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JWT Token Decoder & Validator - Complete Guide

Decode, validate, and analyze JWT tokens with detailed header and payload inspection

Comprehensive Tutorial
15-20 min read
Professional Guide

📋 Table of Contents

Complete JWT Decoder Guide: Token Security & Validation

What are JWTs and Why Decode Them?

JSON Web Tokens (JWTs) are compact, URL-safe tokens used for securely transmitting information between parties. Our JWT Decoder provides essential tools for developers to decode, validate, and debug JWT tokens in authentication systems.

Why Our JWT Decoder is Essential:

  • Security Analysis: Inspect token contents and validate signatures
  • Debugging Tools: Identify authentication and authorization issues
  • Token Validation: Verify token integrity and expiration
  • Claims Inspection: Examine user permissions and metadata
  • Multi-Algorithm Support: RSA, ECDSA, HMAC signature verification
  • Real-time Decoding: Instant token analysis as you paste

JWT Structure and Components

JWT Anatomy

JWT Token Structure:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Structure: HEADER.PAYLOAD.SIGNATURE

Header Component

// Base64 decoded header
{
  "alg": "HS256",        // Signing algorithm
  "typ": "JWT",          // Token type
  "kid": "key-id-123"    // Key ID (optional)
}

// Raw base64: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9

Payload Component

// Base64 decoded payload
{
  "sub": "1234567890",           // Subject (user ID)
  "name": "John Doe",            // User name
  "iat": 1516239022,             // Issued at
  "exp": 1516242622,             // Expiration
  "aud": "my-app",               // Audience
  "iss": "auth-server.com",      // Issuer
  "roles": ["admin", "user"]     // Custom claims
}

// Raw base64: eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ

Signature Component

// Signature verification
HMACSHA256(
  base64UrlEncode(header) + "." + base64UrlEncode(payload),
  secret_key
) = SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Decoding JWT Tokens

Step-by-Step Decoding Process

1. Token Input and Parsing

// Paste JWT token into decoder
const token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c";

// Automatic parsing into components
const [header, payload, signature] = token.split('.');

// Base64 decode each component
const decodedHeader = JSON.parse(atob(header));
const decodedPayload = JSON.parse(atob(payload));

2. Header Analysis

Header Analysis:
{
  "algorithm": "HS256",
  "type": "JWT",
  "keyId": "optional-key-id",
  "critical": [],
  "securityLevel": "Standard",
  "algorithmFamily": "HMAC"
}

3. Payload Examination

Payload Details:
{
  "claims": {
    "standard": {
      "sub": "1234567890",
      "iat": 1516239022,
      "exp": 1516242622
    },
    "public": {
      "name": "John Doe",
      "email": "john@example.com"
    },
    "private": {
      "roles": ["admin"],
      "permissions": ["read", "write"]
    }
  },
  "timestamps": {
    "issuedAt": "2018-01-18 01:30:22 UTC",
    "expiresAt": "2018-01-18 02:30:22 UTC",
    "notBefore": "2018-01-18 01:30:22 UTC"
  },
  "status": "Valid",
  "timeToExpiration": "45 minutes"
}

Real-Time Decoding Features

Instant Analysis

Real-Time Features:
├── Auto-Detection: Recognizes JWT format automatically
├── Live Parsing: Updates as you type or paste
├── Error Highlighting: Shows malformed token parts
├── Syntax Validation: Validates JSON structure
├── Timestamp Conversion: Human-readable dates
├── Expiration Warnings: Alerts for expired tokens
└── Security Alerts: Flags insecure configurations

Visual Components

┌─────────────────────────────────────────────┐
│ JWT Token Input                             │
├─────────────────────────────────────────────┤
│ Header (Red):   Algorithm, Type, Key ID     │
│ Payload (Blue): Claims, User Data, Times    │
│ Signature (Green): Verification Hash        │
├─────────────────────────────────────────────┤
│ Decoded JSON Output                         │
│ ├── Header: { alg, typ, kid }               │
│ ├── Payload: { sub, iat, exp, ... }        │
│ └── Signature: Verification Status          │
└─────────────────────────────────────────────┘

