JWT for Beginners
1. JWT Overview:
JSON Web Tokens (JWTs) are a compact, URL-safe means of representing claims between two parties. JWTs are often used for authentication, information exchange, and authorization in web development.
2. JWT Structure:
Header (Base64Url encoded JSON):
- Contains metadata about the type of token (JWT) and the signing algorithm used.
- Example:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
Payload (Base64Url encoded JSON):
- Contains claims. Claims are statements about an entity (typically, the user) and additional data.
- Common claims:
iss
(issuer),exp
(expiration time),sub
(subject),aud
(audience), etc. - Example:
eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ
Signature:
- Created by hashing the encoded Header and encoded Payload with a secret key using the specified algorithm.
- Example:
SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
JWT Example:
- Concatenated Header, Payload, and Signature:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0...SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
- Concatenated Header, Payload, and Signature:
3. Creating a JWT:
- Choose a signing algorithm (e.g., HMAC SHA-256) and obtain a secret key.
- Encode Header and Payload into Base64Url format.
- Concatenate encoded Header, a dot (
.
), and encoded Payload. - Sign the concatenated string using the secret key to create the Signature.
- Concatenate the previous string with another dot (
.
) and the Signature to obtain the final JWT.
4. Verifying a JWT:
- Split JWT into Header, Payload, and Signature.
- Base64Url decode the Header and Payload.
- Verify the Signature using the original Header, Payload, and the secret key.
- Check claims in the Payload for validation.
5. Use Cases:
Authentication:
- After successful login, a server issues a JWT to be sent with subsequent requests.
Information Exchange:
- Secure transmission of information between parties, often used in API communication.
Authorization:
- Include user roles and permissions in the Payload to control access.
6. Security Considerations:
- Always use HTTPS to prevent interception of JWTs.
- Safeguard the secret key; never expose it to clients.
- Be cautious about storing sensitive information in the Payload.
7. Best Practices:
- Keep JWTs short-lived; leverage expiration (
exp
) claims. - Consider token revocation mechanisms for added security.