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Inspect token structure and claims quickly during authentication development.
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Paste a JWT string into the decoder input.
Review decoded header and payload sections.
Inspect claim fields such as issuer, audience, and expiry.
Use findings to validate token generation and verification logic.
Auth integration troubleshooting
Token claim validation
Session expiry debugging
API security QA workflows
JWT debugging pages perform best when they separate token inspection from security assumptions. This page is optimized for that practical engineering intent.
Decoding is helpful for visibility, but token trust decisions must still rely on proper signature verification in your backend.
Never treat decoded payload data as trusted unless signature verification and claim validation are completed server-side.
⚠️ Security Notice: All JWT processing happens in your browser. Never paste production tokens or sensitive secrets. This tool is for development and debugging purposes only. Always verify JWT signatures on your backend server.
Everything you need to know about JSON Web Tokens and our powerful decoder tool
A JSON Web Token (JWT) is a compact and secure way to transmit information between two parties. Think of it like a digital ID card that proves who you are without constantly asking for your username and password.
When you log into a website, the server creates a JWT containing your user information (like your user ID and permissions). This token is then sent to your browser, and your browser includes it with every request to prove you're authenticated. It's much faster than checking the database every single time!
Why are JWTs so popular?
A JWT looks like a long random string, but it's actually three parts separated by dots (.). Each part has a specific purpose. Let's break it down:
The header tells us what type of token this is and how it's signed. It's like the envelope that says "This is a JWT, signed with HS256".
The payload is the actual data - your user ID, name, permissions, expiration time, etc. This is where you store the information you want to share. Think of it as the content of a letter.
Important: The payload is NOT encrypted, only encoded! Anyone can decode it and read it. Never put passwords or credit card numbers here!
The signature is like a tamper-proof seal. It's created by combining the header, payload, and a secret key that only the server knows. If someone tries to change even one character in the header or payload, the signature won't match!
Let's walk through a real-world example of how JWTs are used in a web application:
User logs in
You enter your username and password on a login form and click "Sign In".
Server verifies credentials
The server checks if your username and password are correct by looking them up in the database.
Server creates JWT
If credentials are valid, the server creates a JWT with your user info (ID, name, role) and signs it with a secret key.
Token sent to browser
The server sends this JWT back to your browser, which stores it (usually in localStorage or a cookie).
Browser includes token in requests
Every time you make a request (like loading your profile), the browser includes this JWT in the Authorization header.
Server verifies token
The server checks the signature to make sure the token wasn't tampered with, checks if it's expired, and extracts your user info.
Server processes request
If the token is valid, the server processes your request and sends back the data you asked for.
Let's see how to create and verify JWTs in different programming languages. These are real, working code examples you can use in your projects! Our tool provides these code snippets automatically - just click the "Get Code" button after decoding a token.
Install: npm install jsonwebtoken
const jwt = require('jsonwebtoken');
// Create a JWT
const payload = {
userId: 123,
username: 'johndoe',
role: 'user'
};
const secretKey = 'your-secret-key-keep-it-safe';
const token = jwt.sign(payload, secretKey, { expiresIn: '1h' });
console.log('Generated Token:', token);
// Verify a JWT
try {
const decoded = jwt.verify(token, secretKey);
console.log('Decoded:', decoded);
// You can now access: decoded.userId, decoded.username, etc.
} catch (error) {
console.error('Invalid token:', error.message);
}Install: pip install PyJWT
import jwt
import datetime
# Create a JWT
payload = {
'user_id': 123,
'username': 'johndoe',
'role': 'user',
'exp': datetime.datetime.utcnow() + datetime.timedelta(hours=1)
}
secret_key = 'your-secret-key-keep-it-safe'
token = jwt.encode(payload, secret_key, algorithm='HS256')
print(f'Generated Token: {token}')
# Verify a JWT
try:
decoded = jwt.decode(token, secret_key, algorithms=['HS256'])
print(f'Decoded: {decoded}')
# Access: decoded['user_id'], decoded['username'], etc.
except jwt.ExpiredSignatureError:
print('Token has expired')
except jwt.InvalidTokenError:
print('Invalid token')Don't want to write this code manually? Our JWT Decoder has a "Get Code" button that generates ready-to-use code snippets for all 6 languages (Node.js, Python, Java, PHP, Go, C#). Just decode your token and click the green "Get Code" button!
