# Copyright 2020 Quentin Gliech
# Copyright 2021 The Matrix.org Foundation C.I.C.
#
# 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 inspect
import logging
from typing import TYPE_CHECKING, Any, Dict, Generic, List, Optional, TypeVar, Union
from urllib.parse import urlencode, urlparse

import attr
import pymacaroons
from authlib.common.security import generate_token
from authlib.jose import JsonWebToken, jwt
from authlib.oauth2.auth import ClientAuth
from authlib.oauth2.rfc6749.parameters import prepare_grant_uri
from authlib.oidc.core import CodeIDToken, UserInfo
from authlib.oidc.discovery import OpenIDProviderMetadata, get_well_known_url
from jinja2 import Environment, Template
from pymacaroons.exceptions import (
    MacaroonDeserializationException,
    MacaroonInitException,
    MacaroonInvalidSignatureException,
)
from typing_extensions import TypedDict

from twisted.web.client import readBody
from twisted.web.http_headers import Headers

from synapse.config import ConfigError
from synapse.config.oidc import OidcProviderClientSecretJwtKey, OidcProviderConfig
from synapse.handlers.sso import MappingException, UserAttributes
from synapse.http.site import SynapseRequest
from synapse.logging.context import make_deferred_yieldable
from synapse.types import JsonDict, UserID, map_username_to_mxid_localpart
from synapse.util import Clock, json_decoder
from synapse.util.caches.cached_call import RetryOnExceptionCachedCall
from synapse.util.macaroons import get_value_from_macaroon, satisfy_expiry

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)

# we want the cookie to be returned to us even when the request is the POSTed
# result of a form on another domain, as is used with `response_mode=form_post`.
#
# Modern browsers will not do so unless we set SameSite=None; however *older*
# browsers (including all versions of Safari on iOS 12?) don't support
# SameSite=None, and interpret it as SameSite=Strict:
# https://bugs.webkit.org/show_bug.cgi?id=198181
#
# As a rather painful workaround, we set *two* cookies, one with SameSite=None
# and one with no SameSite, in the hope that at least one of them will get
# back to us.
#
# Secure is necessary for SameSite=None (and, empirically, also breaks things
# on iOS 12.)
#
# Here we have the names of the cookies, and the options we use to set them.
_SESSION_COOKIES = [
    (b"oidc_session", b"HttpOnly; Secure; SameSite=None"),
    (b"oidc_session_no_samesite", b"HttpOnly"),
]


#: A token exchanged from the token endpoint, as per RFC6749 sec 5.1. and
#: OpenID.Core sec 3.1.3.3.
class Token(TypedDict):
    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.
class JWKS(TypedDict):
    keys: List[JWK]


class OidcHandler:
    """Handles requests related to the OpenID Connect login flow."""

    def __init__(self, hs: "HomeServer"):
        self._sso_handler = hs.get_sso_handler()

        provider_confs = hs.config.oidc.oidc_providers
        # we should not have been instantiated if there is no configured provider.
        assert provider_confs

        self._token_generator = OidcSessionTokenGenerator(hs)
        self._providers: Dict[str, "OidcProvider"] = {
            p.idp_id: OidcProvider(hs, self._token_generator, p) for p in provider_confs
        }

    async def load_metadata(self) -> None:
        """Validate the config and load the metadata from the remote endpoint.

        Called at startup to ensure we have everything we need.
        """
        for idp_id, p in self._providers.items():
            try:
                await p.load_metadata()
                if not p._uses_userinfo:
                    await p.load_jwks()
            except Exception as e:
                raise Exception(
                    "Error while initialising OIDC provider %r" % (idp_id,)
                ) from e

    async def handle_oidc_callback(self, request: SynapseRequest) -> None:
        """Handle an incoming request to /_synapse/client/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._sso_handler.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 know the session is legit, we then delegate to the OIDC Provider
        implementation, which will exchange the code with the provider and complete the
        login/authentication.

