Implement OpenID Connect-based login (#7256)

2 files changed, 1000 insertions, 2 deletions

diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 7613e5b6ab..f8d2331bf1 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -126,13 +126,13 @@ class AuthHandler(BaseHandler):
         # It notifies the user they are about to give access to their matrix account
         # to the client.
         self._sso_redirect_confirm_template = load_jinja2_templates(
-            hs.config.sso_redirect_confirm_template_dir, ["sso_redirect_confirm.html"],
+            hs.config.sso_template_dir, ["sso_redirect_confirm.html"],
         )[0]
         # The following template is shown during user interactive authentication
         # in the fallback auth scenario. It notifies the user that they are
         # authenticating for an operation to occur on their account.
         self._sso_auth_confirm_template = load_jinja2_templates(
-            hs.config.sso_redirect_confirm_template_dir, ["sso_auth_confirm.html"],
+            hs.config.sso_template_dir, ["sso_auth_confirm.html"],
         )[0]
         # The following template is shown after a successful user interactive
         # authentication session. It tells the user they can close the window.
diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
new file mode 100644
index 0000000000..178f263439
--- /dev/null
+++ b/synapse/handlers/oidc_handler.py
@@ -0,0 +1,998 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 Quentin Gliech
+#
+# 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 json
+import logging
+from typing import Dict, Generic, List, Optional, Tuple, TypeVar
+from urllib.parse import urlencode
+
+import attr
+import pymacaroons
+from authlib.common.security import generate_token
+from authlib.jose import JsonWebToken
+from authlib.oauth2.auth import ClientAuth
+from authlib.oauth2.rfc6749.parameters import prepare_grant_uri
+from authlib.oidc.core import CodeIDToken, ImplicitIDToken, UserInfo
+from authlib.oidc.discovery import OpenIDProviderMetadata, get_well_known_url
+from jinja2 import Environment, Template
+from pymacaroons.exceptions import (
+    MacaroonDeserializationException,
+    MacaroonInvalidSignatureException,
+)
+from typing_extensions import TypedDict
+
+from twisted.web.client import readBody
+
+from synapse.config import ConfigError
+from synapse.http.server import finish_request
+from synapse.http.site import SynapseRequest
+from synapse.push.mailer import load_jinja2_templates
+from synapse.server import HomeServer
+from synapse.types import UserID, map_username_to_mxid_localpart
+
+logger = logging.getLogger(__name__)
+
+SESSION_COOKIE_NAME = b"oidc_session"
+
+#: A token exchanged from the token endpoint, as per RFC6749 sec 5.1. and
+#: OpenID.Core sec 3.1.3.3.
+Token = TypedDict(
+    "Token",
+    {
+        "access_token": str,
+        "token_type": str,
+        "id_token": Optional[str],
+        "refresh_token": Optional[str],
+        "expires_in": int,
+        "scope": Optional[str],
+    },
+)
+
+#: A JWK, as per RFC7517 sec 4. The type could be more precise than that, but
+#: there is no real point of doing this in our case.
+JWK = Dict[str, str]
+
+#: A JWK Set, as per RFC7517 sec 5.
+JWKS = TypedDict("JWKS", {"keys": List[JWK]})
+
+
+class OidcError(Exception):
+    """Used to catch errors when calling the token_endpoint
+    """
+
+    def __init__(self, error, error_description=None):
+        self.error = error
+        self.error_description = error_description
+
+    def __str__(self):
+        if self.error_description:
+            return "{}: {}".format(self.error, self.error_description)
+        return self.error
+
+
+class MappingException(Exception):
+    """Used to catch errors when mapping the UserInfo object
+    """
+
+
+class OidcHandler:
+    """Handles requests related to the OpenID Connect login flow.
