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+# Add CLARIAH authentication to your service
+
+(...)
+
+
+The following two Open ID Process Flows are documented in this readme:
+
+1. **Authentication Flow**: User authenticates to your service
+2. **Authorization Code Flow**: Your service calls other CLARIAH services **on behalf of authenticated users**
+
+If your service stands on itself, meaning it does not rely on any other external resources for loading/saving data, the **Authentication Flow** is enough and fairly simple to incorporate. 
+
+If however your service or users need to call external APIs after authenticating, the **Authentication Code Flow** needs to be implemented as well. Also your external resources/services/APIs also need to be adapted to make sure they can handle the `access_tokens` provided by the this process flow.
+
+## Considerations
+
+Before going ahead with the implementation, please consider the following:
+
+- Does my service load external resources that require user authentication?
+- What institutions are allowed to access my service?
+- Are there institutions of which only certain known users are allowed access?
+
+The first consideration was mentioned in the previous section. The remaining two can be addressed by whitelisting/blacklisting institutions and/or individual users within institutions. Examples for this are included further on. TODO ref 
+
+## Prerequisites
+
+The following example implementations require the following:
+
+- Python 3.8 or higher
+- [Authlib](https://docs.authlib.org/en/latest/)
+- [Flask](https://flask.palletsprojects.com/en/2.2.x/)
+- Permanent session cookie
+
+In Flask a permanent session cookie can be configured as follows:
+
+```python
+# Note: These settings only work if running on HTTPS! (https://github.com/onelogin/python-saml)
+SESSION_COOKIE_NAME = "__Host-xlab-mediasuite"
+SESSION_COOKIE_HTTPONLY = True
+SESSION_COOKIE_PATH = "/"
+SESSION_COOKIE_SECURE = True
+SESSION_COOKIE_SAMESITE = "Lax"
+
+# Note: These settings only work on HTTP (so without a web proxy with a TLS cert)
+# SESSION_COOKIE_HTTPONLY = False
+# SESSION_COOKIE_PATH = "/"
+# SESSION_COOKIE_SECURE = True
+# SESSION_COOKIE_SAMESITE = "None"
+```
+
+```python
+@app.before_request
+def make_session_permanent():
+    # ensure the session is permanent (to avoid killing the session on browser closing)
+    session.permanent = True
+    app.logger.info(app.session_interface.get_expiration_time(app, session))
+```
+
+# Implementation
+
+For both the Authentication Flow and the Authorization Code Flow an Open ID client needs to be instantiated.
+
+Flask is one of the web servers supported by Authlib and by adding the following keys to your Flask config, the OAuth client can fetch the following settings conveniently:
+
+```python
+ # NOTE: a CLIENT ID and SECRET must be requested from the CLARIAH organisation
+CLARIAH_CLIENT_ID = "your_service_client_id"
+CLARIAH_CLIENT_SECRET = "your_service_client_secret"
+```
+
+With these settings in place the client can be instantiated as follows:
+
+```python
+from flask import Flask
+from authlib.integrations.flask_client import OAuth
+from authlib.integrations.base_client.errors import OAuthError  # used in later examples
+
+# basic example of initiating the Flask app
+app = Flask(__name__)
+
+auth_server_metadata_url = 'https://CLARIAH_AUTH_SERVER/.well-known/openid-configuration'
+
+# initiate the OIDC (OAuth) client
+oauth = OAuth(app)  # pass the Flask app here
+oauth.register(
+    name="clariah",  # CLARIAH_CLIENT_ID and CLARIAH_CLIENT_SECRET are found this way
+    server_metadata_url=auth_server_metadata_url,  # points to CLARIAH SATOSA auth server
+    client_kwargs={"scope": "openid email profile"},  # parameters to enforce open ID connect
+)
+```
+
+If you follow the [auth_server_metadata_url](https://CLARIAH_AUTH_SERVER/.well-known/openid-configuration), you can see how the `register` function obtains all the relevant endpoints required for both the Authentication and Authorization Code Flows.
+
+## Authentication Flow
+
+Let's assume you have configured the following routes in your Flask application:
+
+```python
+# add the following routs to your Flask app
+app.add_url_rule("/login", view_func=login, methods=["GET"])
+app.add_url_rule(
+    "/oidc/redirect", view_func=oidc_auth_redirect, methods=["GET"]
+)
+
+def login(self):
+    redirect_user_to_uri = request.headers.get("Referer") 
+    return do_login(oauth, redirect_user_to_uri)
+
+def oidc_auth_redirect(self):
+    return oidc_auth_redirect(oauth)
+```
+
+
+The following function shows how the **Authentication Flow** can be triggered. Pass the `oauth` client from the previous code example. 
