Add support for SingleSignOnService with HTTP-POST binding

Warning, this changes the return type of `prepare_for_authentication`
by including the chosen binding, and opens the door for supporting
other SingleSignOnService bindings.

src/saml2/client.py

@@ -42,7 +42,7 @@ class Saml2Client(Base):
     """ The basic pySAML2 service provider class """
 
     def prepare_for_authenticate(self, entityid=None, relay_state="",
-                                 binding=saml2.BINDING_HTTP_REDIRECT, vorg="",
+                                 binding=None, vorg="",
                                  nameid_format=None,
                                  scoping=None, consent=None, extensions=None,
                                  sign=None,
@@ -64,20 +64,31 @@ class Saml2Client(Base):
         :return: session id and AuthnRequest info
         """
 
-        destination = self._sso_location(entityid, binding)
+        expected_binding = binding
 
-        reqid, req = self.create_authn_request(destination, vorg, scoping,
+        for binding in [BINDING_HTTP_REDIRECT, BINDING_HTTP_POST]:
-                                               response_binding, nameid_format,
+            if expected_binding and binding != expected_binding:
-                                               consent=consent,
+                continue
-                                               extensions=extensions, sign=sign,
-                                               **kwargs)
-        _req_str = "%s" % req
 
-        logger.info("AuthNReq: %s" % _req_str)
+            destination = self._sso_location(entityid, binding)
+            logger.info("destination to provider: %s" % destination)
 
-        info = self.apply_binding(binding, _req_str, destination, relay_state)
+            reqid, request = self.create_authn_request(
+                destination, vorg, scoping, response_binding, nameid_format,
+                consent=consent,
+                extensions=extensions, sign=sign,
+                **kwargs)
 
-        return reqid, info
+            _req_str = str(request)
+
+            logger.info("AuthNReq: %s" % _req_str)
+
+            http_info = self.apply_binding(binding, _req_str, destination,
+                                           relay_state)
+
+            return reqid, binding, http_info
+        else:
+            raise SignonError("No binding available for singon")
 
     def global_logout(self, name_id, reason="", expire=None, sign=None):
         """ More or less a layer of indirection :-/

src/saml2/client_base.py

@@ -71,6 +71,10 @@ class VerifyError(SAMLError):
     pass
 
 
+class SignonError(SAMLError):
+    pass
+
+
 class LogoutError(SAMLError):
     pass
 

tests/test_51_client.py

@@ -619,10 +619,11 @@ class TestClientWithDummy():
     def test_do_authn(self):
         binding = BINDING_HTTP_REDIRECT
         response_binding = BINDING_HTTP_POST
-        sid, http_args = self.client.prepare_for_authenticate(
+        sid, auth_binding, http_args = self.client.prepare_for_authenticate(
             IDP, "http://www.example.com/relay_state",
             binding=binding, response_binding=response_binding)
 
+        assert binding == auth_binding
         assert isinstance(sid, basestring)
         assert len(http_args) == 4
         assert http_args["headers"][0][0] == "Location"
@@ -669,11 +670,13 @@ class TestClientWithDummy():
     def test_post_sso(self):
         binding = BINDING_HTTP_POST
         response_binding = BINDING_HTTP_POST
-        sid, http_args = self.client.prepare_for_authenticate(
+        sid, auth_binding, http_args = self.client.prepare_for_authenticate(
             "urn:mace:example.com:saml:roland:idp", relay_state="really",
             binding=binding, response_binding=response_binding)
         _dic = unpack_form(http_args["data"][3])
 
+        assert binding == auth_binding
+
         req = self.server.parse_authn_request(_dic["SAMLRequest"], binding)
         resp_args = self.server.response_args(req.message, [response_binding])
         assert resp_args["binding"] == response_binding