+    """
+
+    def __init__(self, hs: HomeServer):
+        self._callback_url = hs.config.oidc_callback_url  # type: str
+        self._scopes = hs.config.oidc_scopes  # type: List[str]
+        self._client_auth = ClientAuth(
+            hs.config.oidc_client_id,
+            hs.config.oidc_client_secret,
+            hs.config.oidc_client_auth_method,
+        )  # type: ClientAuth
+        self._client_auth_method = hs.config.oidc_client_auth_method  # type: str
+        self._subject_claim = hs.config.oidc_subject_claim
+        self._provider_metadata = OpenIDProviderMetadata(
+            issuer=hs.config.oidc_issuer,
+            authorization_endpoint=hs.config.oidc_authorization_endpoint,
+            token_endpoint=hs.config.oidc_token_endpoint,
+            userinfo_endpoint=hs.config.oidc_userinfo_endpoint,
+            jwks_uri=hs.config.oidc_jwks_uri,
+        )  # type: OpenIDProviderMetadata
+        self._provider_needs_discovery = hs.config.oidc_discover  # type: bool
+        self._user_mapping_provider = hs.config.oidc_user_mapping_provider_class(
+            hs.config.oidc_user_mapping_provider_config
+        )  # type: OidcMappingProvider
+        self._skip_verification = hs.config.oidc_skip_verification  # type: bool
+
+        self._http_client = hs.get_proxied_http_client()
+        self._auth_handler = hs.get_auth_handler()
+        self._registration_handler = hs.get_registration_handler()
+        self._datastore = hs.get_datastore()
+        self._clock = hs.get_clock()
+        self._hostname = hs.hostname  # type: str
+        self._server_name = hs.config.server_name  # type: str
+        self._macaroon_secret_key = hs.config.macaroon_secret_key
+        self._error_template = load_jinja2_templates(
+            hs.config.sso_template_dir, ["sso_error.html"]
+        )[0]
+
+        # identifier for the external_ids table
+        self._auth_provider_id = "oidc"
+
+    def _render_error(
+        self, request, error: str, error_description: Optional[str] = None
+    ) -> None:
+        """Renders the error template and respond with it.
+
+        This is used to show errors to the user. The template of this page can
+        be found under ``synapse/res/templates/sso_error.html``.
+
+        Args:
+            request: The incoming request from the browser.
+                We'll respond with an HTML page describing the error.
+            error: A technical identifier for this error. Those include
+                well-known OAuth2/OIDC error types like invalid_request or
+                access_denied.
+            error_description: A human-readable description of the error.
+        """
+        html_bytes = self._error_template.render(
+            error=error, error_description=error_description
+        ).encode("utf-8")
+
+        request.setResponseCode(400)
+        request.setHeader(b"Content-Type", b"text/html; charset=utf-8")
+        request.setHeader(b"Content-Length", b"%i" % len(html_bytes))
+        request.write(html_bytes)
+        finish_request(request)
+
+    def _validate_metadata(self):
+        """Verifies the provider metadata.
+
+        This checks the validity of the currently loaded provider. Not
+        everything is checked, only:
+
+          - ``issuer``
+          - ``authorization_endpoint``
+          - ``token_endpoint``
+          - ``response_types_supported`` (checks if "code" is in it)
+          - ``jwks_uri``
+
+        Raises:
+            ValueError: if something in the provider is not valid
+        """
+        # Skip verification to allow non-compliant providers (e.g. issuers not running on a secure origin)
+        if self._skip_verification is True:
+            return
+
+        m = self._provider_metadata
+        m.validate_issuer()
+        m.validate_authorization_endpoint()
+        m.validate_token_endpoint()
+
+        if m.get("token_endpoint_auth_methods_supported") is not None:
+            m.validate_token_endpoint_auth_methods_supported()
+            if (
+                self._client_auth_method
+                not in m["token_endpoint_auth_methods_supported"]
+            ):
+                raise ValueError(
+                    '"{auth_method}" not in "token_endpoint_auth_methods_supported" ({supported!r})'.format(
+                        auth_method=self._client_auth_method,
+                        supported=m["token_endpoint_auth_methods_supported"],
+                    )
+                )
+
+        if m.get("response_types_supported") is not None:
+            m.validate_response_types_supported()
+
+            if "code" not in m["response_types_supported"]:
+                raise ValueError(
+                    '"code" not in "response_types_supported" (%r)'
+                    % (m["response_types_supported"],)
+                )
+
+        # If the openid scope was not requested, we need a userinfo endpoint to fetch user infos
+        if self._uses_userinfo:
+            if m.get("userinfo_endpoint") is None:
+                raise ValueError(
+                    'provider has no "userinfo_endpoint", even though it is required because the "openid" scope is not requested'
+                )
+        else:
+            # If we're not using userinfo, we need a valid jwks to validate the ID token
+            if m.get("jwks") is None:
+                if m.get("jwks_uri") is not None:
+                    m.validate_jwks_uri()
+                else:
+                    raise ValueError('"jwks_uri" must be set')
+
+    @property
+    def _uses_userinfo(self) -> bool:
+        """Returns True if the ``userinfo_endpoint`` should be used.