JWT Validation and Verification

Signature Verification

HMAC Signatures (HS256, HS384, HS512)

// HMAC signature verification
const crypto = require('crypto');

function verifyHMACSignature(token, secret) {
  const [header, payload, signature] = token.split('.');
  const data = `${header}.${payload}`;
  
  // Generate expected signature
  const expectedSignature = crypto
    .createHmac('sha256', secret)
    .update(data)
    .digest('base64url');
    
  return signature === expectedSignature;
}

// Usage in decoder
Secret Key: your-256-bit-secret
Algorithm: HS256
Status: ✅ Signature Valid / ❌ Signature Invalid

RSA Signatures (RS256, RS384, RS512)

// RSA signature verification requires public key
const publicKey = `
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA4f5wg5l2hKsTeNem/V41
fGnJm6gOdrj8ym3rFkEjWT2btf3EMs8wAIzgVvN4qTmWCsKg8PEfJ3kV5dQ5...
-----END PUBLIC KEY-----`;

function verifyRSASignature(token, publicKey) {
  const [header, payload, signature] = token.split('.');
  const data = `${header}.${payload}`;
  
  return crypto.verify(
    'sha256',
    Buffer.from(data),
    publicKey,
    Buffer.from(signature, 'base64url')
  );
}

Claims Validation

Standard Claims Verification

Standard Claims Validation:
{
  "iss": {
    "claim": "https://auth.example.com",
    "expected": "https://auth.example.com",
    "status": "✅ Valid",
    "description": "Token issuer matches expected"
  },
  "aud": {
    "claim": "my-app",
    "expected": ["my-app", "api-gateway"],
    "status": "✅ Valid",
    "description": "Audience claim includes expected value"
  },
  "exp": {
    "claim": 1516242622,
    "current": 1516239022,
    "status": "✅ Valid",
    "timeRemaining": "60 minutes",
    "description": "Token not expired"
  },
  "nbf": {
    "claim": 1516239022,
    "current": 1516239022,
    "status": "✅ Valid", 
    "description": "Token is active (not before time passed)"
  }
}

Custom Claims Validation

Custom Validation Rules:
{
  "roles": {
    "claim": ["admin", "user"],
    "required": ["user"],
    "status": "✅ Valid",
    "description": "User has required role"
  },
  "permissions": {
    "claim": ["read", "write", "delete"],
    "required": ["read"],
    "status": "✅ Valid",
    "description": "User has required permissions"
  },
  "email_verified": {
    "claim": true,
    "required": true,
    "status": "✅ Valid",
    "description": "Email verification required and present"
  }
}

Security Validation Checks

Algorithm Security Assessment

Algorithm Security Analysis:
{
  "algorithm": "HS256",
  "securityLevel": "Medium",
  "recommendations": [
    "✅ Using secure HMAC-SHA256 algorithm",
    "⚠️ Consider upgrading to RS256 for public/private key security",
    "✅ No 'none' algorithm vulnerability"
  ],
  "vulnerabilities": [],
  "bestPractices": {
    "keyLength": "✅ Sufficient (>= 256 bits)",
    "algorithm": "✅ Secure algorithm family",
    "implementation": "✅ Standard implementation"
  }
}

Security Best Practices

Common JWT Vulnerabilities

Algorithm Confusion Attacks

// Dangerous: Algorithm can be manipulated
{
  "alg": "none",     // ❌ No signature verification
  "typ": "JWT"
}

// Secure: Specific algorithm enforcement
{
  "alg": "RS256",    // ✅ Strong RSA signature
  "typ": "JWT"
}

