Implement SAML2 authentication (#4267)

This implements both a SAML2 metadata endpoint (at
`/_matrix/saml2/metadata.xml`), and a SAML2 response receiver (at
`/_matrix/saml2/authn_response`). If the SAML2 response matches what's been
configured, we complete the SSO login flow by redirecting to the client url
(aka `RelayState` in SAML2 jargon) with a login token.

What we don't yet have is anything to build a SAML2 request and redirect the
user to the identity provider. That is left as an exercise for the reader.

synapse/rest/saml2/response_resource.py

@@ -0,0 +1,71 @@
+# -*- coding: utf-8 -*-
+#
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+
+import saml2
+from saml2.client import Saml2Client
+
+from twisted.web.resource import Resource
+from twisted.web.server import NOT_DONE_YET
+
+from synapse.api.errors import CodeMessageException
+from synapse.http.server import wrap_html_request_handler
+from synapse.http.servlet import parse_string
+from synapse.rest.client.v1.login import SSOAuthHandler
+
+logger = logging.getLogger(__name__)
+
+
+class SAML2ResponseResource(Resource):
+    """A Twisted web resource which handles the SAML response"""
+
+    isLeaf = 1
+
+    def __init__(self, hs):
+        Resource.__init__(self)
+
+        self._saml_client = Saml2Client(hs.config.saml2_sp_config)
+        self._sso_auth_handler = SSOAuthHandler(hs)
+
+    def render_POST(self, request):
+        self._async_render_POST(request)
+        return NOT_DONE_YET
+
+    @wrap_html_request_handler
+    def _async_render_POST(self, request):
+        resp_bytes = parse_string(request, 'SAMLResponse', required=True)
+        relay_state = parse_string(request, 'RelayState', required=True)
+
+        try:
+            saml2_auth = self._saml_client.parse_authn_request_response(
+                resp_bytes, saml2.BINDING_HTTP_POST,
+            )
+        except Exception as e:
+            logger.warning("Exception parsing SAML2 response", exc_info=1)
+            raise CodeMessageException(
+                400, "Unable to parse SAML2 response: %s" % (e,),
+            )
+
+        if saml2_auth.not_signed:
+            raise CodeMessageException(400, "SAML2 response was not signed")
+
+        if "uid" not in saml2_auth.ava:
+            raise CodeMessageException(400, "uid not in SAML2 response")
+
+        username = saml2_auth.ava["uid"][0]
+        return self._sso_auth_handler.on_successful_auth(
+            username, request, relay_state,
+        )