Why OAuth 2.0 flow in HLD? - Purpose & Use Cases

The Big Idea

What if you could share your house keys without ever giving out your real keys?

The Scenario

Imagine you want to let your friend borrow your house keys to water your plants while you're away. You give them a copy of your house key directly. Now, they can enter your house anytime, even when you don't want them to.

The Problem

Giving out your main key (like your password) to every app or service is risky. It's slow to manage, easy to lose control, and if someone misuses it, your whole account is at risk. You have to change your password everywhere if something goes wrong.

The Solution

OAuth 2.0 flow acts like a special guest pass system. Instead of sharing your main key, you give apps a limited-time, limited-access pass. This pass lets them do only what you allow, and you can revoke it anytime without changing your main password.

Before vs After

✗ Before

username = input('Enter username')
password = input('Enter password')
login(username, password)

✓ After

access_token = get_oauth_token()
use_token(access_token)

What It Enables

OAuth 2.0 flow enables secure, controlled, and easy sharing of your account access without exposing your password.

Real Life Example

When you sign into a new app using your Google or Facebook account, OAuth 2.0 flow is working behind the scenes to safely share your identity without giving away your password.

Key Takeaways

Manual sharing of passwords is risky and hard to manage.

OAuth 2.0 flow provides limited, revocable access tokens instead of passwords.

This makes app integrations safer and easier for users and developers.

Practice

1. What is the main purpose of the OAuth 2.0 Authorization Code flow?

easy

A. To replace user authentication with biometric data

B. To store user passwords safely on the server

C. To allow apps to access user data securely without sharing passwords

D. To encrypt all user data before sending it

Solution

1. Step 1: Understand OAuth 2.0 Authorization Code flow

   This flow lets apps get permission to access user data without needing the user's password directly.

2. Step 2: Identify the main goal

   The goal is to keep user credentials safe while allowing apps to act on behalf of the user.

3. Final Answer:

   To allow apps to access user data securely without sharing passwords -> Option C

4. Quick Check:

   OAuth 2.0 Authorization Code flow = Secure app access without passwords [OK]

Hint: Remember: OAuth avoids sharing passwords directly [OK]

Common Mistakes:

• Confusing OAuth with password storage
• Thinking OAuth encrypts all user data
• Assuming OAuth replaces authentication methods

2. Which of the following is the correct sequence in the OAuth 2.0 Authorization Code flow?

easy

A. User logs in -> App receives access token -> App requests authorization code

B. App sends password -> User grants access -> App receives token

C. App directly receives access token -> User logs in -> Access granted

D. User logs in -> App gets authorization code -> App exchanges code for access token

Solution

1. Step 1: Recall the OAuth 2.0 Authorization Code flow steps

   User logs in and authorizes the app, which then receives an authorization code.

2. Step 2: Understand token exchange

   The app exchanges the authorization code for an access token to access user data.

3. Final Answer:

   User logs in -> App gets authorization code -> App exchanges code for access token -> Option D

4. Quick Check:

   Authorization Code flow sequence = Login, code, token [OK]

Hint: Authorization code comes before access token [OK]

Common Mistakes:

• Thinking app gets token before authorization code
• Confusing password sending with OAuth flow
• Mixing order of login and token receipt

3. Given this OAuth 2.0 flow snippet, what is the expected output after exchanging the authorization code?

POST /token
Headers: Content-Type: application/x-www-form-urlencoded
Body: grant_type=authorization_code&code=abc123&redirect_uri=https://app.com/callback&client_id=client123&client_secret=secretXYZ

Response:
{
  "access_token": "token456",
  "token_type": "Bearer",
  "expires_in": 3600
}

medium

A. An access token 'token456' valid for 3600 seconds

B. An error because client_secret is missing

C. A refresh token instead of access token

D. No token, only a confirmation message

Solution

1. Step 1: Analyze the token request

   The app sends a POST request with grant_type authorization_code and includes the code and client credentials.

2. Step 2: Interpret the response

   The response contains an access_token 'token456', token_type 'Bearer', and expires_in 3600 seconds, meaning the token is valid for one hour.

3. Final Answer:

   An access token 'token456' valid for 3600 seconds -> Option A

4. Quick Check:

   Token response includes access_token and expiry [OK]

Hint: Token response includes access_token and expiry time [OK]

Common Mistakes:

• Expecting refresh token instead of access token
• Missing client_secret causes error
• Confusing confirmation message with token response

4. A developer implements OAuth 2.0 Authorization Code flow but the app never receives an access token after exchanging the code. What is the most likely cause?

medium

A. The app forgot to include the client_id in the authorization request

B. The redirect URI used in the token request does not match the one registered

C. The user did not enter their password correctly

D. The access token expired before the request

Solution

1. Step 1: Identify common OAuth token exchange errors

   One frequent error is a mismatch in redirect URI between authorization and token requests.

2. Step 2: Understand impact of redirect URI mismatch

   If the redirect URI differs, the authorization server rejects the token request, so no access token is returned.

3. Final Answer:

   The redirect URI used in the token request does not match the one registered -> Option B

4. Quick Check:

   Redirect URI mismatch blocks token issuance [OK]

Hint: Check redirect URI matches exactly in all requests [OK]

Common Mistakes:

• Blaming user password errors for token issues
• Ignoring client_id in authorization request
• Assuming token expires before it is issued

5. You are designing a system that uses OAuth 2.0 Authorization Code flow for a mobile app and a web app. Which approach best secures the access tokens and refresh tokens?

hard

A. Use Authorization Code flow with PKCE for the mobile app and standard Authorization Code flow for the web app

B. Use Implicit flow for both apps to simplify token handling

C. Store access tokens in local storage for both apps to allow easy access

D. Send access tokens directly in URL query parameters for both apps

Solution

1. Step 1: Understand security needs for mobile and web apps

   Mobile apps cannot keep client secrets safe, so PKCE (Proof Key for Code Exchange) enhances security for Authorization Code flow.

2. Step 2: Choose best practices for token storage and flow

   Web apps can use standard Authorization Code flow with client secret. Implicit flow is deprecated due to security risks. Storing tokens in local storage or URLs exposes them to attacks.

3. Final Answer:

   Use Authorization Code flow with PKCE for the mobile app and standard Authorization Code flow for the web app -> Option A

4. Quick Check:

   PKCE secures mobile OAuth; standard flow suits web apps [OK]

Hint: Use PKCE for mobile apps, standard flow for web apps [OK]

Common Mistakes:

• Using Implicit flow despite security risks
• Storing tokens insecurely in local storage
• Passing tokens in URL parameters