Our JWT Decoder is packed with powerful features to make working with JWTs easy. Let's explore each one:
Just paste your JWT token in the text box and it automatically decodes! You'll see the header, payload, and signature split into three color-coded sections. We show both JSON format and a Claims Table view for easier reading.
Use the dropdown menu to select from 12 different algorithms (HS256, RS256, ES256, etc.). Click "Generate Example" to create a sample token for that algorithm - perfect for learning or testing!
If your token has an expiration time (exp claim), you'll see a live countdown timer showing exactly how much time is left. The color changes from green to yellow to red as expiration approaches - so you never get caught with an expired token!
Click the yellow "Performance" button to see how your token performs. We show decode/encode times, size breakdown (header vs payload vs signature), network impact projections, and compare against industry benchmarks. Learn if your token is too large!
Need to save your decoded token? Click "Export" to download in JSON, CSV, XML, YAML, or even a professional PDF report! Perfect for documentation or sharing with your team.
Got a large token? Click "Optimize" to get smart suggestions on how to reduce its size. We'll show you exactly how many bytes you can save by shortening claim names, removing unnecessary data, or using a different algorithm. You can even preview the optimized token before using it!
Need to compare two tokens side-by-side? Click "Compare" to paste a second token and see exactly what changed. We highlight added, removed, and modified claims in different colors. Super useful for debugging token refresh issues or checking what changed between environments!
Click the red "Security" button to scan your token for vulnerabilities. We check for weak algorithms, expired tokens, missing critical claims, sensitive data in the payload, and more. You'll get a security score from 0-100 plus specific recommendations to improve your token's security.
Click "Get Code" to generate ready-to-use code snippets for verifying your token. Choose from Node.js, Python, Java, PHP, Go, or C#. Each snippet includes installation instructions and is ready to copy-paste into your project!
Need to debug with your team? Click "Share" to generate a shareable link. You can choose to mask sensitive data (like emails or passwords) before sharing. The link never expires and your token is encoded in the URL - no data stored on our servers!
Everyone makes mistakes when learning JWT. Here are the most common ones and how to avoid them:
Problem: Putting passwords, credit card numbers, or API keys directly in the JWT payload.
Solution: Remember, JWTs are encoded, NOT encrypted! Anyone can decode them and read the payload. Only include non-sensitive information like user IDs, usernames, and permissions. Use our Security Analyzer to check for sensitive data!
Problem: Accepting tokens without verifying if they've expired.
Solution: Always check the "exp" claim and reject expired tokens. Our Live Expiration Countdown shows exactly when your token will expire, and the Security Analyzer will warn you about expired tokens!
Problem: Using the "none" algorithm or a weak secret key (like "secret" or "password123").
Solution: Always use a strong algorithm (HS256 minimum) and a long, random secret key (at least 256 bits for HS256). Our Security Analyzer flags weak algorithms and short secrets!
Problem: Including too much data in the payload, resulting in tokens over 1KB or even 4KB+.
Solution: Keep your tokens lean! Only include essential data. Use our Performance Metrics to check token size and get benchmarked against industry standards. If your token is large, click "Optimize" for suggestions on reducing its size.
Problem: Just decoding the JWT without verifying its signature, allowing tampered tokens to be accepted.
Solution: Always verify the signature on the server side! Our Code Generator provides complete examples including signature verification for all 6 programming languages.
Problem: Storing tokens in browser localStorage makes them vulnerable to XSS (Cross-Site Scripting) attacks.
Solution: For maximum security, store JWTs in httpOnly cookies that JavaScript can't access. If you must use localStorage, implement strong Content Security Policy (CSP) headers to prevent XSS attacks.
Now that you understand JWTs, scroll up and paste a token to try our interactive features! Use all the powerful tools we've built - Performance Metrics, Security Analyzer, Size Optimizer, Code Generator, and more. Your tokens are processed locally in your browser for maximum security.