JWT Decoder & Parser

JWT Decoder - Complete Guide

What is a JWT Token?

JSON Web Token (JWT) is an open standard (RFC 7519) for securely transmitting information between parties as a JSON object. JWTs are commonly used for authentication and information exchange in web applications and APIs. A JWT consists of three parts separated by dots: Header.Payload.Signature. Each part is Base64URL encoded, making the token URL-safe and easy to transmit.

How to Decode JWT Online

Our online JWT decoder allows you to decode and parse JSON Web Tokens instantly without any setup or installation. Simply paste your JWT token (the long string starting with "eyJ") into the decoder, and it will automatically parse the header, payload, and signature. The decoder validates the token structure and displays all claims, including user information, expiration time, issued time, and algorithm type. All decoding happens client-side in your browser for maximum security.

JWT Structure Explained

Header: Contains the token type (typ) and signing algorithm (alg) like HS256, RS256, or ES256. The header tells systems how to verify the token signature.

Payload: Contains the claims - statements about the user and additional data. Standard claims include "sub" (subject), "iss" (issuer), "exp" (expiration), "iat" (issued at), and "aud" (audience). Custom claims can also be added.

Signature: Created by encoding the header and payload, then signing with a secret key or private key. The signature ensures the token hasn't been tampered with.

Checking JWT Expiration

JWT tokens include an "exp" (expiration) claim in the payload that specifies when the token expires. Our decoder automatically checks if a token is expired by comparing the exp claim with the current time. Expired tokens are marked with a clear indicator showing the exact expiration date and time. The decoder also shows the "iat" (issued at) claim to see when the token was originally created. This is essential for debugging authentication issues and ensuring tokens are valid before making API requests.

Common JWT Algorithms

JWTs support multiple signing algorithms:
HS256 (HMAC SHA-256): Symmetric algorithm using a shared secret key. Fast and simple but requires secure key distribution.
RS256 (RSA SHA-256): Asymmetric algorithm using public/private key pairs. More secure for distributed systems where multiple services need to verify tokens.
ES256 (ECDSA SHA-256): Elliptic curve digital signature algorithm. Provides strong security with smaller key sizes and faster operations.

JWT Security Best Practices

When working with JWT tokens, never share them publicly or commit them to version control. Tokens often contain sensitive user information and can grant access to your applications. Always use HTTPS to transmit tokens to prevent interception. Implement proper token expiration (exp claim) with reasonable timeframes - typically 15 minutes to 1 hour for access tokens. Use refresh tokens for extended sessions. Validate the token signature server-side before trusting any claims. Our client-side decoder is safe for development and debugging since tokens never leave your browser.

Use Cases for JWT Decoder

Development & Debugging: Quickly inspect tokens during development to verify claims, check expiration, and debug authentication issues.
API Testing: Decode tokens from API responses to verify the server is sending correct user information and permissions.
Security Auditing: Audit JWT implementations to ensure proper algorithm selection, claim structure, and expiration policies.
Learning: Understand how JWT authentication works by examining real tokens and their structure.

Free JWT Decoder Tool

Swapcode's JWT decoder is completely free with unlimited use and no registration required. Unlike other tools, our decoder runs entirely in your browser using JavaScript, ensuring your tokens remain completely private. No tokens are sent to servers, logged, or stored. The tool supports all standard JWT algorithms (HS256, RS256, ES256, PS256, etc.) and automatically validates token structure. It's perfect for developers, security researchers, and anyone working with JWT-based authentication systems.