        Args:
            request: the incoming request from the browser.
        """
        # This will always be set by the time Twisted calls us.
        assert request.args is not None

        # The provider might redirect with an error.
        # In that case, just display it as-is.
        if b"error" in request.args:
            # error response from the auth server. see:
            #  https://tools.ietf.org/html/rfc6749#section-4.1.2.1
            #  https://openid.net/specs/openid-connect-core-1_0.html#AuthError
            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.
            logger.log(
                logging.INFO if error == "access_denied" else logging.ERROR,
                "Received OIDC callback with error: %s %s",
                error,
                description,
            )

            self._sso_handler.render_error(request, error, description)
            return

        # otherwise, it is presumably a successful response. see:
        #   https://tools.ietf.org/html/rfc6749#section-4.1.2

        # Fetch the session cookie. See the comments on SESSION_COOKIES for why there
        # are two.

        for cookie_name, _ in _SESSION_COOKIES:
            session: Optional[bytes] = request.getCookie(cookie_name)
            if session is not None:
                break
        else:
            logger.info("Received OIDC callback, with no session cookie")
            self._sso_handler.render_error(
                request, "missing_session", "No session cookie found"
            )
            return

        # Remove the cookies. There is a good chance that if the callback failed
        # once, it will fail next time and the code will already be exchanged.
        # Removing the cookies early avoids spamming the provider with token requests.
        #
        # we have to build the header by hand rather than calling request.addCookie
        # because the latter does not support SameSite=None
        # (https://twistedmatrix.com/trac/ticket/10088)

        for cookie_name, options in _SESSION_COOKIES:
            request.cookies.append(
                b"%s=; Expires=Thu, Jan 01 1970 00:00:00 UTC; %s"
                % (cookie_name, options)
            )

        # Check for the state query parameter
        if b"state" not in request.args:
            logger.info("Received OIDC callback, with no state parameter")
            self._sso_handler.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:
            session_data = self._token_generator.verify_oidc_session_token(
                session, state
            )
        except (MacaroonInitException, MacaroonDeserializationException, KeyError) as e:
            logger.exception("Invalid session for OIDC callback")
            self._sso_handler.render_error(request, "invalid_session", str(e))
            return
        except MacaroonInvalidSignatureException as e:
            logger.exception("Could not verify session for OIDC callback")
            self._sso_handler.render_error(request, "mismatching_session", str(e))
            return

        logger.info("Received OIDC callback for IdP %s", session_data.idp_id)

        oidc_provider = self._providers.get(session_data.idp_id)
        if not oidc_provider:
            logger.error("OIDC session uses unknown IdP %r", oidc_provider)
            self._sso_handler.render_error(request, "unknown_idp", "Unknown IdP")
            return

        if b"code" not in request.args:
            logger.info("Code parameter is missing")
            self._sso_handler.render_error(
                request, "invalid_request", "Code parameter is missing"
            )
            return

        code = request.args[b"code"][0].decode()

        await oidc_provider.handle_oidc_callback(request, session_data, code)


class OidcError(Exception):
    """Used to catch errors when calling the token_endpoint"""

    def __init__(self, error: str, error_description: Optional[str] = None):
        self.error = error
        self.error_description = error_description

    def __str__(self) -> str:
        if self.error_description:
            return f"{self.error}: {self.error_description}"
        return self.error


class OidcProvider:
    """Wraps the config for a single OIDC IdentityProvider

    Provides methods for handling redirect requests and callbacks via that particular
    IdP.
    """

    def __init__(
        self,
        hs: "HomeServer",
        token_generator: "OidcSessionTokenGenerator",
        provider: OidcProviderConfig,
    ):
        self._store = hs.get_datastore()

        self._token_generator = token_generator

        self._config = provider
        self._callback_url: str = hs.config.oidc.oidc_callback_url

