JWT Decoder

This tool allows you to decode JSON Web Tokens (JWT) and inspect their header and payload in a readable format.

Important: This tool only decodes the token. It does not verify the signature and should be used for debugging, learning, and inspection purposes only.

What is a JSON Web Token (JWT)?

A JSON Web Token (JWT) is a compact, URL-safe token format used to securely transfer information between two parties as a JSON object.

JWTs are commonly used in authentication and authorization systems, especially in stateless, distributed applications such as microservices and APIs.

Why JWT Is Widely Used

JWTs enable stateless authentication, meaning the server does not need to store session data.

This makes JWTs ideal for scalable systems where multiple services need to validate user identity without sharing session state.

JWT Structure Explained

A JWT consists of three Base64-encoded parts separated by dots:

• Header – Token type and signing algorithm
• Payload – Claims (user and token data)
• Signature – Ensures token integrity

Example format:

header.payload.signature

JWT Header

The header typically contains:

• alg – Signing algorithm (HS256, RS256, etc.)
• typ – Token type (JWT)

This information tells the system how the token was signed.

JWT Payload (Claims)

The payload contains claims, which are statements about the user or token.

Common standard claims include:

• sub – Subject (user identifier)
• exp – Expiration time
• iat – Issued at
• iss – Issuer

Custom claims are often added to store application-specific data such as roles or permissions.

JWT Signature (Why It Matters)

The signature ensures the token has not been tampered with.

It is created using the header, payload, a secret or private key, and the specified algorithm. Without verifying the signature, a decoded JWT should never be trusted.

Decoding vs Verifying a JWT

Decoding simply converts Base64-encoded data into readable JSON.

Verification checks the signature using a secret or public key. This tool only decodes JWTs and does not perform signature verification.

Common Real-World JWT Use Cases

• User authentication in REST APIs
• Authorization between microservices
• Single Sign-On (SSO)
• Mobile and SPA authentication
• OAuth 2.0 access tokens

JWT in Authentication Flow

A typical JWT authentication flow:

1. User logs in with credentials
2. Server issues a signed JWT
3. Client stores the token (memory or secure storage)
4. Client sends JWT in Authorization header
5. Server verifies token on each request

Token Expiry and Refresh Tokens

JWTs should be short-lived to reduce security risk.

Refresh tokens are often used to obtain new access tokens without forcing users to log in again.

Advantages of JWT

• Stateless and scalable
• Self-contained token
• Language and platform independent
• Works well with microservices

Disadvantages and Limitations

• Token size is larger than session IDs
• Revoking tokens is difficult
• Improper storage can lead to security risks

Because JWTs are stateless, compromised tokens remain valid until they expire unless additional revocation mechanisms are implemented.

Common JWT Security Mistakes

• Storing JWTs in localStorage
• Using long expiration times
• Not validating signature or algorithm
• Including sensitive data in payload

Best Practices When Using JWT

• Always verify signatures on the server
• Use HTTPS exclusively
• Keep tokens short-lived
• Use HttpOnly cookies when possible
• Rotate secrets and keys regularly

Security and Privacy Notice

This JWT decoder does not store, log, or transmit your tokens beyond decoding.

However, JWTs may contain sensitive information. Avoid decoding production tokens in untrusted environments.

Who Should Use This Tool?

• Backend and API developers
• DevOps and platform engineers
• Security engineers
• Students learning authentication systems

If you work with APIs, OAuth, or microservices, this JWT decoder can help you quickly inspect and debug tokens.