lasso/docs/lasso-book/writing-a-c-sp.txt

=======================================
Writing a Liberty service provider in C
=======================================

:author: Frederic Peters
:contact: fpeters@entrouvert.com
:date: $Date$
:revision: $Revision$
:copyright: Copyright © 2004-2007 Entr'ouvert

.. contents:: Table of Contents
.. section-numbering::


Lasso Projects Basics
=====================

Lasso functions are defined in several header files typically located in
``/usr/include/lasso/`` or ``/usr/local/include/lasso/``.  It is possible to
include individual files but in most case it is enough to include the main
``lasso.h``.

The first thing to do is then to call ``lasso_init()``.  Similarly the last
thing will be to call ``lasso_shutdown()``.  The smallest and useless Lasso
project will therefore be::

  #include <lasso/lasso.h>

  int main(int argc, char *argv[])
  {
      lasso_init();
      printf("Hello world.\n");
      lasso_shutdown();
      return 0;
  }

Lasso uses a tool called ``pkg-config`` to know the necessary flags for
compilation and linking.

::

  $ pkg-config lasso --cflags
 -DXMLSEC_CRYPTO=\"openssl\" -DXMLSEC_LIBXML_260=1 -D__XMLSEC_FUNCTION__=__FUNCTION__
 -DXMLSEC_NO_XKMS=1 -DXMLSEC_NO_CRYPTO_DYNAMIC_LOADING=1 -DXMLSEC_CRYPTO_OPENSSL=1
 -I/usr/include/lasso -I/usr/include/libxml2 -I/usr/include/xmlsec1 -I/usr/include/glib-2.0
 -I/usr/lib/glib-2.0/include
  $ pkg-config lasso --libs
 -llasso -lxmlsec1-openssl -lxmlsec1 -lssl -lcrypto -ldl -lgobject-2.0 -lxslt -lxml2
 -lpthread -lz -lm -lglib-2.0


Creating an executable from the previous sample would then a simple matter of
calling ``gcc`` with the right flags.  But there is currently a bug in
XMLSec, the library used by Lasso to provide XML Signature and XML Encryption
support.  It is possible to workaround the bug::

  $ gcc hello.c -o hello $(pkg-config lasso --cflags --libs)
 <command line>:4:16: missing terminating " character
  $ gcc hello.c -o hello $(pkg-config xmlsec1 --cflags --libs | tr -d '\\')
  $ ./hello
 Hello world.


Liberty and Lasso profiles
==========================

Lasso provides the necessary functions to implement Liberty Alliance profiles,
as defined in the `Liberty ID-FF Bindings and Profiles Specification`_.  They
are:

- Single Sign-On and Federation
- Name Registration
- Federation Termination Notification
- Single Logout
- Identity Provider Introduction
- Name Identifier Mapping
- Name Identifier Encryption

Each profile maps to a Lasso object such as ``LassoLogin``, ``LassoLogout``...
Those are initialised with data known about identity and service providers,
available in a ``LassoServer`` object.

The ``LassoServer`` object may be created as follows:

::

  LassoServer *server;
  server = lasso_server_new("sp-metadata.xml",
  		"sp-private-key.pem", NULL, "sp-crt.pem");

  lasso_server_add_provider(server, LASSO_PROVIDER_ROLE_IDP,
  		"idp-metadata.xml", "idp-public-key.pem", "ca-crt.pem");

- ``sp-metadata.xml`` is the Liberty metadata file for the service provider
- ``idp-metadata.xml`` is the Liberty metadata file for the identity provider
- ``sp-private-key.pem`` is the service provider private key; used to sign
  documents
- ``sp-crt.pem`` is the service provider certificate; sent inside signed
  documents
- ``idp-public-key.pem`` is the identity provider public key; used to verify
  signature in documents sent by the identity provider
- ``ca-crt.pem`` is the certificate of the certification authority used by the
  identity provider
- NULL, the third argument, would be used if the private key was protected by a
  password.

It is of course possible to have several calls so ``lasso_server_add_provider``
if there are more than one identity provider.

.. note:: Figures in the previously referred Binding and Profiles specification
          document are quite helpful in figuring out the message passing.


Serialisation
-------------

``LassoServer`` objects can be serialised into XML files::

  gchar *dump;
  FILE *fd;

  dump = lasso_server_dump(server);
  /* write dump into a file, a database, whatever */
  g_free(dump);

.. note:: ``lasso_server_dump`` (and other Lasso dump functions) allocates
          memory through GLib.  ``g_free`` is then the function to use instead
	  of ``free`` to release memory.

