JWT structure (header, payload, signature) in Spring Boot - Mini Project: Build & Apply

JWT structure (header, payload, signature)

📖 Scenario: You are building a simple Spring Boot application that handles JSON Web Tokens (JWT). JWTs have three parts: header, payload, and signature. Understanding how to create and structure these parts is important for secure token handling.

🎯 Goal: Build a basic Spring Boot class that creates a JWT token by defining the header, payload, and signature parts as strings.

📋 What You'll Learn

Create a Map for the JWT header with algorithm and type

Create a Map for the JWT payload with user information

Create a String for the JWT signature placeholder

Combine these parts into a JWT token string separated by dots

💡 Why This Matters

🌍 Real World

JWTs are widely used for secure user authentication and data exchange in web applications.

💼 Career

Understanding JWT structure is essential for backend developers working with authentication and authorization in modern web services.

1

Create JWT header Map

Create a Map called header with these exact entries: "alg" set to "HS256" and "typ" set to "JWT".

Spring Boot

// Your code here

Hint

Use a HashMap to store the header keys and values.

2

Create JWT payload Map

Create a Map called payload with these exact entries: "sub" set to "1234567890", "name" set to "John Doe", and "admin" set to true.

Spring Boot

Map header = new HashMap();
header.put("alg", "HS256");
header.put("typ", "JWT");
// Your code here

Hint

Use a HashMap to store the payload keys and values. Note the value types: String and boolean.

3

Create JWT signature string

Create a String variable called signature and set it to the exact value "signature-placeholder".

Spring Boot

Map header = new HashMap();
header.put("alg", "HS256");
header.put("typ", "JWT");

Map payload = new HashMap();
payload.put("sub", "1234567890");
payload.put("name", "John Doe");
payload.put("admin", true);
// Your code here

Hint

Just assign the exact string to the signature variable.

4

Combine header, payload, and signature into JWT token

Create a String variable called jwtToken that combines the strings "header", "payload", and "signature" separated by dots ("."). Use the exact code: jwtToken = "header.payload.signature";

Spring Boot

Map header = new HashMap();
header.put("alg", "HS256");
header.put("typ", "JWT");

Map payload = new HashMap();
payload.put("sub", "1234567890");
payload.put("name", "John Doe");
payload.put("admin", true);

String signature = "signature-placeholder";
// Your code here

Hint

Combine the three parts as a single string separated by dots.

Practice

1. Which part of a JWT contains information about the algorithm used for signing the token?

easy

A. Payload

B. Header

C. Signature

D. Issuer

Solution

1. Step 1: Understand JWT parts

   A JWT has three parts: header, payload, and signature.

2. Step 2: Identify algorithm info location

   The header contains metadata including the signing algorithm used.

3. Final Answer:

   Header -> Option B

4. Quick Check:

   Algorithm info = Header [OK]

Hint: Algorithm info is always in the JWT header [OK]

Common Mistakes:

• Confusing payload with header
• Thinking signature contains algorithm info
• Assuming issuer is a JWT part

2. Which of the following correctly represents the order of parts in a JWT string?

easy

A. Header.Payload.Signature

B. Signature.Payload.Header

C. Payload.Header.Signature

D. Header.Signature.Payload

Solution

1. Step 1: Recall JWT format

   A JWT is a string with three parts separated by dots.

2. Step 2: Confirm correct order

   The order is header first, then payload, then signature.

3. Final Answer:

   Header.Payload.Signature -> Option A

4. Quick Check:

   JWT order = Header.Payload.Signature [OK]

Hint: JWT parts order: header, payload, then signature [OK]

Common Mistakes:

• Mixing up header and payload order
• Placing signature in the middle
• Assuming signature comes first

3. Given this JWT string: eyJhbGciOiJIUzI1NiJ9.eyJ1c2VyIjoiam9obiJ9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c, what does the middle part represent?

medium

A. Algorithm type

B. Encoded header

C. Signature hash

D. Encoded payload

Solution

1. Step 1: Identify JWT parts by position

   The JWT has three parts separated by dots: header.payload.signature.

2. Step 2: Locate the middle part

   The middle part is the payload, which contains user data encoded in Base64Url.

3. Final Answer:

   Encoded payload -> Option D

4. Quick Check:

   Middle JWT part = Payload [OK]

Hint: Middle JWT part is always the payload [OK]

Common Mistakes:

• Confusing payload with header
• Thinking signature is in the middle
• Assuming algorithm is separate part

4. You receive a JWT but the signature part is missing. What issue will this cause?

medium

A. The token will expire immediately

B. The payload will be unreadable

C. The token cannot be verified for authenticity

D. The header will be invalid JSON

Solution

1. Step 1: Understand the role of signature

   The signature proves the token is authentic and unchanged.

2. Step 2: Consequence of missing signature

   Without the signature, the token cannot be verified and may be tampered with.

3. Final Answer:

   The token cannot be verified for authenticity -> Option C

4. Quick Check:

   Missing signature = No verification [OK]

Hint: Signature missing means no token verification possible [OK]

Common Mistakes:

• Thinking payload becomes unreadable
• Assuming header JSON breaks
• Believing token expires immediately

5. In a Spring Boot application, you want to verify a JWT token. Which sequence correctly describes the verification steps?

hard

A. Decode header and payload, then verify signature using secret key

B. Verify signature first, then decode payload and header

C. Decode signature, then verify payload and header

D. Decode payload only, signature is not needed for verification

Solution

1. Step 1: Decode header and payload

   First, decode the header and payload from Base64Url to read their contents.

2. Step 2: Verify signature using secret key

   Use the secret key and header info to verify the signature matches the token data.

3. Final Answer:

   Decode header and payload, then verify signature using secret key -> Option A

4. Quick Check:

   Decode then verify signature = Correct process [OK]

Hint: Always decode first, then verify signature with secret [OK]

Common Mistakes:

• Trying to verify signature before decoding
• Ignoring signature verification
• Decoding signature as if it contains data