Documentation: document dex scopes, claims, and client features

Documentation/custom-scopes-claims-clients.md

+# Custom scopes, claims and client features
+
+This document describes the set of OAuth2 and OpenID Connect features implemented by dex.
+
+## Scopes
+
+The following is the exhaustive list of scopes supported by dex:
+
+| Name | Description |
+| ---- | ------------|
+| `openid` | Required scope for all login requests. |
+| `email` | ID token claims should include the end user's email and if that email was verified by an upstream provider. |
+| `profile` | ID token claims should include the username of the end user. |
+| `groups` | ID token claims should include a list of groups the end user is a member of. |
+| `offline_access` | Token response should include a refresh token. |
+| `audience:server:client_id:( client-id )` | Dynamic scope indicating that the ID token should be issued on behalf of another client. See the _"Cross-client trust and authorized party"_ section below. |
+
+## Custom claims
+
+Beyond the [required OpenID Connect claims][core-claims], and a handful of [standard claims][standard-claims], dex implements the following non-standard claims.
+
+| Name | Description |
+| ---- | ------------|
+| `groups` | A list of strings representing the groups a user is a member of. |
+| `email` | The email of the user. |
+| `email_verified` | If the upstream provider has verified the email. |
+| `name` | User's display name. |
+
+## Cross-client trust and authorized party
+
+Dex has the ability to issue ID tokens to clients on behalf of other clients. In OpenID Connect terms, this means the ID token's `aud` (audience) claim being a different client ID than the client that performed the login.
+
+For example, this feature could be used to allow a web app to generate an ID token on behalf of a command line tool:
+
+```yaml
+staticClients:
+- id: web-app
+  redirectURIs:
+  - 'https://web-app.example.com/callback'
+  name: 'Web app'
+  secret: web-app-secret
+
+- id: cli-app
+  redirectURIs:
+  - 'https://cli-app.example.com/callback'
+  name: 'Command line tool'
+  secret: cli-app-secret
+  # The command line tool lets the web app issue ID tokens on its behalf.
+  trustedPeers:
+  - web-app
+```
+
+Note that the command line tool must explicitly trust the web app using the `trustedPeers` field. The web app can then use the following scope to request an ID token that's issued for the command line tool.
+
+```
+audience:server:client_id:cli-app
+```
+
+The ID token claims will then include the following audience and authorized party:
+
+```
+{
+    "aud": "cli-app",
+    "azp": "web-app",
+    "email": "foo@bar.com",
+    // other claims...
+}
+``` 
+
+[core-claims]: https://openid.net/specs/openid-connect-core-1_0.html#IDToken
+[standard-claims]: https://openid.net/specs/openid-connect-core-1_0.html#StandardClaims

Documentation/openid-connect.md

 # An overview of OpenID Connect
 
-This document attempts to provide an overview of the [OpenID Connect protocol](
-https://openid.net/connect/), a flavor of OAuth2 that dex implements. While
-this document isn't complete we hope it provides a enough to get users up and
-running.
+This document attempts to provide a general overview of the [OpenID Connect protocol](https://openid.net/connect/), a flavor of OAuth2 that dex implements. While this document isn't complete, we hope it provides enough information to get users up and running.
+
+For an overview of custom claims, scopes, and client features implemented by dex, see [this document][scopes-claims-clients].
 
 ## OAuth2
 
@@ -138,3 +137,5 @@ $ curl http://127.0.0.1:5556/keys
   ]
 }
 ```
+
+[scopes-claims-clients]: custom-scopes-claims-clients.md

README.md

@@ -30,6 +30,7 @@ More docs for running dex as a Kubernetes authenticator can be found [here](Docu
 
 * [Getting started](Documentation/getting-started.md)
 * [What's new in v2](Documentation/v2.md)
+* [Custom scopes, claims, and client features](Documentation/custom-scopes-claims-clients.md)
 * [Storage options](Documentation/storage.md)
 * [Intro to OpenID Connect](Documentation/openid-connect.md)
 * [gRPC API](Documentation/api.md)