Introduction to JWT (JSON Web Tokens)

JSON Web Tokens (JWT) are a compact, URL-safe means of representing claims to be transferred between two parties. They are widely used in mobile app development for secure authentication and information exchange. JWTs are particularly popular due to their simplicity, security, and ease of use.

Structure of a JWT

A JWT consists of three parts, separated by dots (‘.’):

  • Header
  • Payload
  • Signature

Header

The header typically consists of two parts: the type of token (JWT) and the signing algorithm being used, such as HMAC SHA256 or RSA.

{
  "alg": "HS256",
  "typ": "JWT"
}

Payload

The payload contains the claims. Claims are statements about an entity (typically, the user) and additional data. There are three types of claims:

  • Registered claims: Predefined claims which are not mandatory but recommended, such as iss (issuer), exp (expiration time), sub (subject), and aud (audience).
  • Public claims: Custom claims created to share information, which should be defined in the IANA JSON Web Token Registry or be collision-resistant.
  • Private claims: Custom claims created to share information between parties that agree on using them and are not registered or public claims.
{
  "sub": "1234567890",
  "name": "John Doe",
  "admin": true
}

Signature

To create the signature part, you have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that.

HMACSHA256(
  base64UrlEncode(header) + "." +
  base64UrlEncode(payload),
  secret
)

How JWT Works in Mobile App Development

JWTs are commonly used for authentication in mobile apps. Here’s a typical flow:

  • The user logs in with their credentials.
  • The server verifies the credentials and issues a JWT signed with a secret key.
  • The client stores the JWT (usually in local storage or secure storage).
  • For subsequent requests, the client sends the JWT in the HTTP Authorization header.
  • The server verifies the JWT and processes the request if the token is valid.

Advantages of Using JWT

JWTs offer several benefits in mobile app development:

  • Compact: JWTs are compact and can be sent via URL, POST parameter, or inside an HTTP header, making them suitable for mobile devices.
  • Self-contained: JWTs contain all the necessary information about the user, reducing the need for multiple database queries.
  • Secure: JWTs can be signed using a secret or a public/private key pair, ensuring data integrity and authenticity.
  • Stateless: JWTs are stateless, meaning the server does not need to store session information, which simplifies scaling.

Best Practices for Using JWT

To ensure the security and efficiency of JWTs in mobile app development, consider the following best practices:

  • Use HTTPS: Always use HTTPS to prevent token interception.
  • Store tokens securely: Store JWTs in secure storage mechanisms provided by the mobile platform.
  • Set expiration times: Always set an expiration time for JWTs to limit their validity period.
  • Validate tokens: Always validate the token signature and claims on the server side.
  • Use strong secrets: Use strong, random secrets for signing tokens to prevent brute-force attacks.

Conclusion

JWTs are a powerful tool for authentication and information exchange in mobile app development. Their compact size, self-contained nature, and security features make them an excellent choice for modern mobile applications. By following best practices, developers can ensure the secure and efficient use of JWTs in their apps.