+
+        This is based on the requested scopes: if the scopes include
+        ``openid``, the provider should give use an ID token containing the
+        user informations. If not, we should fetch them using the
+        ``access_token`` with the ``userinfo_endpoint``.
+        """
+
+        # Maybe that should be user-configurable and not inferred?
+        return "openid" not in self._scopes
+
+    async def load_metadata(self) -> OpenIDProviderMetadata:
+        """Load and validate the provider metadata.
+
+        The values metadatas are discovered if ``oidc_config.discovery`` is
+        ``True`` and then cached.
+
+        Raises:
+            ValueError: if something in the provider is not valid
+
+        Returns:
+            The provider's metadata.
+        """
+        # If we are using the OpenID Discovery documents, it needs to be loaded once
+        # FIXME: should there be a lock here?
+        if self._provider_needs_discovery:
+            url = get_well_known_url(self._provider_metadata["issuer"], external=True)
+            metadata_response = await self._http_client.get_json(url)
+            # TODO: maybe update the other way around to let user override some values?
+            self._provider_metadata.update(metadata_response)
+            self._provider_needs_discovery = False
+
+        self._validate_metadata()
+
+        return self._provider_metadata
+
+    async def load_jwks(self, force: bool = False) -> JWKS:
+        """Load the JSON Web Key Set used to sign ID tokens.
+
+        If we're not using the ``userinfo_endpoint``, user infos are extracted
+        from the ID token, which is a JWT signed by keys given by the provider.
+        The keys are then cached.
+
+        Args:
+            force: Force reloading the keys.
+
+        Returns:
+            The key set
+
+            Looks like this::
+
+                {
+                    'keys': [
+                        {
+                            'kid': 'abcdef',
+                            'kty': 'RSA',
+                            'alg': 'RS256',
+                            'use': 'sig',
+                            'e': 'XXXX',
+                            'n': 'XXXX',
+                        }
+                    ]
+                }
+        """
+        if self._uses_userinfo:
+            # We're not using jwt signing, return an empty jwk set
+            return {"keys": []}
+
+        # First check if the JWKS are loaded in the provider metadata.
+        # It can happen either if the provider gives its JWKS in the discovery
+        # document directly or if it was already loaded once.
+        metadata = await self.load_metadata()
+        jwk_set = metadata.get("jwks")
+        if jwk_set is not None and not force:
+            return jwk_set
+
+        # Loading the JWKS using the `jwks_uri` metadata
+        uri = metadata.get("jwks_uri")
+        if not uri:
+            raise RuntimeError('Missing "jwks_uri" in metadata')
+
+        jwk_set = await self._http_client.get_json(uri)
+
+        # Caching the JWKS in the provider's metadata
+        self._provider_metadata["jwks"] = jwk_set
+        return jwk_set
+
+    async def _exchange_code(self, code: str) -> Token:
+        """Exchange an authorization code for a token.
+
+        This calls the ``token_endpoint`` with the authorization code we
+        received in the callback to exchange it for a token. The call uses the
+        ``ClientAuth`` to authenticate with the client with its ID and secret.
+
+        Args:
+            code: The autorization code we got from the callback.
+
+        Returns:
+            A dict containing various tokens.
+
+            May look like this::
+
+                {
+                    'token_type': 'bearer',
+                    'access_token': 'abcdef',
+                    'expires_in': 3599,
+                    'id_token': 'ghijkl',
+                    'refresh_token': 'mnopqr',
+                }
+
+        Raises:
+            OidcError: when the ``token_endpoint`` returned an error.
+        """
+        metadata = await self.load_metadata()
+        token_endpoint = metadata.get("token_endpoint")
+        headers = {
+            "Content-Type": "application/x-www-form-urlencoded",
+            "User-Agent": self._http_client.user_agent,
+            "Accept": "application/json",
+        }
+
+        args = {
+            "grant_type": "authorization_code",
+            "code": code,
+            "redirect_uri": self._callback_url,
+        }
+        body = urlencode(args, True)
+
+        # Fill the body/headers with credentials
+        uri, headers, body = self._client_auth.prepare(
+            method="POST", uri=token_endpoint, headers=headers, body=body
+        )
+        headers = {k: [v] for (k, v) in headers.items()}
+
+        # Do the actual request
+        # We're not using the SimpleHttpClient util methods as we don't want to
+        # check the HTTP status code and we do the body encoding ourself.
