docs: Sequence diagram for auth code grant (#3574)

#3572

docs/admin/auth-server/oauth-features/auth-code-grant.md

@@ -0,0 +1,76 @@
+---
+tags:
+  - administration
+  - auth-server
+  - oauth
+  - feature
+---
+
+# OAuth 2.0 Authorization Code Grant 
+
+The [Authorization Code grant type](tools.ietf.org/html/rfc6749#section-1.3.1) is used by confidential and public clients to exchange an authorization code for an access token.
+After the user returns to the client via the redirect URL, the application will get the authorization code from the URL and use it to request an access token.
+
+### Sequence Diagram
+
+```mermaid
+
+sequenceDiagram
+title Authorization code flow
+autonumber 1
+
+participant Resource owner User 
+activate Resource owner User
+Client->>Jans AS:Authorization Request
+activate Client 
+activate Jans AS
+Resource owner User ->>Jans AS:User login and consent
+Jans AS-->>Client:Authorization code response
+deactivate Client
+deactivate Jans AS
+
+Client->>Jans AS: Exchange code for Access Token
+activate Client
+activate Jans AS
+Jans AS-->>Client: Access Token with optional Refresh Token
+deactivate Client
+deactivate Jans AS
+
+loop 
+    Client->>Resource Server: Call API with Access Token
+    activate Client
+    activate Resource Server
+    Resource Server-->>Client: Protected Resource
+    deactivate Client
+    deactivate Resource Server
+end
+
+
+Client->>Resource Server: Call API with expired Access Token
+activate Client
+activate Resource Server
+Resource Server-->>Client: Invalid token error
+deactivate Client
+deactivate Resource Server
+
+
+Client->>Jans AS: Resource Token
+activate Client
+activate Jans AS
+Jans AS-->>Client: Access Token with optional Refresh Token
+deactivate Client
+deactivate Jans AS
+deactivate Resource owner User
+```
+
+### Testing
+
+1. https://github.com/JanssenProject/jans/blob/main/jans-auth-server/client/src/test/java/io/jans/as/client/ws/rs/AuthorizationCodeFlowHttpTest.java
+
+2. On a browser access 
+`https://my.jans.server/jans-auth/restv1/authorize?redirect_uri=https://my-redirect-app:8080&client_id=Put_client_id_here&scope=username+openid&response_type=code`
+Based on the default authentication method set (acr), the user will be presented with credentials for login. The OpenID Provider (Gluu Server) verifies the user’s identity and authenticates the user.
+In the back channel the following steps take place :
+- The OpenID Provider (Gluu Server) sends the user back to the application with an authorization code.
+- The application sends the code to the Token Endpoint to receive an Access Token and ID Token in the response.
+- The application uses the ID Token to authorize the user. At this point the application/RP can access the UserInfo endpoint for claims.