Security Check:
├── ❌ 'none' algorithm detected - CRITICAL VULNERABILITY
├── ⚠️ 'HS256' with RSA key - Potential confusion attack
├── ✅ 'RS256' with proper RSA key - Secure
└── ✅ Algorithm whitelist enforced - Secure

Weak Signature Detection

Signature Strength Analysis:
{
  "algorithm": "HS256",
  "keyStrength": "Weak",
  "recommendations": [
    "❌ Secret key too short (< 32 characters)",
    "❌ Common secret detected (e.g., 'secret', '123456')",
    "✅ Use cryptographically secure random key",
    "✅ Minimum 256-bit key for HS256"
  ],
  "score": "2/10 - INSECURE"
}

Token Lifecycle Security

Expiration Management

Token Lifecycle Checks:
{
  "expiration": {
    "hasExp": true,
    "timeRemaining": "15 minutes",
    "status": "⚠️ Expiring Soon",
    "recommendation": "Refresh token within 5 minutes"
  },
  "issuedAt": {
    "hasIat": true,
    "age": "45 minutes", 
    "status": "✅ Valid",
    "recommendation": "Token age acceptable"
  },
  "notBefore": {
    "hasNbf": false,
    "status": "⚠️ Missing",
    "recommendation": "Consider adding 'nbf' claim for security"
  }
}

Audience and Issuer Validation

Trust Boundary Validation:
{
  "issuer": {
    "claim": "https://auth.malicious.com",
    "whitelist": ["https://auth.example.com", "https://sso.company.com"],
    "status": "❌ SECURITY RISK",
    "action": "REJECT TOKEN - Untrusted issuer"
  },
  "audience": {
    "claim": "different-app",
    "expected": "my-app",
    "status": "❌ INVALID",
    "action": "REJECT TOKEN - Wrong audience"
  }
}

Secure Token Handling

Storage Recommendations

Token Storage Security:
{
  "localStorage": {
    "security": "❌ Insecure",
    "vulnerability": "XSS attacks can steal tokens",
    "recommendation": "Avoid for sensitive tokens"
  },
  "sessionStorage": {
    "security": "⚠️ Better than localStorage",
    "vulnerability": "Still vulnerable to XSS",
    "recommendation": "Use for non-sensitive tokens only"
  },
  "httpOnlyCookies": {
    "security": "✅ Secure",
    "benefits": ["XSS protection", "Automatic CSRF protection"],
    "recommendation": "Preferred method for auth tokens"
  },
  "inMemory": {
    "security": "✅ Most Secure",
    "benefits": ["No persistence", "XSS resistant"],
    "drawback": "Lost on page refresh"
  }
}

JWT Algorithms and Signatures

Symmetric Algorithms (HMAC)

HS256 (HMAC-SHA256)

// HMAC-SHA256 implementation
Algorithm: HS256
Key Type: Symmetric (shared secret)
Key Size: Minimum 256 bits (32 bytes)
Security: Medium (depends on secret strength)
Use Case: Internal APIs, single-service authentication

Example Secret Generation:
const crypto = require('crypto');
const secret = crypto.randomBytes(64).toString('hex');
// Generates: a1b2c3d4e5f6... (128 hex characters = 512 bits)

HS384 and HS512

HS384 Configuration:
├── Algorithm: HMAC-SHA384
├── Key Size: Minimum 384 bits
├── Output Size: 384 bits
└── Security Level: High

HS512 Configuration:
├── Algorithm: HMAC-SHA512  
├── Key Size: Minimum 512 bits
├── Output Size: 512 bits
└── Security Level: Very High

Asymmetric Algorithms (RSA/ECDSA)

RS256 (RSA-SHA256)

// RSA-SHA256 configuration
Algorithm: RS256
Key Type: Asymmetric (public/private key pair)
Key Size: Minimum 2048 bits (4096 recommended)
Security: High
Use Case: Distributed systems, microservices

