Record mappings from saml users in an external table

We want to assign unique mxids to saml users based on an incrementing
suffix. For that to work, we need to record the allocated mxid in a separate
table.

synapse/config/saml2_config.py

@@ -12,7 +12,13 @@
 # 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 re
+
 from synapse.python_dependencies import DependencyException, check_requirements
+from synapse.types import (
+    map_username_to_mxid_localpart,
+    mxid_localpart_allowed_characters,
+)
 
 from ._base import Config, ConfigError
 
@@ -36,6 +42,14 @@ class SAML2Config(Config):
 
         self.saml2_enabled = True
 
+        self.saml2_mxid_source_attribute = saml2_config.get(
+            "mxid_source_attribute", "uid"
+        )
+
+        self.saml2_grandfathered_mxid_source_attribute = saml2_config.get(
+            "grandfathered_mxid_source_attribute", "uid"
+        )
+
         import saml2.config
 
         self.saml2_sp_config = saml2.config.SPConfig()
@@ -51,6 +65,12 @@ class SAML2Config(Config):
             saml2_config.get("saml_session_lifetime", "5m")
         )
 
+        mapping = saml2_config.get("mxid_mapping", "hexencode")
+        try:
+            self.saml2_mxid_mapper = MXID_MAPPER_MAP[mapping]
+        except KeyError:
+            raise ConfigError("%s is not a known mxid_mapping" % (mapping,))
+
     def _default_saml_config_dict(self):
         import saml2
 
@@ -58,6 +78,13 @@ class SAML2Config(Config):
         if public_baseurl is None:
             raise ConfigError("saml2_config requires a public_baseurl to be set")
 
+        required_attributes = {"uid", self.saml2_mxid_source_attribute}
+
+        optional_attributes = {"displayName"}
+        if self.saml2_grandfathered_mxid_source_attribute:
+            optional_attributes.add(self.saml2_grandfathered_mxid_source_attribute)
+        optional_attributes -= required_attributes
+
         metadata_url = public_baseurl + "_matrix/saml2/metadata.xml"
         response_url = public_baseurl + "_matrix/saml2/authn_response"
         return {
@@ -69,8 +96,9 @@ class SAML2Config(Config):
                             (response_url, saml2.BINDING_HTTP_POST)
                         ]
                     },
-                    "required_attributes": ["uid"],
-                    "optional_attributes": ["mail", "surname", "givenname"],
+                    "required_attributes": list(required_attributes),
+                    "optional_attributes": list(optional_attributes),
+                    # "name_id_format": saml2.saml.NAMEID_FORMAT_PERSISTENT,
                 }
             },
         }
@@ -146,6 +174,52 @@ class SAML2Config(Config):
           # The default is 5 minutes.
           #
           #saml_session_lifetime: 5m
+
+          # The SAML attribute (after mapping via the attribute maps) to use to derive
+          # the Matrix ID from. 'uid' by default.
+          #
+          #mxid_source_attribute: displayName
+
+          # The mapping system to use for mapping the saml attribute onto a matrix ID.
+          # Options include:
+          #  * 'hexencode' (which maps unpermitted characters to '=xx')
+          #  * 'dotreplace' (which replaces unpermitted characters with '.').
+          # The default is 'hexencode'.
+          #
+          #mxid_mapping: dotreplace
+
+          # In previous versions of synapse, the mapping from SAML attribute to MXID was
+          # always calculated dynamically rather than stored in a table. For backwards-
+          # compatibility, we will look for user_ids matching such a pattern before
+          # creating a new account.
+          #
+          # This setting controls the SAML attribute which will be used for this
+          # backwards-compatibility lookup. Typically it should be 'uid', but if the
+          # attribute maps are changed, it may be necessary to change it.
+          #
+          # The default is 'uid'.
+          #
+          #grandfathered_mxid_source_attribute: upn
         """ % {
             "config_dir_path": config_dir_path
         }
+
+
+DOT_REPLACE_PATTERN = re.compile(
+    ("[^%s]" % (re.escape("".join(mxid_localpart_allowed_characters)),))
+)
+
+
+def dot_replace_for_mxid(username: str) -> str:
+    username = username.lower()
+    username = DOT_REPLACE_PATTERN.sub(".", username)
+
+    # regular mxids aren't allowed to start with an underscore either
+    username = re.sub("^_", "", username)
+    return username
+
+
+MXID_MAPPER_MAP = {
+    "hexencode": map_username_to_mxid_localpart,
+    "dotreplace": dot_replace_for_mxid,
+}