        # Calculate the prefix for OIDC callback paths based on the public_baseurl.
        # We'll insert this into the Path= parameter of any session cookies we set.
        public_baseurl_path = urlparse(hs.config.server.public_baseurl).path
        self._callback_path_prefix = (
            public_baseurl_path.encode("utf-8") + b"_synapse/client/oidc"
        )

        self._oidc_attribute_requirements = provider.attribute_requirements
        self._scopes = provider.scopes
        self._user_profile_method = provider.user_profile_method

        client_secret: Optional[Union[str, JwtClientSecret]] = None
        if provider.client_secret:
            client_secret = provider.client_secret
        elif provider.client_secret_jwt_key:
            client_secret = JwtClientSecret(
                provider.client_secret_jwt_key,
                provider.client_id,
                provider.issuer,
                hs.get_clock(),
            )

        self._client_auth = ClientAuth(
            provider.client_id,
            client_secret,
            provider.client_auth_method,
        )
        self._client_auth_method = provider.client_auth_method

        # cache of metadata for the identity provider (endpoint uris, mostly). This is
        # loaded on-demand from the discovery endpoint (if discovery is enabled), with
        # possible overrides from the config.  Access via `load_metadata`.
        self._provider_metadata = RetryOnExceptionCachedCall(self._load_metadata)

        # cache of JWKs used by the identity provider to sign tokens. Loaded on demand
        # from the IdP's jwks_uri, if required.
        self._jwks = RetryOnExceptionCachedCall(self._load_jwks)

        self._user_mapping_provider = provider.user_mapping_provider_class(
            provider.user_mapping_provider_config
        )
        self._skip_verification = provider.skip_verification
        self._allow_existing_users = provider.allow_existing_users

        self._http_client = hs.get_proxied_http_client()
        self._server_name: str = hs.config.server.server_name

        # identifier for the external_ids table
        self.idp_id = provider.idp_id

        # user-facing name of this auth provider
        self.idp_name = provider.idp_name

        # MXC URI for icon for this auth provider
        self.idp_icon = provider.idp_icon

        # optional brand identifier for this auth provider
        self.idp_brand = provider.idp_brand

        self._sso_handler = hs.get_sso_handler()

        self._sso_handler.register_identity_provider(self)

    def _validate_metadata(self, m: OpenIDProviderMetadata) -> None:
        """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.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"],)
                )

        # Ensure there's a userinfo endpoint to fetch from if it is required.
        if self._uses_userinfo:
            if m.get("userinfo_endpoint") is None:
                raise ValueError(
                    'provider has no "userinfo_endpoint", even though it is required'
                )
        else:
            # If we're not using userinfo, we need a valid jwks to validate the ID token
            m.validate_jwks_uri()

    @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 information. If not, we should fetch them using the
        ``access_token`` with the ``userinfo_endpoint``.
        """

        return (
            "openid" not in self._scopes
            or self._user_profile_method == "userinfo_endpoint"
        )

    async def load_metadata(self, force: bool = False) -> OpenIDProviderMetadata:
        """Return the provider metadata.

        If this is the first call, the metadata is built from the config and from the
        metadata discovery endpoint (if enabled), and then validated. If the metadata
        is successfully validated, it is then cached for future use.

        Args:
            force: If true, any cached metadata is discarded to force a reload.

        Raises:
            ValueError: if something in the provider is not valid

        Returns:
            The provider's metadata.
        """
        if force:
            # reset the cached call to ensure we get a new result
            self._provider_metadata = RetryOnExceptionCachedCall(self._load_metadata)

        return await self._provider_metadata.get()

    async def _load_metadata(self) -> OpenIDProviderMetadata:
        # start out with just the issuer (unlike the other settings, discovered issuer
        # takes precedence over configured issuer, because configured issuer is
        # required for discovery to take place.)
        #
        metadata = OpenIDProviderMetadata(issuer=self._config.issuer)

        # load any data from the discovery endpoint, if enabled
        if self._config.discover:
            url = get_well_known_url(self._config.issuer, external=True)
            metadata_response = await self._http_client.get_json(url)
            metadata.update(metadata_response)