Key Generation Example:
openssl genpkey -algorithm RSA -out private.pem -pkcs8 -aes256 -pass pass:mypassword -aes-256-cbc -keysize 4096
openssl rsa -pubout -in private.pem -out public.pem -passin pass:mypassword

Public Key Distribution: Safe to share
Private Key: Keep absolutely secret

ES256 (ECDSA-SHA256)

ECDSA Configuration:
├── Algorithm: ES256 (P-256 curve)
├── Key Size: 256 bits (much smaller than RSA)
├── Performance: Faster than RSA
├── Security: Equivalent to 3072-bit RSA
└── Use Case: Mobile apps, IoT devices

Curve Options:
- ES256: P-256 curve (secp256r1)
- ES384: P-384 curve (secp384r1) 
- ES512: P-521 curve (secp521r1)

Algorithm Comparison

Performance Comparison

Algorithm Performance (operations per second):
┌─────────┬──────────┬──────────┬─────────────┐
│Algorithm│ Sign     │ Verify   │ Key Size    │
├─────────┼──────────┼──────────┼─────────────┤
│ HS256   │ 50,000   │ 50,000   │ 256 bits    │
│ HS512   │ 45,000   │ 45,000   │ 512 bits    │
│ RS256   │ 2,000    │ 20,000   │ 2048+ bits  │
│ RS512   │ 1,800    │ 18,000   │ 2048+ bits  │
│ ES256   │ 8,000    │ 4,000    │ 256 bits    │
│ ES512   │ 1,200    │ 600      │ 521 bits    │
└─────────┴──────────┴──────────┴─────────────┘

Security Comparison

Security Analysis:
{
  "HMAC": {
    "keyDistribution": "❌ Shared secret required",
    "scalability": "❌ Difficult in distributed systems", 
    "performance": "✅ Very fast",
    "quantumResistance": "❌ Vulnerable to quantum attacks"
  },
  "RSA": {
    "keyDistribution": "✅ Public key can be shared",
    "scalability": "✅ Excellent for distributed systems",
    "performance": "⚠️ Slower signing, fast verification",
    "quantumResistance": "❌ Vulnerable to quantum attacks"
  },
  "ECDSA": {
    "keyDistribution": "✅ Public key can be shared", 
    "scalability": "✅ Good for distributed systems",
    "performance": "✅ Balanced signing/verification",
    "quantumResistance": "❌ Vulnerable to quantum attacks"
  }
}

Common JWT Claims

Registered Claims (Standard)

Core Temporal Claims

Temporal Claims:
{
  "iat": {
    "name": "Issued At",
    "value": 1516239022,
    "readable": "2018-01-18 01:30:22 UTC",
    "description": "When token was issued",
    "validation": "Should not be in future"
  },
  "exp": {
    "name": "Expiration Time", 
    "value": 1516242622,
    "readable": "2018-01-18 02:30:22 UTC",
    "description": "When token expires",
    "validation": "Must be in future for valid token"
  },
  "nbf": {
    "name": "Not Before",
    "value": 1516239022,
    "readable": "2018-01-18 01:30:22 UTC", 
    "description": "Token not valid before this time",
    "validation": "Current time must be after nbf"
  }
}

Identity and Scope Claims

Identity Claims:
{
  "sub": {
    "name": "Subject",
    "value": "user:1234567890", 
    "description": "Unique user identifier",
    "validation": "Should be immutable user ID"
  },
  "iss": {
    "name": "Issuer",
    "value": "https://auth.example.com",
    "description": "Token issuer URL",
    "validation": "Must match trusted issuer list"
  },
  "aud": {
    "name": "Audience", 
    "value": ["my-app", "api-gateway"],
    "description": "Intended token recipients",
    "validation": "Current service must be in audience"
  },
  "jti": {
    "name": "JWT ID",
    "value": "unique-token-id-12345",
    "description": "Unique token identifier", 
    "validation": "Used for token revocation/tracking"
  }
}

Public Claims (Standardized)

