Add docs on "OIDC setup guidelines" (#321)

* Add docs on "OIDC setup guidelines"

related: #320

---------

Co-authored-by: Matthias Mohr <[email protected]>

README.md

@@ -45,6 +45,7 @@ Example: <https://editor.openeo.org?server=https://earthengine.openeo.org&discov
 Here we collection information for back-end implementors that want to improve the experience with the Web Editor by fine-tuning their implementation.
 
 * [GeoTiff / COG support](docs/geotiff.md)
+* [OIDC setup](docs/oidc.md)
 * and more...
 
 ## Contributions

docs/oidc.md

@@ -0,0 +1,72 @@
+# OIDC setup guidelines for usage with the Web Editor
+
+The standard authentication mechanism in openEO is OpenID Connect (OIDC).
+The openEO Web Editor, being a standard web application, uses the corresponding OIDC flows such as the
+[Authorization Code Flow](https://openid.net/specs/openid-connect-core-1_0.html#CodeFlowAuth)
+to authenticate the user.
+
+To provide an optimal OIDC authentication user experience in the openEO Web Editor,
+it is important for an openEO back-end to properly set up a couple of things:
+
+- Determine what **OIDC provider(s)** to support.
+  One can choose from existing OIDC provider services,
+  or set up their own custom OIDC provider,
+  (e.g. using [Keycloak](https://www.keycloak.org/)).
+
+- For at least one (and preferably all) of the supported OIDC providers:
+  create an OIDC client that can be used as the **default OIDC client**
+  by users that do not manage their own OIDC client.
+
+  How this OIDC client has to be configured practically,
+  heavily depends on the OIDC provider,
+  but here are some general guidelines and constraints
+  for the OIDC client configuration:
+
+  - (At least) support the **Authorization Code Flow** without client secrets
+    (which is sometimes labeled as a "public client").
+    Support for PKCE (Proof Key for Code Exchange) is recommended.
+
+  - **Allow-list the proper redirect URIs** related to your openEO Web Editor deployment.
+
+    For example, if you host the web editor at `https://editor.openeo.example.org/`,
+    you should allow the redirect URI `https://editor.openeo.example.org`.
+
+    Note that the redirect URIs should be allow-listed *without trailing slashes*.
+
+    Make sure to cover all possible Web Editor domains you want to support with the client.
+    For example, consider allow-listing:
+    - `http://localhost:8080` for local development of the web editor
+    - `https://editor.openeo.org` so that your back-end can be used with the official openEO Web Editor.
+
+  - **Allow-list the proper origins** of your openEO Web Editor deployment.
+    (In Keycloak based providers, this setting is typically called "Web Origins".)
+    This ensures that the OIDC provider sets the proper **CORS headers**
+    so that the openEO Web Editor web app can access the tokens after authentication.
+    In the rare case that you host the web editor and the OIDC provider on the same domain,
+    you probably don't have to allow-list any origins.
+
+    For example, if you host the web editor at `https://openeo.example.org/editor`,
+    you should allow the origin `https://openeo.example.org`.
+
+    Note that an origin by definition is only scheme + domain and optionally a port.
+    Don't include a path (like `/editor` in the example above),
+    not even a trailing slash.
+
+    As with the redirect URIs, consider including:
+    - `http://localhost:8080`
+    - `https://editor.openeo.org`
+
+- Handle the `GET /credentials/oidc` endpoint in your openEO back-end,
+  based on the OIDC providers and OIDC clients discussed above.
+
+  Apart from the full details discussed
+  in the [`GET /credentials/oidc` specification](https://api.openeo.org/#tag/Account-Management/operation/authenticate-oidc)
+  consider these additional notes on the `default_clients` items:
+
+  - Include the appropriate grant type under the `grant_types` field:
+    - `authorization_code` for the Authorization Code Flow
+    - `authorization_code+pkce` for the Authorization Code Flow with PKCE
+    - `implicit` for the Implicit Flow (**discouraged**)
+  - List the same redirect URIs discussed above again under the `redirect_urls` field.
+    This listing allows the openEO Web Editor to hide authentication options
+    that won't work because redirect URIs configuration.