        # override any discovered data with any settings in our config
        if self._config.authorization_endpoint:
            metadata["authorization_endpoint"] = self._config.authorization_endpoint

        if self._config.token_endpoint:
            metadata["token_endpoint"] = self._config.token_endpoint

        if self._config.userinfo_endpoint:
            metadata["userinfo_endpoint"] = self._config.userinfo_endpoint

        if self._config.jwks_uri:
            metadata["jwks_uri"] = self._config.jwks_uri

        self._validate_metadata(metadata)

        return 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 force:
            # reset the cached call to ensure we get a new result
            self._jwks = RetryOnExceptionCachedCall(self._load_jwks)
        return await self._jwks.get()

    async def _load_jwks(self) -> JWKS:
        metadata = await self.load_metadata()

        # Load the JWKS using the `jwks_uri` metadata.
        uri = metadata.get("jwks_uri")
        if not uri:
            # this should be unreachable: load_metadata validates that
            # there is a jwks_uri in the metadata if _uses_userinfo is unset
            raise RuntimeError('Missing "jwks_uri" in metadata')

        jwk_set = await self._http_client.get_json(uri)

        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.

        See:
           https://tools.ietf.org/html/rfc6749#section-3.2
           https://openid.net/specs/openid-connect-core-1_0.html#TokenEndpoint

        Args:
            code: The authorization 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")
        raw_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, raw_headers, body = self._client_auth.prepare(
            method="POST", uri=token_endpoint, headers=raw_headers, body=body
        )
        headers = Headers({k: [v] for (k, v) in raw_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 make_deferred_yieldable(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_decoder.decode(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_decoder.decode(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: %r", resp)
            raise OidcError("server_error", description)

        return resp

    async def _fetch_userinfo(self, token: Token) -> UserInfo:
        """Fetch user information 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.
        """
        logger.debug("Using the OAuth2 access_token to request userinfo")
        metadata = await self.load_metadata()

        resp = await self._http_client.get_json(
            metadata["userinfo_endpoint"],
            headers={"Authorization": ["Bearer {}".format(token["access_token"])]},
        )

        logger.debug("Retrieved user info from userinfo endpoint: %r", resp)

        return UserInfo(resp)

    async def _parse_id_token(self, token: Token, nonce: str) -> CodeIDToken:
        """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:
            The decoded claims in the ID token.
        """
        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"]

        alg_values = metadata.get("id_token_signing_alg_values_supported", ["RS256"])
        jwt = JsonWebToken(alg_values)

        claim_options = {"iss": {"values": [metadata["issuer"]]}}

        id_token = token["id_token"]
        logger.debug("Attempting to decode JWT id_token %r", id_token)

        # 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(
                id_token,
                key=jwk_set,
                claims_cls=CodeIDToken,
                claims_options=claim_options,
                claims_params=claims_params,
            )
        except ValueError:
            logger.info("Reloading JWKS after decode error")
            jwk_set = await self.load_jwks(force=True)  # try reloading the jwks
            claims = jwt.decode(
                id_token,
                key=jwk_set,
                claims_cls=CodeIDToken,
                claims_options=claim_options,
                claims_params=claims_params,
            )

        logger.debug("Decoded id_token JWT %r; validating", claims)

        claims.validate(leeway=120)  # allows 2 min of clock skew

        return claims

    async def handle_redirect_request(
        self,
        request: SynapseRequest,
        client_redirect_url: Optional[bytes],
        ui_auth_session_id: Optional[str] = None,
    ) -> str:
        """Handle an incoming request to /login/sso/redirect

        It returns a redirect 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/client/oidc/callback``
          - ``scope``: the list of scopes set in ``oidc_config.scopes``
          - ``state``: a random string
          - ``nonce``: a random string

        In addition generating a redirect URL, 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 (or None for UI Auth)
            ui_auth_session_id: The session ID of the ongoing UI Auth (or
                None if this is a login).

        Returns:
            The redirect URL to the authorization endpoint.

