JWT Decoder & Signature Validator
Decode JSON Web Tokens and validate HS256 signatures instantly. 100% client-side, secure, and private.
JWT Token
Secret Key (Optional)
Provide a secret key to verify HS256 signatures. Leave empty to skip verification.
Paste a JWT token above to decode and analyze it
What is a JWT (JSON Web Token)? Complete Guide
JSON Web Tokens (JWT) are an industry-standard method (RFC 7519) for securely representing claims between two parties. Unlike traditional session-based authentication where the server stores user state, JWTs enable stateless authentication—the token itself contains all necessary information to verify a user's identity and permissions.
A JWT is a compact, URL-safe string consisting of three Base64url-encoded sections separated by dots: header.payload.signature. This structure allows JWTs to be easily transmitted via HTTP headers, URL parameters, or POST request bodies, making them ideal for modern web applications, mobile apps, and microservices architectures.
How JWT Authentication Works: When a user successfully logs in, the authentication server generates a JWT containing the user's ID, roles, and permissions. This token is sent to the client, which stores it (typically in memory or httpOnly cookies). For subsequent requests, the client includes the JWT in the Authorization header. The server validates the signature and checks claims like expiration before granting access—no database lookup required.
Critical Security Note: JWTs are encoded, NOT encrypted. Anyone can decode the header and payload to read their contents. Never store passwords, API keys, credit card numbers, or other sensitive data in JWT payloads. Only the signature provides security by preventing tampering. Always transmit JWTs over HTTPS to prevent interception, and implement proper expiration times to limit the damage if a token is compromised.
Understanding JWT Structure: Header, Payload, and Signature
Header
Contains metadata about the token: token type (always "JWT") and the cryptographic algorithm used for signing (e.g., HS256, RS256, ES256).
{"alg": "HS256", "typ": "JWT"}Payload
Contains claims—statements about the user and additional metadata. Common claims include sub (subject/user ID), exp (expiration), iat (issued at), and custom claims like roles or permissions.
{"sub": "user123", "exp": 1735689600}Signature
Ensures token integrity and authenticity. Created by signing the encoded header and payload with a secret key. Verification proves the token hasn't been tampered with.
HMACSHA256(base64(header) + "." + base64(payload), secret)JWT vs. Session-Based Authentication: Which Should You Use?
JWT Advantages
- ✓Stateless: No server-side session storage required—scales horizontally easily
- ✓Cross-domain: Works seamlessly across different domains and services
- ✓Mobile-friendly: Perfect for mobile apps and SPAs—no cookies needed
- ✓Microservices: Ideal for distributed systems and API gateways
- ✓Performance: No database lookups for authentication validation
Session Advantages
- ✓Instant revocation: Can immediately invalidate sessions server-side
- ✓Smaller payload: Only session ID transmitted—reduces bandwidth
- ✓Sensitive data: Keep all user data server-side for better security
- ✓Traditional auth: Well-established pattern with mature tooling
- ✓httpOnly cookies: Better protection against XSS attacks
Recommendation: Use JWTs for stateless APIs, microservices, mobile applications, and cross-domain authentication. Use sessions for traditional web applications where you need instant revocation and when most traffic is same-domain. Many modern applications use a hybrid approach: short-lived JWTs (15 minutes) paired with refresh tokens stored server-side.
Real-World JWT Use Cases and Applications
API Authentication & Authorization
REST and GraphQL APIs use JWTs in Authorization headers to authenticate requests and control access to resources. JWTs can contain user roles and permissions, enabling fine-grained authorization without database queries. Perfect for microservices where each service validates tokens independently using a shared secret or public key.
Single Sign-On (SSO)
JWTs enable SSO across multiple applications and domains. After authenticating once with an identity provider (like Auth0, Okta, or Azure AD), users receive a JWT that grants access to all integrated applications without additional logins. Common in enterprise environments and SaaS platforms.
Mobile App Authentication
Mobile applications (iOS, Android, React Native) use JWTs because they don't rely on cookies. After login, apps store the JWT securely (iOS Keychain, Android Keystore) and include it in API requests. Refresh tokens enable long-lived sessions without storing passwords.
Information Exchange
JWTs securely transmit information between parties. For example, password reset links can include a JWT containing the user ID and expiration time. Email verification links use JWTs to confirm email ownership. The signature ensures the data hasn't been tampered with during transmission.
Microservices Architecture
In distributed systems, JWTs allow services to verify requests without calling a central authentication service. An API gateway issues tokens, and downstream services validate them using shared secrets (HS256) or public keys (RS256). This reduces latency and eliminates single points of failure.
Serverless & Edge Computing
Serverless functions (AWS Lambda, Cloudflare Workers) and edge computing benefit from stateless JWT authentication. No need for session storage—functions validate tokens and execute. Ideal for globally distributed applications where session synchronization would be complex and slow.
How to Use the JWT Decoder & Signature Validator
Paste Your JWT Token
Copy your JWT token and paste it into the JWT Token input field. The token will be automatically decoded as you type.
View Decoded Information
Instantly see the decoded Header and Payload in formatted JSON. Check token information like algorithm, expiration, and claims.
Add Secret Key (Optional)
To verify the signature, enter the secret key that was used to sign the token. Only HS256 algorithm is supported for verification.
Check Signature Status
View the signature verification result. The tool will show if the signature is valid, invalid, or not verified.
JWT Security Best Practices for Production
🔒Use Strong Secret Keys and Algorithms
For HS256, use randomly generated secrets at least 256 bits (32 characters) long. Never use predictable secrets like "secret" or "password123". Consider using RS256 for better key management in distributed systems. Always reject tokens with algorithm "none".
⏰Implement Short Expiration Times
Set JWT expiration (exp claim) to 15-30 minutes for access tokens. Use refresh tokens (stored server-side) for extended sessions. Short-lived tokens limit damage if compromised— attackers have a small window before tokens expire.
🛡️Validate All Claims Server-Side
Never trust client-side validation. Server must verify: signature is valid, token hasn't expired (exp), token isn't used before valid time (nbf), issuer (iss) matches expected value, audience (aud) matches your application, and algorithm matches expected algorithm.
🍪Store Tokens Securely
Never store JWTs in localStorage (vulnerable to XSS). Use httpOnly, secure, SameSite cookies for web apps. For SPAs, store in memory with proper refresh token rotation. Mobile apps should use platform secure storage (iOS Keychain, Android Keystore).
🔄Implement Token Revocation Strategy
Since JWTs are stateless, implement token blacklisting for logout and compromised tokens. Maintain a revocation list with expired token IDs, or use short expiration times so tokens naturally expire quickly. Include a token version (jti claim) that can be invalidated.
🌐Always Use HTTPS
Transmit JWTs only over HTTPS/TLS encrypted connections. Man-in-the-middle attacks can intercept tokens sent over HTTP. Use HSTS headers to enforce HTTPS. For mobile apps, implement certificate pinning for additional security.
JWT Frequently Asked Questions
Expert answers to common questions about JWT decoding, verification, and security