JWT structure and flow in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

A JWT has three parts separated by dots: Header, Payload, and Signature.

beginner

What information does the JWT Header contain?

The Header usually contains the token type (JWT) and the signing algorithm used, like HS256 or RS256.

beginner

What is stored in the Payload of a JWT?

The Payload contains claims or data about the user and token, like user ID, expiration time, and roles.

intermediate

What is the purpose of the Signature in a JWT?

The Signature ensures the token is not altered. It is created by signing the Header and Payload with a secret or private key.

intermediate

Describe the typical flow of JWT in a REST API authentication.

1. User logs in and server creates a JWT.
2. Server sends JWT to user.
3. User sends JWT with each request.
4. Server verifies JWT signature and data.
5. If valid, server processes request; if not, rejects it.

Which part of the JWT contains the user's data like user ID?

A Payload

B Header

C Signature

D Secret

The Payload holds the user's data and claims.

What does the Signature in a JWT protect against?

A User authentication

B Token expiration

C Data tampering

D Token size

The Signature ensures the token has not been changed.

When does the server verify the JWT?

A With each request containing the token

B When the token is created

C When the user logs in

D Only once per session

The server checks the JWT on every request to confirm validity.

What is the format of a JWT?

A JSON object

B Base64 encoded string with three parts separated by dots

C XML document

D Plain text string

JWT is a Base64 encoded string with Header, Payload, and Signature separated by dots.

Which algorithm might be specified in the JWT Header?

A SHA256

B MD5

C AES

D HS256

HS256 is a common signing algorithm used in JWT headers.

Explain the structure of a JWT and what each part does.

Think about how the token is built and why each part is important.

Describe the flow of JWT in a REST API from login to request validation.

Imagine how a user proves who they are using the token.

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