+        response = await self._http_client.request(
+            method="POST", uri=uri, data=body.encode("utf-8"), headers=headers,
+        )
+
+        # This is used in multiple error messages below
+        status = "{code} {phrase}".format(
+            code=response.code, phrase=response.phrase.decode("utf-8")
+        )
+
+        resp_body = await readBody(response)
+
+        if response.code >= 500:
+            # In case of a server error, we should first try to decode the body
+            # and check for an error field. If not, we respond with a generic
+            # error message.
+            try:
+                resp = json.loads(resp_body.decode("utf-8"))
+                error = resp["error"]
+                description = resp.get("error_description", error)
+            except (ValueError, KeyError):
+                # Catch ValueError for the JSON decoding and KeyError for the "error" field
+                error = "server_error"
+                description = (
+                    (
+                        'Authorization server responded with a "{status}" error '
+                        "while exchanging the authorization code."
+                    ).format(status=status),
+                )
+
+            raise OidcError(error, description)
+
+        # Since it is a not a 5xx code, body should be a valid JSON. It will
+        # raise if not.
+        resp = json.loads(resp_body.decode("utf-8"))
+
+        if "error" in resp:
+            error = resp["error"]
+            # In case the authorization server responded with an error field,
+            # it should be a 4xx code. If not, warn about it but don't do
+            # anything special and report the original error message.
+            if response.code < 400:
+                logger.debug(
+                    "Invalid response from the authorization server: "
+                    'responded with a "{status}" '
+                    "but body has an error field: {error!r}".format(
+                        status=status, error=resp["error"]
+                    )
+                )
+
+            description = resp.get("error_description", error)
+            raise OidcError(error, description)
+
+        # Now, this should not be an error. According to RFC6749 sec 5.1, it
+        # should be a 200 code. We're a bit more flexible than that, and will
+        # only throw on a 4xx code.
+        if response.code >= 400:
+            description = (
+                'Authorization server responded with a "{status}" error '
+                'but did not include an "error" field in its response.'.format(
+                    status=status
+                )
+            )
+            logger.warning(description)
+            # Body was still valid JSON. Might be useful to log it for debugging.
+            logger.warning("Code exchange response: {resp!r}".format(resp=resp))
+            raise OidcError("server_error", description)
+
+        return resp
+
+    async def _fetch_userinfo(self, token: Token) -> UserInfo:
+        """Fetch user informations from the ``userinfo_endpoint``.
+
+        Args:
+            token: the token given by the ``token_endpoint``.
+                Must include an ``access_token`` field.
+
+        Returns:
+            UserInfo: an object representing the user.
+        """
+        metadata = await self.load_metadata()
+
+        resp = await self._http_client.get_json(
+            metadata["userinfo_endpoint"],
+            headers={"Authorization": ["Bearer {}".format(token["access_token"])]},
+        )
+
+        return UserInfo(resp)
+
+    async def _parse_id_token(self, token: Token, nonce: str) -> UserInfo:
+        """Return an instance of UserInfo from token's ``id_token``.
+
+        Args:
+            token: the token given by the ``token_endpoint``.
+                Must include an ``id_token`` field.
+            nonce: the nonce value originally sent in the initial authorization
+                request. This value should match the one inside the token.
+
+        Returns:
+            An object representing the user.
+        """
+        metadata = await self.load_metadata()
+        claims_params = {
+            "nonce": nonce,
+            "client_id": self._client_auth.client_id,
+        }
+        if "access_token" in token:
+            # If we got an `access_token`, there should be an `at_hash` claim
+            # in the `id_token` that we can check against.
