JWT (JSON Web Token) Complete Guide: Structure, Security, and Use Cases

JWT is the most widely used authentication method in modern web applications. But why do we use it? This article covers everything developers need to know, from JWT structure to security best practices.

What is JWT?

JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed.

Structure of JWT

JWT consists of three parts separated by dots (.):

• Header: Token type (JWT) and hashing algorithm (e.g., HMAC SHA256)
• Payload: Actual data called Claims (User ID, Expiration time, etc.)
• Signature: Encrypted string proving the token hasn't been tampered with

1. Header

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

2. Payload

{
  "sub": "1234567890",
  "name": "John Doe",
  "iat": 1516239022
}

When should you use JWT?

• Authentication: Once the user is logged in, each subsequent request will include the JWT. The server doesn't need to maintain sessions, offering great scalability.
• Information Exchange: Since they can be signed using public/private key pairs, you can be sure the senders are who they say they are.

Security Best Practices

1. No Sensitive Data: The payload can be decoded by anyone. Never put passwords or social security numbers in it.
2. Use HTTPS: Always use SSL/TLS to prevent token interception.
3. Short Expiration: Set a short expiration time (exp) and use a Refresh Token strategy to minimize damage if a token is stolen.