API authentication (OAuth, JWT, API keys) in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of API authentication?

API authentication ensures that only authorized users or systems can access the API, protecting data and services from unauthorized use.

beginner

Explain OAuth in simple terms.

OAuth is a way to let users give permission to apps to access their data without sharing passwords. It uses tokens to grant limited access.

intermediate

What is a JWT and what does it contain?

JWT (JSON Web Token) is a compact token that contains encoded information like user identity and permissions, signed to prevent tampering.

intermediate

How do API keys differ from OAuth tokens?

API keys are simple static strings used to identify the client, while OAuth tokens are dynamic, time-limited, and can carry user permissions.

beginner

Why is it important to use HTTPS with API authentication?

HTTPS encrypts data between client and server, protecting sensitive authentication tokens and credentials from being stolen by attackers.

Which of the following is a key feature of OAuth?

A Allows users to share passwords with third-party apps

B Uses tokens to grant limited access without sharing passwords

C Is a static API key used for client identification

D Encrypts data between client and server

OAuth uses tokens to let apps access user data without sharing passwords.

What does a JWT typically include?

A User credentials in plain text

B Encrypted user password

C Encoded user identity and permissions with a signature

D A static API key

JWT contains encoded user info and permissions, signed to prevent tampering.

API keys are best described as:

A Static strings identifying the client

B Dynamic tokens with expiration

C Encrypted user passwords

D OAuth authorization codes

API keys are static strings used to identify clients.

Why should API authentication always use HTTPS?

A To speed up API responses

B To store tokens on the client

C To allow anonymous access

D To encrypt authentication data and prevent interception

HTTPS encrypts data, protecting tokens and credentials from attackers.

Which authentication method allows users to grant limited access to their data without sharing passwords?

A OAuth

B Basic Authentication

C API keys

D JWT

OAuth lets users grant limited access using tokens without sharing passwords.

Describe how OAuth works to authenticate API requests.

Think about how apps ask for permission to use your data.

Explain the differences between API keys, JWT, and OAuth tokens.

Consider how each method handles security and access control.

Practice

1. Which of the following is the primary purpose of API authentication?

easy

A. To verify the identity of the client accessing the API

B. To encrypt the data sent between client and server

C. To improve the speed of API responses

D. To log all API requests for analytics

Solution

1. Step 1: Understand the role of authentication

   Authentication is about confirming who is making the request, not about encryption or speed.

2. Step 2: Identify the correct purpose

   Verifying the client's identity ensures only authorized users access the API.

3. Final Answer:

   To verify the identity of the client accessing the API -> Option A

4. Quick Check:

   Authentication = Verify identity [OK]

Hint: Authentication means confirming who you are [OK]

Common Mistakes:

• Confusing authentication with encryption
• Thinking authentication speeds up API
• Mixing authentication with logging

2. Which of the following is the correct format of a JWT token?

easy

A. header.payload.signature

B. username:password

C. api_key=12345

D. token|timestamp|user

Solution

1. Step 1: Recall JWT structure

   A JWT token consists of three parts separated by dots: header, payload, and signature.

2. Step 2: Match the correct format

   Only the format with three dot-separated parts matches JWT.

3. Final Answer:

   header.payload.signature -> Option A

4. Quick Check:

   JWT = header.payload.signature [OK]

Hint: JWT always has three parts separated by dots [OK]

Common Mistakes:

• Confusing JWT with basic auth format
• Using equal signs instead of dots
• Mixing API key format with JWT

3. Given this OAuth flow: A client requests an access token using a refresh token. Which response indicates a successful token refresh?

medium

A. {"refresh_token": "xyz789"}

B. {"error": "invalid_grant"}

C. {"access_token": null}

D. {"access_token": "abc123", "expires_in": 3600}

Solution

1. Step 1: Understand OAuth token refresh response

   A successful refresh returns a new access token and expiry time.

2. Step 2: Identify the success response

   {"access_token": "abc123", "expires_in": 3600} contains a valid access token and expiry, indicating success.

3. Final Answer:

   {"access_token": "abc123", "expires_in": 3600} -> Option D

4. Quick Check:

   Success = new access token returned [OK]

Hint: Success returns new access token, not error or null [OK]

Common Mistakes:

• Confusing error response with success
• Expecting refresh token in response
• Assuming null access token means success

4. A developer uses an API key in the URL query string for authentication but notices security warnings. What is the main issue?

medium

A. API keys cannot be used for authentication

B. API keys must be encrypted with RSA

C. API keys in URLs can be logged or leaked easily

D. API keys expire too quickly

Solution

1. Step 1: Analyze API key usage in URLs

   URLs are often logged by servers, browsers, and proxies, risking key exposure.

2. Step 2: Identify the security risk

   Embedding API keys in URLs can leak them unintentionally, causing security warnings.

3. Final Answer:

   API keys in URLs can be logged or leaked easily -> Option C

4. Quick Check:

   API key in URL = risk of leakage [OK]

Hint: Never put API keys in URLs to avoid leaks [OK]

Common Mistakes:

• Thinking API keys must be encrypted always
• Believing API keys can't authenticate
• Assuming keys expire too fast

5. You design an API that uses JWT for authentication. To improve security, which combination is best practice?

hard

A. Store JWTs in localStorage and never expire them

B. Use short-lived JWTs with HTTPS and validate signature on each request

C. Send JWTs in URL query parameters for easy access

D. Use JWTs without signature to reduce overhead

Solution

1. Step 1: Identify secure JWT handling practices

   Short-lived tokens reduce risk if stolen; HTTPS protects data in transit; signature validation ensures token integrity.

2. Step 2: Evaluate options for security

   Use short-lived JWTs with HTTPS and validate signature on each request follows all best practices; others expose security risks like token theft or tampering.

3. Final Answer:

   Use short-lived JWTs with HTTPS and validate signature on each request -> Option B

4. Quick Check:

   Secure JWT = short life + HTTPS + signature check [OK]

Hint: Short life + HTTPS + signature check = secure JWT [OK]

Common Mistakes:

• Storing JWTs insecurely in localStorage
• Sending tokens in URLs risking leaks
• Skipping signature validation