        """

        state = generate_token()
        nonce = generate_token()

        if not client_redirect_url:
            client_redirect_url = b""

        cookie = self._token_generator.generate_oidc_session_token(
            state=state,
            session_data=OidcSessionData(
                idp_id=self.idp_id,
                nonce=nonce,
                client_redirect_url=client_redirect_url.decode(),
                ui_auth_session_id=ui_auth_session_id or "",
            ),
        )

        # Set the cookies. See the comments on _SESSION_COOKIES for why there are two.
        #
        # we have to build the header by hand rather than calling request.addCookie
        # because the latter does not support SameSite=None
        # (https://twistedmatrix.com/trac/ticket/10088)

        for cookie_name, options in _SESSION_COOKIES:
            request.cookies.append(
                b"%s=%s; Max-Age=3600; Path=%s; %s"
                % (
                    cookie_name,
                    cookie.encode("utf-8"),
                    self._callback_path_prefix,
                    options,
                )
            )

        metadata = await self.load_metadata()
        authorization_endpoint = metadata.get("authorization_endpoint")
        return 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,
        )

    async def handle_oidc_callback(
        self, request: SynapseRequest, session_data: "OidcSessionData", code: str
    ) -> None:
        """Handle an incoming request to /_synapse/client/oidc/callback

        By this time we have already validated the session on the synapse side, and
        now need to do the provider-specific operations. This includes:

          - 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.
            session_data: the session data, extracted from our cookie
            code: The authorization code we got from the callback.
        """
        # Exchange the code with the provider
        try:
            logger.debug("Exchanging OAuth2 code for a token")
            token = await self._exchange_code(code)
        except OidcError as e:
            logger.exception("Could not exchange OAuth2 code")
            self._sso_handler.render_error(request, e.error, e.error_description)
            return

        logger.debug("Successfully obtained OAuth2 token data: %r", token)

        # If there is an id_token, it should be validated, regardless of the
        # userinfo endpoint is used or not.
        if token.get("id_token") is not None:
            try:
                id_token = await self._parse_id_token(token, nonce=session_data.nonce)
                sid = id_token.get("sid")
            except Exception as e:
                logger.exception("Invalid id_token")
                self._sso_handler.render_error(request, "invalid_token", str(e))
                return
        else:
            id_token = None
            sid = None

        # Now that we have a token, get the userinfo either from the `id_token`
        # claims or by fetching the `userinfo_endpoint`.
        if self._uses_userinfo:
            try:
                userinfo = await self._fetch_userinfo(token)
            except Exception as e:
                logger.exception("Could not fetch userinfo")
                self._sso_handler.render_error(request, "fetch_error", str(e))
                return
        elif id_token is not None:
            userinfo = UserInfo(id_token)
        else:
            logger.error("Missing id_token in token response")
            self._sso_handler.render_error(
                request, "invalid_token", "Missing id_token in token response"
            )
            return

        # first check if we're doing a UIA
        if session_data.ui_auth_session_id:
            try:
                remote_user_id = self._remote_id_from_userinfo(userinfo)
            except Exception as e:
                logger.exception("Could not extract remote user id")
                self._sso_handler.render_error(request, "mapping_error", str(e))
                return

            return await self._sso_handler.complete_sso_ui_auth_request(
                self.idp_id, remote_user_id, session_data.ui_auth_session_id, request
            )

        # otherwise, it's a login
        logger.debug("Userinfo for OIDC login: %s", userinfo)

        # Ensure that the attributes of the logged in user meet the required
        # attributes by checking the userinfo against attribute_requirements
        # In order to deal with the fact that OIDC userinfo can contain many
        # types of data, we wrap non-list values in lists.
        if not self._sso_handler.check_required_attributes(
            request,
            {k: v if isinstance(v, list) else [v] for k, v in userinfo.items()},
            self._oidc_attribute_requirements,
        ):
            return

        # Call the mapper to register/login the user
        try:
            await self._complete_oidc_login(
                userinfo, token, request, session_data.client_redirect_url, sid
            )
        except MappingException as e:
            logger.exception("Could not map user")
            self._sso_handler.render_error(request, "mapping_error", str(e))

    async def _complete_oidc_login(
        self,
        userinfo: UserInfo,
        token: Token,
        request: SynapseRequest,
        client_redirect_url: str,
        sid: Optional[str],
    ) -> None:
        """Given a UserInfo response, complete the login flow

        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 and allow_existing_users
        is disabled, raise an exception.

        Otherwise, render a redirect back to the client_redirect_url with a loginToken.

