Writing a Liberty service provider in C

Author: Frederic Peters
Contact: fpeters@entrouvert.com
Date: 2006-09-21
Revision: 1.41
Copyright: Copyright © 2004, 2005, 2006 Entr'ouvert

1   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[])
    printf("Hello world.\n");
    return 0;

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

 $ pkg-config lasso --cflags
-I/usr/include/lasso -I/usr/include/libxml2 -I/usr/include/xmlsec1 -I/usr/include/glib-2.0
 $ 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.

2   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.


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

2.1   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 */


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);


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

3   Liberty Metadata Files

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

<?xml version="1.0"?>
    providerID="https://sp.example.com/" xmlns="urn:liberty:metadata:2003-08">
    <AssertionConsumerServiceURL id="AssertionConsumerServiceURL1" isDefault="true">

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.

4   Single Sign-On and Federation Profile


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.


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)->consent =

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.


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);

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);

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);

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

/* code to store identity_dump and session_dump */

Finally the login object can then be destroyed:


And a success web page displayed.

5   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);

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.

6   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 */

7   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


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