User Profile Claims

Profile Claims (OpenID Connect):
{
  "name": "John Doe",
  "given_name": "John",
  "family_name": "Doe", 
  "middle_name": "Michael",
  "nickname": "Johnny",
  "preferred_username": "johndoe",
  "profile": "https://example.com/users/johndoe",
  "picture": "https://example.com/avatar/johndoe.jpg",
  "website": "https://johndoe.com",
  "email": "john@example.com",
  "email_verified": true,
  "gender": "male",
  "birthdate": "1990-01-01",
  "zoneinfo": "America/New_York",
  "locale": "en-US",
  "phone_number": "+1-555-123-4567",
  "phone_number_verified": true
}

Address Claims

Address Claim:
{
  "address": {
    "formatted": "123 Main St\nAnytown, CA 12345\nUSA",
    "street_address": "123 Main St",
    "locality": "Anytown",
    "region": "CA", 
    "postal_code": "12345",
    "country": "USA"
  }
}

Private Claims (Custom)

Authorization Claims

Custom Authorization Claims:
{
  "roles": ["admin", "user", "moderator"],
  "permissions": [
    "users:read",
    "users:write", 
    "posts:delete",
    "settings:admin"
  ],
  "groups": ["staff", "beta-users"],
  "scopes": ["read:profile", "write:posts", "admin:users"],
  "tenant": "company-123",
  "department": "engineering",
  "clearance_level": "confidential"
}

Application-Specific Claims

Application Claims:
{
  "app_metadata": {
    "subscription": "premium",
    "features": ["advanced-search", "api-access"],
    "limits": {
      "api_calls_per_day": 10000,
      "storage_gb": 100
    }
  },
  "user_metadata": {
    "preferences": {
      "theme": "dark",
      "language": "en",
      "timezone": "UTC"
    },
    "onboarding_completed": true,
    "last_login": "2023-09-01T10:30:00Z"
  }
}

Troubleshooting JWT Issues

Common JWT Problems

Signature Verification Failures

Problem: "Invalid signature" error
Causes:
├── Wrong secret key used for HMAC algorithms
├── Public/private key mismatch for RSA/ECDSA
├── Algorithm mismatch (header vs verification)
├── Token tampering or corruption
└── Clock skew between issuer and verifier

Debugging Steps:
1. Verify the exact secret/key being used
2. Check algorithm in header matches verification
3. Ensure token hasn't been modified
4. Test with known good token from same issuer
5. Check for trailing spaces or encoding issues

Expiration and Timing Issues

Problem: "Token expired" error
Analysis:
{
  "tokenExp": "2023-09-01 10:30:00 UTC",
  "currentTime": "2023-09-01 10:35:00 UTC",
  "timeDiff": "+5 minutes",
  "status": "❌ Expired",
  "clockSkew": "±30 seconds allowed",
  "recommendation": "Refresh token or check system clocks"
}

Problem: "Token not yet valid" (nbf claim)
Analysis:
{
  "tokenNbf": "2023-09-01 11:00:00 UTC", 
  "currentTime": "2023-09-01 10:55:00 UTC",
  "timeDiff": "-5 minutes",
  "status": "❌ Not yet valid",
  "recommendation": "Wait or check token issuance time"
}

Malformed Token Issues

Problem: "Malformed JWT" error
Common Issues:
├── Missing dots (should have exactly 2 dots)
├── Invalid Base64 encoding in header/payload
├── Non-JSON content in header/payload
├── Special characters not URL-safe encoded
└── Empty segments

Token Validation:
const tokenParts = token.split('.');
if (tokenParts.length !== 3) {
  throw new Error('JWT must have 3 parts separated by dots');
}

try {
  JSON.parse(atob(tokenParts[0])); // Header
  JSON.parse(atob(tokenParts[1])); // Payload
} catch (e) {
  throw new Error('Invalid JSON in JWT header or payload');
}