        Args:
            userinfo: an object representing the user
            token: a dict with the tokens obtained from the provider
            request: The request to respond to
            client_redirect_url: The redirect URL passed in by the client.

        Raises:
            MappingException: if there was an error while mapping some properties
        """
        try:
            remote_user_id = self._remote_id_from_userinfo(userinfo)
        except Exception as e:
            raise MappingException(
                "Failed to extract subject from OIDC response: %s" % (e,)
            )

        # Older mapping providers don't accept the `failures` argument, so we
        # try and detect support.
        mapper_signature = inspect.signature(
            self._user_mapping_provider.map_user_attributes
        )
        supports_failures = "failures" in mapper_signature.parameters

        async def oidc_response_to_user_attributes(failures: int) -> UserAttributes:
            """
            Call the mapping provider to map the OIDC userinfo and token to user attributes.

            This is backwards compatibility for abstraction for the SSO handler.
            """
            if supports_failures:
                attributes = await self._user_mapping_provider.map_user_attributes(
                    userinfo, token, failures
                )
            else:
                # If the mapping provider does not support processing failures,
                # do not continually generate the same Matrix ID since it will
                # continue to already be in use. Note that the error raised is
                # arbitrary and will get turned into a MappingException.
                if failures:
                    raise MappingException(
                        "Mapping provider does not support de-duplicating Matrix IDs"
                    )

                attributes = await self._user_mapping_provider.map_user_attributes(  # type: ignore
                    userinfo, token
                )

            return UserAttributes(**attributes)

        async def grandfather_existing_users() -> Optional[str]:
            if self._allow_existing_users:
                # If allowing existing users we want to generate a single localpart
                # and attempt to match it.
                attributes = await oidc_response_to_user_attributes(failures=0)

                if attributes.localpart is None:
                    # If no localpart is returned then we will generate one, so
                    # there is no need to search for existing users.
                    return None

                user_id = UserID(attributes.localpart, self._server_name).to_string()
                users = await self._store.get_users_by_id_case_insensitive(user_id)
                if users:
                    # If an existing matrix ID is returned, then use it.
                    if len(users) == 1:
                        previously_registered_user_id = next(iter(users))
                    elif user_id in users:
                        previously_registered_user_id = user_id
                    else:
                        # Do not attempt to continue generating Matrix IDs.
                        raise MappingException(
                            "Attempted to login as '{}' but it matches more than one user inexactly: {}".format(
                                user_id, users
                            )
                        )

                    return previously_registered_user_id

            return None

        # Mapping providers might not have get_extra_attributes: only call this
        # method if it exists.
        extra_attributes = None
        get_extra_attributes = getattr(
            self._user_mapping_provider, "get_extra_attributes", None
        )
        if get_extra_attributes:
            extra_attributes = await get_extra_attributes(userinfo, token)

        await self._sso_handler.complete_sso_login_request(
            self.idp_id,
            remote_user_id,
            request,
            client_redirect_url,
            oidc_response_to_user_attributes,
            grandfather_existing_users,
            extra_attributes,
            auth_provider_session_id=sid,
        )

    def _remote_id_from_userinfo(self, userinfo: UserInfo) -> str:
        """Extract the unique remote id from an OIDC UserInfo block

        Args:
            userinfo: An object representing the user given by the OIDC provider
        Returns:
            remote user id
        """
        remote_user_id = self._user_mapping_provider.get_remote_user_id(userinfo)
        # Some OIDC providers use integer IDs, but Synapse expects external IDs
        # to be strings.
        return str(remote_user_id)


# number of seconds a newly-generated client secret should be valid for
CLIENT_SECRET_VALIDITY_SECONDS = 3600

# minimum remaining validity on a client secret before we should generate a new one
CLIENT_SECRET_MIN_VALIDITY_SECONDS = 600


class JwtClientSecret:
    """A class which generates a new client secret on demand, based on a JWK

    This implementation is designed to comply with the requirements for Apple Sign in:
    https://developer.apple.com/documentation/sign_in_with_apple/generate_and_validate_tokens#3262048