+            claims_params["access_token"] = token["access_token"]
+            claims_cls = CodeIDToken
+        else:
+            claims_cls = ImplicitIDToken
+
+        alg_values = metadata.get("id_token_signing_alg_values_supported", ["RS256"])
+
+        jwt = JsonWebToken(alg_values)
+
+        claim_options = {"iss": {"values": [metadata["issuer"]]}}
+
+        # Try to decode the keys in cache first, then retry by forcing the keys
+        # to be reloaded
+        jwk_set = await self.load_jwks()
+        try:
+            claims = jwt.decode(
+                token["id_token"],
+                key=jwk_set,
+                claims_cls=claims_cls,
+                claims_options=claim_options,
+                claims_params=claims_params,
+            )
+        except ValueError:
+            jwk_set = await self.load_jwks(force=True)  # try reloading the jwks
+            claims = jwt.decode(
+                token["id_token"],
+                key=jwk_set,
+                claims_cls=claims_cls,
+                claims_options=claim_options,
+                claims_params=claims_params,
+            )
+
+        claims.validate(leeway=120)  # allows 2 min of clock skew
+        return UserInfo(claims)
+
+    async def handle_redirect_request(
+        self, request: SynapseRequest, client_redirect_url: bytes
+    ) -> None:
+        """Handle an incoming request to /login/sso/redirect
+
+        It redirects the browser to the authorization endpoint with a few
+        parameters:
+
+          - ``client_id``: the client ID set in ``oidc_config.client_id``
+          - ``response_type``: ``code``
+          - ``redirect_uri``: the callback URL ; ``{base url}/_synapse/oidc/callback``
+          - ``scope``: the list of scopes set in ``oidc_config.scopes``
+          - ``state``: a random string
+          - ``nonce``: a random string
+
+        In addition to redirecting the client, we are setting a cookie with
+        a signed macaroon token containing the state, the nonce and the
+        client_redirect_url params. Those are then checked when the client
+        comes back from the provider.
+
+
+        Args:
+            request: the incoming request from the browser.
+                We'll respond to it with a redirect and a cookie.
+            client_redirect_url: the URL that we should redirect the client to
+                when everything is done
+        """
+
+        state = generate_token()
+        nonce = generate_token()
+
+        cookie = self._generate_oidc_session_token(
+            state=state, nonce=nonce, client_redirect_url=client_redirect_url.decode(),
+        )
+        request.addCookie(
+            SESSION_COOKIE_NAME,
+            cookie,
+            path="/_synapse/oidc",
+            max_age="3600",
+            httpOnly=True,
+            sameSite="lax",
+        )
+
+        metadata = await self.load_metadata()
+        authorization_endpoint = metadata.get("authorization_endpoint")
+        uri = prepare_grant_uri(
+            authorization_endpoint,
+            client_id=self._client_auth.client_id,
+            response_type="code",
+            redirect_uri=self._callback_url,
+            scope=self._scopes,
+            state=state,
+            nonce=nonce,
+        )
+        request.redirect(uri)
+        finish_request(request)
+
+    async def handle_oidc_callback(self, request: SynapseRequest) -> None:
+        """Handle an incoming request to /_synapse/oidc/callback
+
+        Since we might want to display OIDC-related errors in a user-friendly
+        way, we don't raise SynapseError from here. Instead, we call
+        ``self._render_error`` which displays an HTML page for the error.
+
+        Most of the OpenID Connect logic happens here:
+
+          - first, we check if there was any error returned by the provider and
+            display it
+          - then we fetch the session cookie, decode and verify it
+          - the ``state`` query parameter should match with the one stored in the
+            session cookie
+          - once we known this session is legit, exchange the code with the
+            provider using the ``token_endpoint`` (see ``_exchange_code``)
+          - once we have the token, use it to either extract the UserInfo from
+            the ``id_token`` (``_parse_id_token``), or use the ``access_token``
+            to fetch UserInfo from the ``userinfo_endpoint``
+            (``_fetch_userinfo``)
+          - map those UserInfo to a Matrix user (``_map_userinfo_to_user``) and
+            finish the login
+
+        Args:
+            request: the incoming request from the browser.
+        """
+
+        # The provider might redirect with an error.
+        # In that case, just display it as-is.
+        if b"error" in request.args:
+            error = request.args[b"error"][0].decode()
+            description = request.args.get(b"error_description", [b""])[0].decode()
+
+            # Most of the errors returned by the provider could be due by
+            # either the provider misbehaving or Synapse being misconfigured.
+            # The only exception of that is "access_denied", where the user
+            # probably cancelled the login flow. In other cases, log those errors.
+            if error != "access_denied":
+                logger.error("Error from the OIDC provider: %s %s", error, description)
+
+            self._render_error(request, error, description)
+            return
+
+        # Fetch the session cookie
+        session = request.getCookie(SESSION_COOKIE_NAME)
+        if session is None:
+            logger.info("No session cookie found")
+            self._render_error(request, "missing_session", "No session cookie found")
+            return
+
+        # Remove the cookie. There is a good chance that if the callback failed
+        # once, it will fail next time and the code will already be exchanged.
