (unix.hostname@domain, for	example,
      "unix.expire.lcs.mit.eduATour.edu").  Putting the correct princi-
      pal name in the .Xauthority file causes Xlib to generate the
      appropriate authorization	information using the secure RPC library.

      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.




  X(1X), xdm(1X), xauth(1X), xhost(1X),	Xserver(1X)