    It looks like those requirements are based on https://tools.ietf.org/html/rfc7523,
    but it's worth noting that we still put the generated secret in the "client_secret"
    field (or rather, whereever client_auth_method puts it) rather than in a
    client_assertion field in the body as that RFC seems to require.
    """

    def __init__(
        self,
        key: OidcProviderClientSecretJwtKey,
        oauth_client_id: str,
        oauth_issuer: str,
        clock: Clock,
    ):
        self._key = key
        self._oauth_client_id = oauth_client_id
        self._oauth_issuer = oauth_issuer
        self._clock = clock
        self._cached_secret = b""
        self._cached_secret_replacement_time = 0

    def __str__(self) -> str:
        # if client_auth_method is client_secret_basic, then ClientAuth.prepare calls
        # encode_client_secret_basic, which calls "{}".format(secret), which ends up
        # here.
        return self._get_secret().decode("ascii")

    def __bytes__(self) -> bytes:
        # if client_auth_method is client_secret_post, then ClientAuth.prepare calls
        # encode_client_secret_post, which ends up here.
        return self._get_secret()

    def _get_secret(self) -> bytes:
        now = self._clock.time()

        # if we have enough validity on our existing secret, use it
        if now < self._cached_secret_replacement_time:
            return self._cached_secret

        issued_at = int(now)
        expires_at = issued_at + CLIENT_SECRET_VALIDITY_SECONDS

        # we copy the configured header because jwt.encode modifies it.
        header = dict(self._key.jwt_header)

        # see https://tools.ietf.org/html/rfc7523#section-3
        payload = {
            "sub": self._oauth_client_id,
            "aud": self._oauth_issuer,
            "iat": issued_at,
            "exp": expires_at,
            **self._key.jwt_payload,
        }
        logger.info(
            "Generating new JWT for %s: %s %s", self._oauth_issuer, header, payload
        )
        self._cached_secret = jwt.encode(header, payload, self._key.key)
        self._cached_secret_replacement_time = (
            expires_at - CLIENT_SECRET_MIN_VALIDITY_SECONDS
        )
        return self._cached_secret


class OidcSessionTokenGenerator:
    """Methods for generating and checking OIDC Session cookies."""

    def __init__(self, hs: "HomeServer"):
        self._clock = hs.get_clock()
        self._server_name = hs.hostname
        self._macaroon_secret_key = hs.config.key.macaroon_secret_key

    def generate_oidc_session_token(
        self,
        state: str,
        session_data: "OidcSessionData",
        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.
            session_data: data to include in the session token.
            duration_in_ms: An optional duration for the token in milliseconds.
                Defaults to an hour.

        Returns:
            A signed macaroon token with the session information.
        """
        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("idp_id = %s" % (session_data.idp_id,))
        macaroon.add_first_party_caveat("nonce = %s" % (session_data.nonce,))
        macaroon.add_first_party_caveat(
            "client_redirect_url = %s" % (session_data.client_redirect_url,)
        )
        macaroon.add_first_party_caveat(
            "ui_auth_session_id = %s" % (session_data.ui_auth_session_id,)
        )
        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: bytes, state: str
    ) -> "OidcSessionData":
        """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 data extracted from the session cookie

        Raises:
            KeyError if an expected caveat is missing from the macaroon.
        """
        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("idp_id = "))
        v.satisfy_general(lambda c: c.startswith("client_redirect_url = "))
        v.satisfy_general(lambda c: c.startswith("ui_auth_session_id = "))
        satisfy_expiry(v, self._clock.time_msec)

        v.verify(macaroon, self._macaroon_secret_key)

        # Extract the session data from the token.
        nonce = get_value_from_macaroon(macaroon, "nonce")
        idp_id = get_value_from_macaroon(macaroon, "idp_id")
        client_redirect_url = get_value_from_macaroon(macaroon, "client_redirect_url")
        ui_auth_session_id = get_value_from_macaroon(macaroon, "ui_auth_session_id")
        return OidcSessionData(
            nonce=nonce,
            idp_id=idp_id,
            client_redirect_url=client_redirect_url,
            ui_auth_session_id=ui_auth_session_id,
        )