It is then really easy to get back properly constructed objects::

  LassoServer *server;
  gchar *dump;

  /* restore dump from file, database, whatever */
  server = lasso_server_new_from_dump(dump);

.. warning:: The server dump only contains the file names; not the actual file
             contents.  Files should not be moved afterwards.



Liberty Metadata Files
======================


They are descriptions of a provider containing ``providerID`` and various
normative URLs::

  <?xml version="1.0"?>
  <EntityDescriptor
      providerID="https://sp.example.com/" xmlns="urn:liberty:metadata:2003-08">
    <SPDescriptor>
      <SingleLogoutServiceURL>https://sp.example.com/singleLogout</SingleLogoutServiceURL>
      <SingleLogoutProtocolProfile>http://projectliberty.org/profiles/slo-idp-soap</SingleLogoutProtocolProfile>
      <SoapEndpoint>https://sp.example.com/soapEndpoint</SoapEndpoint>
      <AssertionConsumerServiceURL id="AssertionConsumerServiceURL1" isDefault="true">
        https://sp.example.com/assertionConsumer
      </AssertionConsumerServiceURL>
      <AuthnRequestsSigned>true</AuthnRequestsSigned>
    </SPDescriptor>
  </EntityDescriptor>

Describe a service provider (with providerID ``https://sp.example.com``) whose
single logout service URL is ``https://sp.example.com/singleLogout``.  Refer to
the Liberty Alliance specifications for details.


Single Sign-On and Federation Profile
=====================================

.. warning:: The source code presented in this section has for sole purpose
             to explain the different steps necessary to implement this
	     profile; they notably lack proper error checking.  See `Proper
	     Error Checking`_ for details on error checking.


As a first step the user points its browser to the service provider to the
login URL; the service provider must then respond with an HTTP 302 Redirect
response, pointing the user browser to the identity provider single sign on
service.

.. note:: the login URL is not normative; any name will do.


``server`` is a ``LassoServer`` as seen earlier and ``idpProviderId`` is a
string with the identity provider Id (the string must match a providerID
defined in the metadata file).

::

  LassoLogin *login;
  
  login = lasso_login_new(server);
  lasso_login_init_authn_request(login, idpProviderId, LASSO_HTTP_METHOD_REDIRECT);
  
  LASSO_LIB_AUTHN_REQUEST(LASSO_PROFILE(login)->request)->ForceAuthn = TRUE;
  LASSO_LIB_AUTHN_REQUEST(LASSO_PROFILE(login)->request)->IsPassive = FALSE;
  LASSO_LIB_AUTHN_REQUEST(LASSO_PROFILE(login)->request)->NameIDPolicy =
      strdup(LASSO_LIB_NAMEID_POLICY_TYPE_FEDERATED);
  LASSO_LIB_AUTHN_REQUEST(LASSO_PROFILE(login)->request)->consent =
      strdup(LASSO_LIB_CONSENT_OBTAINED);
  lasso_login_build_authn_request_msg(login);


You can now redirect the user to the URL defined in ``LASSO_PROFILE(login)->msg_url``; for
example, in a CGI::
  
  printf("Location: %s\n", LASSO_PROFILE(login)->msg_url);


The user then logs in on the identity provider which ultimately redirects back
to the service provider; to the assertion consumer URL.  A SAML artifact is
passed in the query parameter.

.. note:: the assertion consumer URL is defined by Liberty; it must be declared
   in the ``AssertionConsumerServiceURL`` element of the metadata file.

::

  LassoLogin *login;
  
  login = lasso_login_new(server);
  lasso_login_init_request(login, query_string, LASSO_HTTP_METHOD_REDIRECT);
  lasso_login_build_request_msg(login);

The service provider must check this artifact using a SOAP request to the
identity provider.  The URL is ``LASSO_PROFILE(login)->msg_url`` while the
request is ``LASSO_PROFILE(login)->msg_body``.  The request must succeed with
an HTTP 200 status code; let's consider its content is put in the ``answer``,
the next statement would be::

  lasso_login_process_response_msg(login, answer);

The users are defined by a ``nameIdentifier`` (accessible through
``LASSO_PROFILE(login)->nameIdentifier``).  Those typically map to users
and sessions in some database on the service provider.  If existing; the
session should probably contains a ``session_dump`` element and the user a
``identity_dump`` element.  See `Database Considerations`_ below for more
information.

It is now time to get them out of the database and apply them to the ``login``
object.