+        # Removing it early avoids spamming the provider with token requests.
+        request.addCookie(
+            SESSION_COOKIE_NAME,
+            b"",
+            path="/_synapse/oidc",
+            expires="Thu, Jan 01 1970 00:00:00 UTC",
+            httpOnly=True,
+            sameSite="lax",
+        )
+
+        # Check for the state query parameter
+        if b"state" not in request.args:
+            logger.info("State parameter is missing")
+            self._render_error(request, "invalid_request", "State parameter is missing")
+            return
+
+        state = request.args[b"state"][0].decode()
+
+        # Deserialize the session token and verify it.
+        try:
+            nonce, client_redirect_url = self._verify_oidc_session_token(session, state)
+        except MacaroonDeserializationException as e:
+            logger.exception("Invalid session")
+            self._render_error(request, "invalid_session", str(e))
+            return
+        except MacaroonInvalidSignatureException as e:
+            logger.exception("Could not verify session")
+            self._render_error(request, "mismatching_session", str(e))
+            return
+
+        # Exchange the code with the provider
+        if b"code" not in request.args:
+            logger.info("Code parameter is missing")
+            self._render_error(request, "invalid_request", "Code parameter is missing")
+            return
+
+        logger.info("Exchanging code")
+        code = request.args[b"code"][0].decode()
+        try:
+            token = await self._exchange_code(code)
+        except OidcError as e:
+            logger.exception("Could not exchange code")
+            self._render_error(request, e.error, e.error_description)
+            return
+
+        # Now that we have a token, get the userinfo, either by decoding the
+        # `id_token` or by fetching the `userinfo_endpoint`.
+        if self._uses_userinfo:
+            logger.info("Fetching userinfo")
+            try:
+                userinfo = await self._fetch_userinfo(token)
+            except Exception as e:
+                logger.exception("Could not fetch userinfo")
+                self._render_error(request, "fetch_error", str(e))
+                return
+        else:
+            logger.info("Extracting userinfo from id_token")
+            try:
+                userinfo = await self._parse_id_token(token, nonce=nonce)
+            except Exception as e:
+                logger.exception("Invalid id_token")
+                self._render_error(request, "invalid_token", str(e))
+                return
+
+        # Call the mapper to register/login the user
+        try:
+            user_id = await self._map_userinfo_to_user(userinfo, token)
+        except MappingException as e:
+            logger.exception("Could not map user")
+            self._render_error(request, "mapping_error", str(e))
+            return
+
+        # and finally complete the login
+        await self._auth_handler.complete_sso_login(
+            user_id, request, client_redirect_url
+        )
+
+    def _generate_oidc_session_token(
+        self,
+        state: str,
+        nonce: str,
+        client_redirect_url: str,
+        duration_in_ms: int = (60 * 60 * 1000),
+    ) -> str:
+        """Generates a signed token storing data about an OIDC session.
+
+        When Synapse initiates an authorization flow, it creates a random state
+        and a random nonce. Those parameters are given to the provider and
+        should be verified when the client comes back from the provider.
+        It is also used to store the client_redirect_url, which is used to
+        complete the SSO login flow.
+
+        Args:
+            state: The ``state`` parameter passed to the OIDC provider.
+            nonce: The ``nonce`` parameter passed to the OIDC provider.
+            client_redirect_url: The URL the client gave when it initiated the
+                flow.
+            duration_in_ms: An optional duration for the token in milliseconds.
+                Defaults to an hour.
+
+        Returns:
+            A signed macaroon token with the session informations.
+        """
+        macaroon = pymacaroons.Macaroon(
+            location=self._server_name, identifier="key", key=self._macaroon_secret_key,
+        )
+        macaroon.add_first_party_caveat("gen = 1")
+        macaroon.add_first_party_caveat("type = session")
+        macaroon.add_first_party_caveat("state = %s" % (state,))
+        macaroon.add_first_party_caveat("nonce = %s" % (nonce,))
+        macaroon.add_first_party_caveat(
+            "client_redirect_url = %s" % (client_redirect_url,)
+        )
+        now = self._clock.time_msec()
+        expiry = now + duration_in_ms
+        macaroon.add_first_party_caveat("time < %d" % (expiry,))
+        return macaroon.serialize()
+
+    def _verify_oidc_session_token(self, session: str, state: str) -> Tuple[str, str]:
+        """Verifies and extract an OIDC session token.