@attr.s(frozen=True, slots=True, auto_attribs=True)
class OidcSessionData:
    """The attributes which are stored in a OIDC session cookie"""

    # the Identity Provider being used
    idp_id: str

    # The `nonce` parameter passed to the OIDC provider.
    nonce: str

    # The URL the client gave when it initiated the flow. ("" if this is a UI Auth)
    client_redirect_url: str

    # The session ID of the ongoing UI Auth ("" if this is a login)
    ui_auth_session_id: str


class UserAttributeDict(TypedDict):
    localpart: Optional[str]
    display_name: Optional[str]
    emails: List[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, failures: int
    ) -> UserAttributeDict:
        """Map a `UserInfo` object 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
            failures: How many times a call to this function with this
                UserInfo has resulted in a failure.

        Returns:
            A dict containing the ``localpart`` and (optionally) the ``display_name``
        """
        raise NotImplementedError()

    async def get_extra_attributes(self, userinfo: UserInfo, token: Token) -> JsonDict:
        """Map a `UserInfo` object into additional attributes passed to the client during login.

        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 additional attributes. Must be JSON serializable.
        """
        return {}


# Used to clear out "None" values in templates
def jinja_finalize(thing: Any) -> Any:
    return thing if thing is not None else ""


env = Environment(finalize=jinja_finalize)


@attr.s(slots=True, frozen=True, auto_attribs=True)
class JinjaOidcMappingConfig:
    subject_claim: str
    localpart_template: Optional[Template]
    display_name_template: Optional[Template]
    email_template: Optional[Template]
    extra_attributes: Dict[str, 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")

        def parse_template_config(option_name: str) -> Optional[Template]:
            if option_name not in config:
                return None
            try:
                return env.from_string(config[option_name])
            except Exception as e:
                raise ConfigError("invalid jinja template", path=[option_name]) from e

        localpart_template = parse_template_config("localpart_template")
        display_name_template = parse_template_config("display_name_template")
        email_template = parse_template_config("email_template")

        extra_attributes = {}  # type Dict[str, Template]
        if "extra_attributes" in config:
            extra_attributes_config = config.get("extra_attributes") or {}
            if not isinstance(extra_attributes_config, dict):
                raise ConfigError("must be a dict", path=["extra_attributes"])

            for key, value in extra_attributes_config.items():
                try:
                    extra_attributes[key] = env.from_string(value)
                except Exception as e:
                    raise ConfigError(
                        "invalid jinja template", path=["extra_attributes", key]
                    ) from e

        return JinjaOidcMappingConfig(
            subject_claim=subject_claim,
            localpart_template=localpart_template,
            display_name_template=display_name_template,
            email_template=email_template,
            extra_attributes=extra_attributes,
        )

    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, failures: int
    ) -> UserAttributeDict:
        localpart = None

        if self._config.localpart_template:
            localpart = self._config.localpart_template.render(user=userinfo).strip()

            # Ensure only valid characters are included in the MXID.
            localpart = map_username_to_mxid_localpart(localpart)

            # Append suffix integer if last call to this function failed to produce
            # a usable mxid.
            localpart += str(failures) if failures else ""

        def render_template_field(template: Optional[Template]) -> Optional[str]:
            if template is None:
                return None
            return template.render(user=userinfo).strip()

        display_name = render_template_field(self._config.display_name_template)
        if display_name == "":
            display_name = None

        emails: List[str] = []
        email = render_template_field(self._config.email_template)
        if email:
            emails.append(email)

        return UserAttributeDict(
            localpart=localpart, display_name=display_name, emails=emails
        )

    async def get_extra_attributes(self, userinfo: UserInfo, token: Token) -> JsonDict:
        extras: Dict[str, str] = {}
        for key, template in self._config.extra_attributes.items():
            try:
                extras[key] = template.render(user=userinfo).strip()
            except Exception as e:
                # Log an error and skip this value (don't break login for this).
                logger.error("Failed to render OIDC extra attribute %s: %s" % (key, e))
        return extras