::

  if (session_dump != NULL) {
      lasso_profile_set_session_from_dump(LASSO_PROFILE(login), session_dump);
  }
  if (identity_dump != NULL) {
      lasso_profile_set_identity_from_dump(LASSO_PROFILE(login), identity_dump);
  }
  lasso_login_accept_sso(login);

After ``lasso_login_accept_sso`` the session and the identity are updated (or
created) and should then be saved.  If the identity has not recognised by the
service provider an account will probably have to be created on the service
provider; this is a good opportunity to ask the user for more information.

You can get respective dumps like this::

  LassoIdentity *identity;
  LassoSession *session;
  char *identity_dump = NULL, *session_dump = NULL;

  if (lasso_profile_is_identity_dirty(LASSO_PROFILE(login))) {
      identity = lasso_profile_get_identity(LASSO_PROFILE(login));
      identity_dump = lasso_identity_dump(identity);
      lasso_identity_destroy(identity);
  }

  if (lasso_profile_is_session_dirty(LASSO_PROFILE(login))) {
      session = lasso_profile_get_session(LASSO_PROFILE(login));
      session_dump = lasso_session_dump(session);
      lasso_session_destroy(session);
  }

  /* code to store identity_dump and session_dump */

Finally the ``login`` object can then be destroyed::

  lasso_login_destroy(login);

And a success web page displayed.


Single Logout Profile
=====================

There are different single logout profiles; some initiated on the identity
provider, others initiated on the service provider, using either HTTP redirects
or SOAP requests.

This part is about a logout using SOAP and initiated on the service provider.

::

  LassoLogout *logout;

  logout = lasso_logout_new(lassoServer);


Identity and session dumps should be restored to prepare the logout request.

::

  if (session_dump != NULL) {
      lasso_profile_set_session_from_dump(LASSO_PROFILE(logout), session_dump);
  }
  if (identity_dump != NULL) {
      lasso_profile_set_identity_from_dump(LASSO_PROFILE(logout), identity_dump);
  }
  
  lasso_logout_init_request(logout, idpProviderId, LASSO_HTTP_METHOD_SOAP);
  lasso_logout_build_request_msg(logout);


The service provider must then make a SOAP request to the identity provider;
``msg_url`` and ``msg_body``.  You should then pass the answer to Lasso::

  lasso_logout_process_response_msg(logout, answer);

And save back session and user dump; the process is similar as the one at the
end of the single sign on profile.


Proper Error Checking
=====================

Most Lasso functions returns 0 on success and a negative number on failure.  It
is strongly advised to check this return code on each call.

::

  int rc;

  rc = lasso_logout_process_response_msg(logout, answer)
  if (rc) {
      fprintf(stderr, "Lasso Error: %d\n", rc);
      /* handling error; most probably bailing out */
  }


Database Considerations
=======================

Lasso has been designed to let the service provider keep on using existing
databases.  Typically there is already a table describing users; just add an
identity dump column to the existing table:

=======    ========================================    ==============
User Id    existing data (name, address...)            Identity dump
=======    ========================================    ==============
1          ...                                         <Identity> ...
2          ...                                         <Identity> ...
=======    ========================================    ==============

Mapping between existing users and name identifiers sent by the identity
provider can be done with a simple table.

===============    =======
Name Identifier    User Id
===============    =======
AQWWRRS...         1
CGFASDE...         2
YYSSSDS...         1
===============    =======

.. note:: A separate table is needed because one user Id could map
          to several name identifiers; in case there are several identity
	  providers.

Sessions are also commonly stored in databases; just add a session dump column
to the existing session table:

==========    =================    =============
Session Id    misc session data    Session dump
==========    =================    =============
6744066       ...                  <Session> ...
3338824       ...                  <Session> ...
==========    =================    =============

Likewise sessions should be mapped to name identifiers.

===============    ==========
Name Identifier    Session Id
===============    ==========
AQWWRRS...         3338824
===============    ==========



API Reference
=============

- LassoLogin_
- LassoLogout_
- LassoIdentity_
- LassoServer_
- LassoSession_




.. _Liberty ID-FF Bindings and Profiles Specification:
   http://www.projectliberty.org/liberty/content/download/319/2369/file/draft-liberty-idff-bindings-profiles-1.2-errata-v2.0.pdf

.. _LassoLogin: /documentation/api-reference/lassologin.html
.. _LassoLogout: /documentation/api-reference/lassologout.html
.. _LassoIdentity: /documentation/api-reference/lassoidentity.html
.. _LassoServer: /documentation/api-reference/lassoserver.html
.. _LassoSession: /documentation/api-reference/lassosession.html