+
+        This verifies that a given session token was issued by this homeserver
+        and extract the nonce and client_redirect_url caveats.
+
+        Args:
+            session: The session token to verify
+            state: The state the OIDC provider gave back
+
+        Returns:
+            The nonce and the client_redirect_url for this session
+        """
+        macaroon = pymacaroons.Macaroon.deserialize(session)
+
+        v = pymacaroons.Verifier()
+        v.satisfy_exact("gen = 1")
+        v.satisfy_exact("type = session")
+        v.satisfy_exact("state = %s" % (state,))
+        v.satisfy_general(lambda c: c.startswith("nonce = "))
+        v.satisfy_general(lambda c: c.startswith("client_redirect_url = "))
+        v.satisfy_general(self._verify_expiry)
+
+        v.verify(macaroon, self._macaroon_secret_key)
+
+        # Extract the `nonce` and `client_redirect_url` from the token
+        nonce = self._get_value_from_macaroon(macaroon, "nonce")
+        client_redirect_url = self._get_value_from_macaroon(
+            macaroon, "client_redirect_url"
+        )
+
+        return nonce, client_redirect_url
+
+    def _get_value_from_macaroon(self, macaroon: pymacaroons.Macaroon, key: str) -> str:
+        """Extracts a caveat value from a macaroon token.
+
+        Args:
+            macaroon: the token
+            key: the key of the caveat to extract
+
+        Returns:
+            The extracted value
+
+        Raises:
+            Exception: if the caveat was not in the macaroon
+        """
+        prefix = key + " = "
+        for caveat in macaroon.caveats:
+            if caveat.caveat_id.startswith(prefix):
+                return caveat.caveat_id[len(prefix) :]
+        raise Exception("No %s caveat in macaroon" % (key,))
+
+    def _verify_expiry(self, caveat: str) -> bool:
+        prefix = "time < "
+        if not caveat.startswith(prefix):
+            return False
+        expiry = int(caveat[len(prefix) :])
+        now = self._clock.time_msec()
+        return now < expiry
+
+    async def _map_userinfo_to_user(self, userinfo: UserInfo, token: Token) -> str:
+        """Maps a UserInfo object to a mxid.
+
+        UserInfo should have a claim that uniquely identifies users. This claim
+        is usually `sub`, but can be configured with `oidc_config.subject_claim`.
+        It is then used as an `external_id`.
+
+        If we don't find the user that way, we should register the user,
+        mapping the localpart and the display name from the UserInfo.
+
+        If a user already exists with the mxid we've mapped, raise an exception.
+
+        Args:
+            userinfo: an object representing the user
+            token: a dict with the tokens obtained from the provider
+
+        Raises:
+            MappingException: if there was an error while mapping some properties
+
+        Returns:
+            The mxid of the user
+        """
+        try:
+            remote_user_id = self._user_mapping_provider.get_remote_user_id(userinfo)
+        except Exception as e:
+            raise MappingException(
+                "Failed to extract subject from OIDC response: %s" % (e,)
+            )
+
+        logger.info(
+            "Looking for existing mapping for user %s:%s",
+            self._auth_provider_id,
+            remote_user_id,
+        )
+
+        registered_user_id = await self._datastore.get_user_by_external_id(
+            self._auth_provider_id, remote_user_id,
+        )
+
+        if registered_user_id is not None:
+            logger.info("Found existing mapping %s", registered_user_id)
+            return registered_user_id
+
+        try:
+            attributes = await self._user_mapping_provider.map_user_attributes(
+                userinfo, token
+            )
+        except Exception as e:
+            raise MappingException(
+                "Could not extract user attributes from OIDC response: " + str(e)
+            )
+
+        logger.debug(
+            "Retrieved user attributes from user mapping provider: %r", attributes
+        )
+
+        if not attributes["localpart"]:
+            raise MappingException("localpart is empty")
+
+        localpart = map_username_to_mxid_localpart(attributes["localpart"])
+
+        user_id = UserID(localpart, self._hostname)
+        if await self._datastore.get_users_by_id_case_insensitive(user_id.to_string()):
+            # This mxid is taken
+            raise MappingException(
+                "mxid '{}' is already taken".format(user_id.to_string())
+            )
+
+        # It's the first time this user is logging in and the mapped mxid was
+        # not taken, register the user
+        registered_user_id = await self._registration_handler.register_user(
+            localpart=localpart, default_display_name=attributes["display_name"],
+        )
+
+        await self._datastore.record_user_external_id(
+            self._auth_provider_id, remote_user_id, registered_user_id,
+        )
+        return registered_user_id
+
+
+UserAttribute = TypedDict(
+    "UserAttribute", {"localpart": str, "display_name": Optional[str]}
+)
+C = TypeVar("C")
+
+
+class OidcMappingProvider(Generic[C]):
+    """A mapping provider maps a UserInfo object to user attributes.
