JWT Decode, Verify & Generate

Related

Use Cases

• Decode JWT online: quickly split a token during OAuth 2.0 / OIDC login debugging to inspect Header algorithm and Payload Claims
• Verify JWT signatures: use a Secret / public key to validate token signatures in real time and decide whether a 401 is 'bad signature', 'expired', or 'algorithm mismatch'
• Generate test JWTs during integration: issue a JWT in the browser with a test key pair, no need to ask the backend to temporarily change code to mint a token
• PEM ↔ JWK conversion: the OIDC IdP gave you JWK but the server needs PEM (or vice versa) — convert in place instead of writing an OpenSSL script
• Diagnose 401 Unauthorized: step through expiry, alg-vs-gateway match, key match, and PEM formatting one at a time
• Compare RS256 vs PS256: many OIDC providers default to RS256 while AWS SigV4 prefers PS256 — switch algorithms with the same RSA key pair and compare results
• Learn JWT internals: change the alg, edit the Payload, swap the key in the browser and watch the signature break in real time — far faster than reading the RFC

Features

• Decode + Verify + Generate in one place: paste a token to split Header/Payload/Signature, switch algorithm and key to verify the signature, edit Claims and re-sign to produce a new token — covers the full JWT troubleshooting chain
• All 13 algorithms supported: HS256/HS384/HS512 (HMAC), RS256/RS384/RS512 (RSA-PKCS1-v1_5), ES256/ES384/ES512 (ECDSA), PS256/PS384/PS512 (RSA-PSS), EdDSA — covers OAuth 2.0, OIDC, JWS, API gateways and enterprise SSO
• PEM and JWK formats: RSA, ECDSA and Ed25519 keys accept PEM strings (with -----BEGIN PUBLIC KEY----- headers) and JWK JSON (kid/kty/n/e fields) — drop in an OIDC /.well-known/jwks.json output directly
• Multiple HMAC secret input forms: raw UTF-8 string, hex bytes, or Base64/Base64URL-encoded secret — switch with one click, so 'looks the same but signature fails' never happens
• Auto key pair generation: RSA-2048, RSA-4096, P-256, P-384 and Ed25519 key pairs generated in-browser, presented in both PEM and JWK formats for immediate test use
• Semantic Claims highlighting: exp / iat / nbf auto-converted to human-readable time with expired / not-yet-valid / issued status highlighted; alg / typ / kid in Header labeled individually
• 100% client-side, zero upload: every parse, verify and sign runs through the browser's native Web Crypto API — tokens, keys and payload never leave the browser, matching the security principle of 'never paste production tokens into random online tools'
• One-click copy: Header, Payload, Signature and the newly generated token are each independently copyable for documentation, tickets and curl commands

How to Use

1. Paste or type the token: drop the JWT string into the input box; the tool automatically splits Header, Payload and Signature and pretty-prints each as JSON
2. Select the algorithm: switch among HS256 / RS256 / ES256 / PS256 / EdDSA; the tool infers from Header.alg and flags any mismatch
3. Import a key: paste an HMAC secret, a PEM string, or a JWK JSON; for asymmetric algorithms, click 'Generate key pair' to get a test key immediately
4. Verify or re-sign: in Decode mode verify with a public key and see '✅ signature valid / ❌ signature invalid'; in Encode mode edit Claims and sign with a private key to produce a new token

FAQ

What is JWT Decode, Verify & Generate?

Code Examples

const crypto = require('node:crypto')

function b64url(input) {
  return Buffer.from(input)
    .toString('base64')
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
    .replace(/=+$/, '')
}

function hmacSign(data, secret) {
  return b64url(crypto.createHmac('sha256', secret).update(data).digest())
}

function sign(payload, secret) {
  const header = b64url(JSON.stringify({ alg: 'HS256', typ: 'JWT' }))
  const body = b64url(JSON.stringify(payload))
  const signature = hmacSign(`${header}.${body}`, secret)
  return `${header}.${body}.${signature}`
}

function verify(token, secret) {
  const [h, p, s] = token.split('.')
  const expected = hmacSign(`${h}.${p}`, secret)
  const a = Buffer.from(s)
  const b = Buffer.from(expected)
  return a.length === b.length && crypto.timingSafeEqual(a, b)
}

const SECRET = 'my-super-secret-key'
const payload = { userId: 42, role: 'admin', exp: Math.floor(Date.now() / 1000) + 3600 }
const token = sign(payload, SECRET)

console.log(token)
console.log(verify(token, SECRET))       // true
console.log(verify(token, 'wrong-key'))  // false

