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/config/saml2_config.py

@@ -0,0 +1,110 @@
+# -*- 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.
+
+from ._base import Config, ConfigError
+
+
+class SAML2Config(Config):
+    def read_config(self, config):
+        self.saml2_enabled = False
+
+        saml2_config = config.get("saml2_config")
+
+        if not saml2_config or not saml2_config.get("enabled", True):
+            return
+
+        self.saml2_enabled = True
+
+        import saml2.config
+        self.saml2_sp_config = saml2.config.SPConfig()
+        self.saml2_sp_config.load(self._default_saml_config_dict())
+        self.saml2_sp_config.load(saml2_config.get("sp_config", {}))
+
+        config_path = saml2_config.get("config_path", None)
+        if config_path is not None:
+            self.saml2_sp_config.load_file(config_path)
+
+    def _default_saml_config_dict(self):
+        import saml2
+
+        public_baseurl = self.public_baseurl
+        if public_baseurl is None:
+            raise ConfigError(
+                "saml2_config requires a public_baseurl to be set"
+            )
+
+        metadata_url = public_baseurl + "_matrix/saml2/metadata.xml"
+        response_url = public_baseurl + "_matrix/saml2/authn_response"
+        return {
+            "entityid": metadata_url,
+
+            "service": {
+                "sp": {
+                    "endpoints": {
+                        "assertion_consumer_service": [
+                            (response_url, saml2.BINDING_HTTP_POST),
+                        ],
+                    },
+                    "required_attributes": ["uid"],
+                    "optional_attributes": ["mail", "surname", "givenname"],
+                },
+            }
+        }
+
+    def default_config(self, config_dir_path, server_name, **kwargs):
+        return """
+        # Enable SAML2 for registration and login. Uses pysaml2.
+        #
+        # saml2_config:
+        #
+        #   # The following is the configuration for the pysaml2 Service Provider.
+        #   # See pysaml2 docs for format of config.
+        #   #
+        #   # Default values will be used for the 'entityid' and 'service' settings,
+        #   # so it is not normally necessary to specify them unless you need to
+        #   # override them.
+        #
+        #   sp_config:
+        #     # point this to the IdP's metadata. You can use either a local file or
+        #     # (preferably) a URL.
+        #     metadata:
+        #       # local: ["saml2/idp.xml"]
+        #       remote:
+        #         - url: https://our_idp/metadata.xml
+        #
+        #     # The following is just used to generate our metadata xml, and you
+        #     # may well not need it, depending on your setup. Alternatively you
+        #     # may need a whole lot more detail - see the pysaml2 docs!
+        #
+        #     description: ["My awesome SP", "en"]
+        #     name: ["Test SP", "en"]
+        #
+        #     organization:
+        #       name: Example com
+        #       display_name:
+        #         - ["Example co", "en"]
+        #       url: "http://example.com"
+        #
+        #     contact_person:
+        #       - given_name: Bob
+        #         sur_name: "the Sysadmin"
+        #         email_address": ["admin@example.com"]
+        #         contact_type": technical
+        #
+        #   # Instead of putting the config inline as above, you can specify a
+        #   # separate pysaml2 configuration file:
+        #   #
+        #   # config_path: "%(config_dir_path)s/sp_conf.py"
+        """ % {"config_dir_path": config_dir_path}