+
+```python
+def do_login(self, oauth, redirect_user_to_uri=None):
+    # remember from which page the user started login for redirection later
+    session["user_requested_url"] = redirect_user_to_uri 
+    redirect_uri = url_for(
+        "oidc_auth_redirect", _external=True, _scheme="https"
+    )
+
+    # use the Oauth client to start the Authentication Flow
+    return oauth.clariah.authorize_redirect(
+        redirect_uri=redirect_uri,
+        claims=json.dumps(
+            {
+                "id_token": {
+                    "edupersontargetedid": {"essential": True},
+                    "schac_home_organisation": {"essential": True},
+                    "nickname": {"essential": True},
+                    "email": {"essential": True},
+                    "eppn": {"essential": True},
+                    "idp": {"essential": True},
+                },
+                "userinfo": {
+                    "edupersontargetedid": {"essential": True},
+                    "schac_home_organisation": {"essential": True},
+                    "nickname": {"essential": True},
+                    "email": {"essential": True},
+                    "eppn": {"essential": True},
+                    "idp": {"essential": True},
+                },
+            }
+        ),
+    )
+```
+
+When the user authenticates SATOSA calls the configured `redirect_uri`. In case your service does not require a an Authorization Code Flow, it's enough implement `oidc_auth_redirect` like this:
+
+
+```python
+def oidc_auth_redirect(self, oauth, idp_whitelist, idp_user_whitelist):
+
+    # check if the authentication server returned an error (see NOTE 1)
+    if not __check_openid_server_error(request):
+        return __redirect_to_login_failed(  # (see NOTE 2)
+            "The authentucation server returned an error, preventing your logging in"
+        )
+
+    #TODO write some code that fetches just the user_info to check the whitelists
+```
+
+## Authorization Code Flow (ACF)
+
+To wire up the ACF, implement the `oidc_auth_redirect` along the following lines:
+
+
+```python
+# example idp_whitelist
+idp_whitelist = [
+    "https://some-idp.example.com",
+    "https://some-other-idp.example.com"
+]
+
+# example idp_user_whitelist
+idp_user_whitelist =  {
+    "https://some-idp.example.com": [
+        "user1@some-idp.example.com",
+        "user2@some-idp.example.com"
+    ]
+}
+
+# called by SATOSA after the user has tried to authenticate at the IdP
+def oidc_auth_redirect(self, oauth, idp_whitelist, idp_user_whitelist):
+
+    # check if the authentication server returned an error (see NOTE 1)
+    if not __check_openid_server_error(request):
+        return __redirect_to_login_failed(  # (see NOTE 2)
+            "The authentucation server returned an error, preventing your logging in"
+        )
+
+    # the user is authenticated, now request an access token for the APIs
+    token = None
+    try:
+        # Authlib takes care of the underlying Authorization Code Flow and returns 
+        # a token, containing namely: the access_token, refresh_token and user_info, but more as well
+        token = oauth.clariah.authorize_access_token()
+    except OAuthError:
+        logger.exception("Error: could not fetch access_token")
+
+    if not token:
+        return __redirect_to_login_failed(
+            "The server failed to fullfil your login request"
+        )
+
+    if not token.get("access_token", None):
+        return __redirect_to_login_failed(
+            "The server failed to obtain an access token"
+        )
+
+    if not token.get("userinfo", None):
+        return __redirect_to_login_failed(
+            "The server failed to obtain any user info"
+        )
+
+    # grab the access_token (for APIs) and userinfo (UI, IdP whitelist check)
+    access_token = token["access_token"]
+    user_info = token["userinfo"]
+    refresh_token = token["refresh_token"]
+    expires_in = token["expires_in"]
+
+    # check if the user's idp is whitelisted
+    allowed_idp = get_allowed_user_idp(user_info, idp_whitelist)
+    if not allowed_idp:
+        return __redirect_to_login_failed(
+            "The organisation you are authenticating from is not authorized to access this service"
+        )
+
+    # check if the user is specically whitelisted for a certain institution
+    if not check_idp_user_whitelist(
+        user_info, allowed_idp, idp_user_whitelist
+    ):
+        return __redirect_to_login_failed(
+            "You are not allowed to login via this organisation; please contact the service owner to request access"
+        )
+
+    # all is good, make sure to keep the vital information in the session cookie
+    session["oidcIsAuthenticated"] = True
+    session["access_token"] = access_token
+    session["userinfo"] = user_info
+    session["refresh_token"] = refresh_token
+    session["expires_in"] = expires_in
+    session["token_obtained"] = int(time())
+
+    # now finally redirect the user to the lastly visited page before logging in
+    return __redirect_to_requested_url()
+```
+
+**Note 1**: The `check_openid_server_error` can be implemented with [this](https://www.oauth.com/oauth2-servers/authorization/the-authorization-response/) in mind.
+
+**Note 2**: The `__redirect_to_login_failed` function should redirect the user to a page that shows the provided string as a nicely legible message.
+
+
+### Refreshing the access_token
+
+To ensure `access_tokens` are refreshed before expiring, e.g. implement:
+
+```python
+# see https://flask.palletsprojects.com/en/1.1.x/api/ (this will make sessions expire after 31 days)
+@app.before_request
+def make_session_permanent():
+    # ensure the session is permanent (to avoid killing the session on browser closing)
+    session.permanent = True
+    app.logger.info(app.session_interface.get_expiration_time(app, session))
+
+    # check if the OIDC access_token needs refreshing
+    token_refreshed = auth_hub.check_refresh_token()
+    app.logger.info(f"New refresh token issued: {token_refreshed}")
+```
+
+
+# Recommended reading
+
+- [auth0.com: Authorization Code Flow](https://auth0.com/docs/get-started/authentication-and-authorization-flow/authorization-code-flow)
+- [openid.net: Authorization Code Flow](https://openid.net/specs/openid-connect-core-1_0.html#CodeFlowAuth)
+- [Considerations concerning the access_token_lifetime](https://www.oauth.com/oauth2-servers/access-tokens/access-token-lifetime/)
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