<!doctype html>
<html>
  <body>
    <pre id="out"></pre>
    <script>
      const out = document.getElementById('out')

      const b64url = buf =>
        btoa(String.fromCharCode(...new Uint8Array(buf)))
          .replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')

      const b64urlStr = str => b64url(new TextEncoder().encode(str))

      async function sign(payload, secret) {
        const header = b64urlStr(JSON.stringify({ alg: 'HS256', typ: 'JWT' }))
        const body = b64urlStr(JSON.stringify(payload))
        const data = new TextEncoder().encode(`${header}.${body}`)

        const key = await crypto.subtle.importKey(
          'raw',
          new TextEncoder().encode(secret),
          { name: 'HMAC', hash: 'SHA-256' },
          false,
          ['sign']
        )

        const sig = await crypto.subtle.sign('HMAC', key, data)
        return `${header}.${body}.${b64url(sig)}`
      }

      ;(async () => {
        const token = await sign({ user: 'alice', role: 'admin' }, 'browser-demo-secret')
        out.textContent = token
      })()
    </script>
  </body>
</html>

import { jwtVerify, importSPKI } from 'jose'
import { readFile } from 'node:fs/promises'

const token = 'eyJhbGciOiJSUzI1NiIsImtpZCI6IjEifQ.payload.signature'
const publicKeyPem = await readFile('public.pem', 'utf8')

try {
  const { payload, protectedHeader } = await jwtVerify(
    token,
    await importSPKI(publicKeyPem, 'RS256'),
    {
      issuer: 'https://idp.example.com',
      audience: 'my-app',
      algorithms: ['RS256'],     // critical: algorithm whitelist blocks alg:none and HS256-substitution attacks
    }
  )
  console.log('alg =', protectedHeader.alg)
  console.log('sub  =', payload.sub)
} catch (err) {
  console.error('verify failed:', err.code, err.message)
  // common: ERR_JWT_EXPIRED / ERR_JWS_INVALID / ERR_JWS_SIGNATURE_VERIFICATION_FAILED
}

import jwt

token = 'eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0In0.SflKxw'

# Parse (no signature check, just look at Header and Payload)
print(jwt.get_unverified_header(token))
# {'alg': 'HS256', 'typ': 'JWT'}

print(jwt.decode(token, options={'verify_signature': False}))
# {'sub': '1234'}

# Full verification
claims = jwt.decode(
    token,
    'my-super-secret-key',
    algorithms=['HS256'],
    audience='my-app',
    issuer='my-service',
)
print(claims['sub'])

Supported Video Formats

Output Example

A real MP3 file encoded to a Data URI — copy-ready:

Header (Base64URL):  eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
Payload(Base64URL):  eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkphbmUgRG9lIiwiaWF0IjoxNzE2MjM5MDIyfQ
Signature:           kZJfaYjK3iCkVFL5EL9zGRZ5SmD8_x2h6B5c7pVFfVGo

Header decoded:
  {
    "alg": "HS256",
    "typ": "JWT"
  }

Payload decoded:
  {
    "sub": "1234567890",
    "name": "Jane Doe",
    "iat": 1716239022        // 2024-05-20 14:23:42 UTC
  }

Privacy & Security

This JWT workbench runs 100% in your browser, backed by the browser's native Web Crypto API. The token you paste, the Header, the Payload, the Signature, the HMAC Secret, any RSA/ECDSA private keys, any generated key pairs, and every signature/verify computation stay on your device — they never travel to a server, are never logged, never analyzed, and never cached. The page can be used offline once loaded. Best practice: never paste long-lived production tokens or production private keys into any online tool, including this one. This tool is appropriate for debugging, integration testing, learning, and minting test tokens — production secrets and private keys should be handled in a controlled, local environment.

Authoritative References

• IETFRFC 7519: JSON Web Token (JWT)
• IETFRFC 7515: JSON Web Signature (JWS)
• IETFRFC 7517: JSON Web Key (JWK)
• Auth0JWT.io — official debugger and library index
• OWASPOWASP JSON Web Token Cheat Sheet
• IETFJOSE working group overview

Troubleshooting