JWT structure and flow in Rest API - Mini Project: Build & Apply

Understanding JWT Structure and Flow

📖 Scenario: You are building a simple REST API that uses JSON Web Tokens (JWT) to manage user authentication. JWTs are small pieces of data that securely transmit information between a client and a server.In this project, you will create a basic JWT structure, configure a secret key, generate a token with user data, and finally display the token to understand how JWTs work.

🎯 Goal: Build a simple program that creates a JWT token with user information and a secret key, then prints the token. This will help you understand the JWT structure and flow in a REST API context.

📋 What You'll Learn

Create a dictionary with user data

Define a secret key string

Generate a JWT token using the user data and secret key

Print the generated JWT token

💡 Why This Matters

🌍 Real World

JWTs are widely used in web applications to securely transmit user identity and permissions between clients and servers without storing session data on the server.

💼 Career

Understanding JWT structure and flow is essential for backend developers, API developers, and security engineers working on authentication and authorization systems.

1

Create user data dictionary

Create a dictionary called user_data with these exact entries: 'user_id': 101, 'username': 'alice', and 'role': 'admin'.

Rest API

# Create the user_data dictionary with user_id, username, and role
# Your code here

Hint

Use curly braces {} to create a dictionary with keys and values.

2

Define the secret key

Create a string variable called secret_key and set it to 'mysecret123'.

Rest API

user_data = {'user_id': 101, 'username': 'alice', 'role': 'admin'}
# Define the secret_key string
# Your code here

Hint

Assign the exact string 'mysecret123' to the variable secret_key.

3

Generate the JWT token

Import the jwt module and use jwt.encode() to create a token called token using user_data and secret_key. Use algorithm='HS256' as the encoding algorithm.

Rest API

user_data = {'user_id': 101, 'username': 'alice', 'role': 'admin'}
secret_key = 'mysecret123'
# Import jwt and generate the token using jwt.encode
# Your code here

Hint

Use import jwt at the top. Then call jwt.encode() with the user data, secret key, and algorithm.

4

Print the JWT token

Write a print() statement to display the token variable.

Rest API

import jwt

user_data = {'user_id': 101, 'username': 'alice', 'role': 'admin'}
secret_key = 'mysecret123'

token = jwt.encode(user_data, secret_key, algorithm='HS256')
# Print the token
# Your code here

Hint

Use print(token) to show the JWT token string.

Practice

1. What are the three main parts of a JWT (JSON Web Token)?

easy

A. Header, Payload, Signature

B. Username, Password, Token

C. Request, Response, Token

D. Key, Value, Token

Solution

1. Step 1: Understand JWT structure basics

   A JWT is made of three parts separated by dots.

2. Step 2: Identify the parts

   The three parts are Header (metadata), Payload (claims), and Signature (verification).

3. Final Answer:

   Header, Payload, Signature -> Option A

4. Quick Check:

   JWT parts = Header, Payload, Signature [OK]

Hint: Remember JWT has 3 parts separated by dots [OK]

Common Mistakes:

• Confusing JWT parts with user credentials
• Thinking JWT has only two parts
• Mixing up token with request/response

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

easy

A. header|payload|signature

B. header-payload-signature

C. header.payload.signature

D. header_payload_signature

Solution

1. Step 1: Recall JWT encoding format

   JWT parts are base64url encoded and joined by dots.

2. Step 2: Identify correct separator

   The correct separator between parts is a dot ('.').

3. Final Answer:

   header.payload.signature -> Option C

4. Quick Check:

   JWT format uses dots '.' [OK]

Hint: JWT parts are joined by dots '.' [OK]

Common Mistakes:

• Using dashes or underscores instead of dots
• Confusing with other token formats
• Not encoding parts properly

3. Given this JWT payload: {"sub":"1234567890","name":"John Doe","iat":1516239022}, what does the iat field represent?

medium

A. Issuer of the token

B. Issued at time

C. Expiration time

D. Subject identifier

Solution

1. Step 1: Understand JWT standard claims

   Common claims include 'sub' (subject), 'iat' (issued at), 'exp' (expiration), and 'iss' (issuer).

2. Step 2: Identify meaning of 'iat'

   'iat' stands for 'issued at' and marks the time the token was created.

3. Final Answer:

   Issued at time -> Option B

4. Quick Check:

   'iat' = issued at time [OK]

Hint: 'iat' means when token was issued [OK]

Common Mistakes:

• Confusing 'iat' with expiration time
• Mixing 'sub' and 'iss' claims
• Assuming 'iat' is issuer

4. You receive a JWT but the signature verification fails. What is the most likely cause?

medium

A. The secret key used to sign the token is different

B. The token payload is empty

C. The header is missing

D. The token is not base64 encoded

Solution

1. Step 1: Understand signature verification

   The signature is created using a secret key and the header and payload.

2. Step 2: Identify cause of verification failure

   If the secret key used to verify differs from the signing key, verification fails.

3. Final Answer:

   The secret key used to sign the token is different -> Option A

4. Quick Check:

   Signature fails if secret keys differ [OK]

Hint: Signature fails if secret keys don't match [OK]

Common Mistakes:

• Assuming empty payload causes signature failure
• Thinking missing header always breaks signature
• Confusing encoding with signature verification

5. In a REST API, after a user logs in, the server issues a JWT. Which step correctly describes the flow for authenticating future requests using this JWT?

hard

A. Client sends JWT in URL query; server ignores signature and trusts token

B. Client sends username and password with every request; server creates new JWT each time

C. Server stores JWT in database and checks it on each request

D. Client sends JWT in Authorization header; server verifies signature and extracts user info

Solution

1. Step 1: Understand JWT usage in REST API

   After login, server issues JWT to client to prove identity without resending credentials.

2. Step 2: Identify correct authentication flow

   Client sends JWT in Authorization header; server verifies signature and extracts user info to authenticate.

3. Final Answer:

   Client sends JWT in Authorization header; server verifies signature and extracts user info -> Option D

4. Quick Check:

   JWT sent in header and verified by server [OK]

Hint: JWT goes in Authorization header, server verifies signature [OK]

Common Mistakes:

• Sending credentials every request instead of JWT
• Storing JWT server-side defeats statelessness
• Ignoring signature verification risks security