Authorization code flow in Rest API - Step-by-Step Execution

Concept Flow - Authorization code flow

User requests login

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Redirect to Authorization Server

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User logs in and consents

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Authorization Server sends code

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Client sends code + client secret to Token Endpoint

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Token Endpoint validates and returns Access Token

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Client uses Access Token to access resource

This flow shows how a client app gets an authorization code, exchanges it for an access token, and then accesses protected resources.

Execution Sample

Rest API

1. User -> Client: Request login
2. Client -> Auth Server: Redirect with client_id
3. Auth Server -> User: Login & consent
4. Auth Server -> Client: Authorization code
5. Client -> Auth Server: Code + client_secret
6. Auth Server -> Client: Access token
7. Client -> Resource Server: Access token

Step-by-step message exchange in the authorization code flow.

Execution Table

Step	Actor	Action	Data Sent	Response/Result
1	User -> Client	User requests login	Login request	Client prepares redirect
2	Client -> Auth Server	Redirect user to auth server	client_id, redirect_uri, scope	Auth server shows login page
3	User -> Auth Server	User logs in and consents	User credentials, consent	Auth server validates user
4	Auth Server -> Client	Send authorization code	Authorization code	Client receives code
5	Client -> Auth Server	Exchange code for token	Authorization code, client_secret	Auth server validates and returns access token
6	Auth Server -> Client	Return access token	Access token	Client stores token
7	Client -> Resource Server	Access resource	Access token	Resource server returns data
8	-	End	-	Flow complete

💡 Flow ends after client uses access token to access resource successfully.

Variable Tracker

Variable	Start	After Step 4	After Step 6	Final
authorization_code	None	Received from auth server	Used and invalidated	None (used)
access_token	None	None	Received from auth server	Stored for resource access
user_logged_in	False	True	True	True

Key Moments - 3 Insights

Why does the client send the authorization code along with the client secret to the token endpoint?

Because the client secret proves the client is authorized to exchange the code for a token, preventing misuse. See execution_table step 5.

Why can't the client use the authorization code directly to access resources?

The authorization code is just a temporary code to get the access token. Only the access token can be used to access resources. See execution_table steps 4 and 7.

What happens if the user does not consent during login?

The authorization server will not issue an authorization code, so the flow stops early. This is implied before step 4 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the client receive the authorization code?

A Step 5

B Step 4

C Step 6

D Step 7

💡 Hint

Check the 'Data Sent' and 'Response/Result' columns at step 4.

According to variable_tracker, what is the state of 'access_token' after step 4?

A Received from auth server

B Stored for resource access

C None

D Used and invalidated

💡 Hint

Look at the 'access_token' row under 'After Step 4' column.

If the client secret is missing when exchanging the code, what would happen in the flow?

A Token endpoint rejects the request

B Access token is still issued

C Authorization code is sent again

D User is asked to login again

💡 Hint

Refer to key_moments about client secret importance and execution_table step 5.

Concept Snapshot

Authorization Code Flow:
1. User logs in via client redirect.
2. Auth server sends code to client.
3. Client exchanges code + secret for access token.
4. Client uses token to access resources.
Key: Code is short-lived, secret proves client identity.

Full Transcript

The Authorization Code Flow is a way for a client app to get permission to access user data securely. First, the user asks to log in. The client sends the user to the authorization server with its ID. The user logs in and agrees to share data. The server sends a temporary code back to the client. The client then sends this code along with its secret to the server's token endpoint. If valid, the server returns an access token. The client uses this token to get data from the resource server. This flow protects user data by requiring the client secret and exchanging a short-lived code for a token.