Debugging Tools and Techniques

Step-by-Step Debugging

JWT Debugging Checklist:
□ 1. Verify token format (3 parts separated by dots)
□ 2. Check header for algorithm and type
□ 3. Decode payload and verify required claims
□ 4. Validate expiration and timing claims
□ 5. Verify signature with correct key/secret
□ 6. Check audience and issuer claims
□ 7. Validate custom claims if present
□ 8. Test with known working configuration

Logging and Monitoring

Debug Information to Log:
{
  "tokenInfo": {
    "algorithm": "RS256",
    "issuer": "https://auth.example.com", 
    "subject": "user-12345",
    "audience": ["my-app"],
    "issuedAt": "2023-09-01T10:00:00Z",
    "expiresAt": "2023-09-01T11:00:00Z"
  },
  "validationResults": {
    "signatureValid": true,
    "notExpired": true,
    "audienceMatch": true,
    "issuerTrusted": true,
    "customClaimsValid": true
  },
  "errors": [],
  "warnings": ["Token expires in 5 minutes"]
}

JWT Testing and Debugging

Testing Strategies

Unit Testing JWT Validation

// Jest test example
describe('JWT Validation', () => {
  const validToken = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...';
  const secret = 'your-256-bit-secret';
  
  test('should validate correct JWT', () => {
    const result = validateJWT(validToken, secret);
    expect(result.valid).toBe(true);
    expect(result.payload.sub).toBe('1234567890');
  });
  
  test('should reject expired JWT', () => {
    const expiredToken = generateExpiredToken();
    const result = validateJWT(expiredToken, secret);
    expect(result.valid).toBe(false);
    expect(result.error).toBe('Token expired');
  });
  
  test('should reject invalid signature', () => {
    const tamperedToken = validToken.slice(0, -5) + 'XXXXX';
    const result = validateJWT(tamperedToken, secret);
    expect(result.valid).toBe(false);
    expect(result.error).toBe('Invalid signature');
  });
});

Integration Testing

// Integration test with real auth server
describe('JWT Integration Tests', () => {
  test('should work with auth server tokens', async () => {
    const response = await fetch('/auth/login', {
      method: 'POST',
      body: JSON.stringify({ username: 'test', password: 'test' })
    });
    
    const { token } = await response.json();
    const decoded = decodeJWT(token);
    
    expect(decoded.header.alg).toBe('RS256');
    expect(decoded.payload.iss).toBe('https://auth.example.com');
    expect(decoded.payload.aud).toContain('my-app');
  });
});

Performance Testing

JWT Processing Benchmarks

// Performance benchmarking
const benchmark = {
  hmacSigning: {
    algorithm: 'HS256',
    iterations: 10000,
    averageTime: '0.02ms',
    tokensPerSecond: 50000
  },
  rsaSigning: {
    algorithm: 'RS256', 
    iterations: 10000,
    averageTime: '0.5ms',
    tokensPerSecond: 2000
  },
  verification: {
    hmac: '0.015ms',
    rsa: '0.05ms',
    ecdsa: '0.25ms'
  }
};

// Load testing
const loadTest = async () => {
  const tokens = generateTestTokens(1000);
  const startTime = Date.now();
  
  const results = await Promise.all(
    tokens.map(token => validateJWT(token))
  );
  
  const endTime = Date.now();
  const duration = endTime - startTime;
  
  console.log(`Validated 1000 tokens in ${duration}ms`);
  console.log(`Rate: ${1000 / (duration / 1000)} tokens/second`);
};

Integration Examples

Node.js Integration

Express.js Middleware

const jwt = require('jsonwebtoken');

// JWT authentication middleware
const authenticateJWT = (req, res, next) => {
  const authHeader = req.headers.authorization;
  const token = authHeader && authHeader.split(' ')[1];
  
  if (!token) {
    return res.status(401).json({ error: 'No token provided' });
  }
  
  jwt.verify(token, process.env.JWT_SECRET, (err, payload) => {
    if (err) {
      return res.status(403).json({ 
        error: 'Invalid token',
        details: err.message 
      });
    }
    
    req.user = payload;
    next();
  });
};