+
+    It should provide the API described by this class.
+    """
+
+    def __init__(self, config: C):
+        """
+        Args:
+            config: A custom config object from this module, parsed by ``parse_config()``
+        """
+
+    @staticmethod
+    def parse_config(config: dict) -> C:
+        """Parse the dict provided by the homeserver's config
+
+        Args:
+            config: A dictionary containing configuration options for this provider
+
+        Returns:
+            A custom config object for this module
+        """
+        raise NotImplementedError()
+
+    def get_remote_user_id(self, userinfo: UserInfo) -> str:
+        """Get a unique user ID for this user.
+
+        Usually, in an OIDC-compliant scenario, it should be the ``sub`` claim from the UserInfo object.
+
+        Args:
+            userinfo: An object representing the user given by the OIDC provider
+
+        Returns:
+            A unique user ID
+        """
+        raise NotImplementedError()
+
+    async def map_user_attributes(
+        self, userinfo: UserInfo, token: Token
+    ) -> UserAttribute:
+        """Map a ``UserInfo`` objects into user attributes.
+
+        Args:
+            userinfo: An object representing the user given by the OIDC provider
+            token: A dict with the tokens returned by the provider
+
+        Returns:
+            A dict containing the ``localpart`` and (optionally) the ``display_name``
+        """
+        raise NotImplementedError()
+
+
+# Used to clear out "None" values in templates
+def jinja_finalize(thing):
+    return thing if thing is not None else ""
+
+
+env = Environment(finalize=jinja_finalize)
+
+
+@attr.s
+class JinjaOidcMappingConfig:
+    subject_claim = attr.ib()  # type: str
+    localpart_template = attr.ib()  # type: Template
+    display_name_template = attr.ib()  # type: Optional[Template]
+
+
+class JinjaOidcMappingProvider(OidcMappingProvider[JinjaOidcMappingConfig]):
+    """An implementation of a mapping provider based on Jinja templates.
+
+    This is the default mapping provider.
+    """
+
+    def __init__(self, config: JinjaOidcMappingConfig):
+        self._config = config
+
+    @staticmethod
+    def parse_config(config: dict) -> JinjaOidcMappingConfig:
+        subject_claim = config.get("subject_claim", "sub")
+
+        if "localpart_template" not in config:
+            raise ConfigError(
+                "missing key: oidc_config.user_mapping_provider.config.localpart_template"
+            )
+
+        try:
+            localpart_template = env.from_string(config["localpart_template"])
+        except Exception as e:
+            raise ConfigError(
+                "invalid jinja template for oidc_config.user_mapping_provider.config.localpart_template: %r"
+                % (e,)
+            )
+
+        display_name_template = None  # type: Optional[Template]
+        if "display_name_template" in config:
+            try:
+                display_name_template = env.from_string(config["display_name_template"])
+            except Exception as e:
+                raise ConfigError(
+                    "invalid jinja template for oidc_config.user_mapping_provider.config.display_name_template: %r"
+                    % (e,)
+                )
+
+        return JinjaOidcMappingConfig(
+            subject_claim=subject_claim,
+            localpart_template=localpart_template,
+            display_name_template=display_name_template,
+        )
+
+    def get_remote_user_id(self, userinfo: UserInfo) -> str:
+        return userinfo[self._config.subject_claim]
+
+    async def map_user_attributes(
+        self, userinfo: UserInfo, token: Token
+    ) -> UserAttribute:
+        localpart = self._config.localpart_template.render(user=userinfo).strip()
+
+        display_name = None  # type: Optional[str]
+        if self._config.display_name_template is not None:
+            display_name = self._config.display_name_template.render(
+                user=userinfo
+            ).strip()
+
+            if display_name == "":
+                display_name = None
+
+        return UserAttribute(localpart=localpart, display_name=display_name)