// Usage
app.get('/protected', authenticateJWT, (req, res) => {
  res.json({ 
    message: 'Access granted',
    user: req.user.sub,
    roles: req.user.roles 
  });
});

RSA Key Management

const fs = require('fs');
const jwt = require('jsonwebtoken');

class JWTManager {
  constructor() {
    this.privateKey = fs.readFileSync('private.pem', 'utf8');
    this.publicKey = fs.readFileSync('public.pem', 'utf8');
  }
  
  generateToken(payload, options = {}) {
    const defaultOptions = {
      algorithm: 'RS256',
      expiresIn: '1h',
      issuer: 'https://auth.example.com',
      audience: 'my-app'
    };
    
    return jwt.sign(payload, this.privateKey, {
      ...defaultOptions,
      ...options
    });
  }
  
  verifyToken(token) {
    try {
      return jwt.verify(token, this.publicKey, {
        algorithms: ['RS256'],
        issuer: 'https://auth.example.com',
        audience: 'my-app'
      });
    } catch (error) {
      throw new Error(`JWT verification failed: ${error.message}`);
    }
  }
}

Frontend Integration

React JWT Hook

import { useState, useEffect, useCallback } from 'react';

const useJWT = () => {
  const [token, setToken] = useState(localStorage.getItem('jwt_token'));
  const [decoded, setDecoded] = useState(null);
  const [isValid, setIsValid] = useState(false);
  
  const decodeToken = useCallback((token) => {
    if (!token) return null;
    
    try {
      const parts = token.split('.');
      if (parts.length !== 3) return null;
      
      const header = JSON.parse(atob(parts[0]));
      const payload = JSON.parse(atob(parts[1]));
      
      // Check expiration
      const now = Math.floor(Date.now() / 1000);
      const isExpired = payload.exp && payload.exp < now;
      
      setDecoded({ header, payload, isExpired });
      setIsValid(!isExpired);
      
      return { header, payload, isExpired };
    } catch (error) {
      console.error('Token decode error:', error);
      setIsValid(false);
      return null;
    }
  }, []);
  
  useEffect(() => {
    if (token) {
      decodeToken(token);
    }
  }, [token, decodeToken]);
  
  const saveToken = useCallback((newToken) => {
    setToken(newToken);
    localStorage.setItem('jwt_token', newToken);
  }, []);
  
  const clearToken = useCallback(() => {
    setToken(null);
    setDecoded(null);
    setIsValid(false);
    localStorage.removeItem('jwt_token');
  }, []);
  
  return {
    token,
    decoded,
    isValid,
    saveToken,
    clearToken,
    decodeToken
  };
};

// Usage in component
const App = () => {
  const { token, decoded, isValid, clearToken } = useJWT();
  
  if (!isValid) {
    return ;
  }
  
  return (
    

      
Welcome, {decoded.payload.name}

      
Roles: {decoded.payload.roles?.join(', ')}

      
    

  );
};

Conclusion

Our JWT Decoder provides comprehensive tools for analyzing, validating, and debugging JSON Web Tokens in your authentication workflows. Whether you're implementing JWT authentication, troubleshooting token issues, or performing security audits, our decoder offers professional-grade capabilities.

Key Benefits:

  • Complete Token Analysis: Decode header, payload, and signature components
  • Security Validation: Verify signatures and detect vulnerabilities
  • Real-time Debugging: Instant feedback on token validity and issues
  • Multi-Algorithm Support: HMAC, RSA, and ECDSA signature verification
  • Claims Inspection: Detailed analysis of standard and custom claims

Ready to decode and validate your JWTs? Try our JWT Decoder today and ensure your token-based authentication is secure and reliable!

Last updated: September 2025 | JWT Decoder Guide | DevToolMint Professional Tools

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