Stop Wasting Time: Master JWT Parser in Under 5 Minutes

Understanding JSON Web Tokens: Decode, Inspect, and Debug

That long string of characters your API returns after login? It's a JSON Web Token (JWT), and it contains more information than you might realize. User IDs, permissions, expiration times—all encoded in three dot-separated segments that look incomprehensible at first glance.

This guide explains how JWTs work, what information they contain, security considerations, and how to use our free parser at pktools.tech to decode tokens instantly.

What Is a JSON Web Token?

A JWT (pronounced "jot") is a compact, URL-safe means of representing claims between two parties. It's the dominant standard for authentication tokens in modern web applications, defined by RFC 7519.

A typical JWT looks like:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Those three sections, separated by dots, are:

• Header: Algorithm and token type
• Payload: Claims (the actual data)
• Signature: Verification that the token hasn't been tampered with

JWT Structure Explained

The Header

The header is a Base64URL-encoded JSON object containing:

• alg: The signing algorithm (HS256, RS256, ES256, etc.)
• typ: Usually "JWT"

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

The Payload (Claims)

The payload contains claims—statements about the user and additional metadata. Claims fall into three categories:

Registered Claims (Standardized)

• iss (Issuer): Who issued the token
• sub (Subject): The user identifier
• aud (Audience): Intended recipient
• exp (Expiration): When the token expires (Unix timestamp)
• iat (Issued At): When the token was created
• nbf (Not Before): Token not valid before this time
• jti (JWT ID): Unique identifier for the token

Public Claims

Custom claims registered with IANA or using collision-resistant names.

Private Claims

Custom claims agreed upon between parties, like user roles or permissions.

{
  "sub": "user123",
  "name": "Jane Developer",
  "role": "admin",
  "iat": 1699564800,
  "exp": 1699651200
}

The Signature

The signature verifies that the header and payload haven't been modified. It's created by:

1. Taking the encoded header and payload
2. Signing them with a secret key (symmetric) or private key (asymmetric)
3. Base64URL-encoding the result

The signature allows servers to validate token integrity without database lookups.

Why JWTs Are Popular

Stateless Authentication

Traditional sessions require server-side storage. JWTs are self-contained—the server doesn't need to remember sessions. This simplifies scaling across multiple servers.

Cross-Domain Support

JWTs work seamlessly in single-page applications, mobile apps, and microservice architectures where cookies are impractical.

Information Sharing

The payload can carry user details, eliminating extra database queries. The user's role, name, and permissions are right there in the token.

How to Use the PKTools JWT Parser

1. Paste your JWT into the input field. The full token, including all three parts.
2. View decoded header: See the algorithm and token type.
3. View decoded payload: All claims are displayed in readable JSON format.
4. Check timestamps: Expiration and issuance times are converted to human-readable dates.

All parsing happens locally in your browser. Your tokens are never transmitted to any server.

JWT Security Considerations

Tokens Are Encoded, Not Encrypted

Base64URL encoding is not encryption. Anyone can decode the header and payload without the secret key. Never store sensitive data (passwords, financial info) in JWT payloads.

Signature Verification Matters

Always verify signatures on the server. An attacker can modify the payload and re-encode it—only signature validation catches tampering.

Short Expiration Times

Set reasonable expiration periods. Access tokens should expire quickly (minutes to hours); use refresh tokens for longer sessions.

The "none" Algorithm Attack

Some libraries accepted tokens with "alg": "none", skipping verification entirely. Modern libraries reject this, but verify your implementation.

Token Storage

• localStorage: Vulnerable to XSS attacks
• HttpOnly cookies: Protected from XSS but susceptible to CSRF
• Memory: Most secure but lost on page refresh

Common JWT Claims Explained

Expiration (exp)

Unix timestamp (seconds since Jan 1, 1970). Our parser converts this to a readable date so you can immediately see when tokens expire.

Issued At (iat)

When the token was created. Useful for identifying stale tokens or calculating token age.

Subject (sub)

The principal (usually user ID) the token represents. This is typically what your application uses for authorization decisions.

Debugging with JWT Parser

Use our parser when:

• Authentication fails: Check if the token has expired
• Permission issues: Verify the role/permission claims
• Integration problems: Confirm the issuer and audience match expectations
• Learning: Understand what your auth system actually returns

JWTs vs. Alternatives

JWTs vs. Session Cookies

Sessions require server storage; JWTs are stateless. JWTs work better for distributed systems; sessions are simpler for traditional web apps.

JWTs vs. PASETO

PASETO (Platform-Agnostic Security Tokens) aims to fix JWT footguns (like the "none" algorithm). It's more opinionated about cryptographic choices but less widely adopted.

JWTs vs. Opaque Tokens

Opaque tokens require server lookup for every request but reveal nothing if stolen. JWTs are self-contained but expose payload data to anyone who captures them.

Frequently Asked Questions

Can I modify a JWT and reuse it?
No. Modifying the payload invalidates the signature. The server will reject tampered tokens (assuming proper signature verification).

Should I refresh tokens before they expire?
Yes. Implement token refresh before expiration to maintain seamless user sessions. Don't wait until the token is already expired.

Can I revoke a JWT?
Not directly—they're stateless. Common approaches include short expiration times, token blacklists, or refresh token revocation.

Is it safe to decode JWTs in the browser?
Decoding reveals payload contents, which shouldn't contain secrets anyway. Our parser runs entirely client-side—your token never leaves your browser.

The Bottom Line

JWTs power authentication across millions of applications. Understanding their structure—header, payload, signature—helps you debug auth issues, implement secure systems, and make informed architectural decisions.

Our pktools.tech JWT Parser decodes tokens instantly, displaying all claims in readable format. Paste your token and see exactly what it contains.

Don't rush through the input - garbage in, garbage out

Take a moment to review settings before processing

Save your results immediately - don't rely on browser history

Test with small samples before processing large datasets

Frequently Asked Questions

Is this tool really free to use? Absolutely! It runs entirely in your browser with no hidden costs.

Can I use this for commercial projects? Yes, there are no restrictions on commercial usage.

How secure is my data? Very secure - all processing happens locally in your browser.

What browsers work best? Modern browsers like Chrome, Firefox, and Safari all work perfectly.

Wrapping Up

Look, JWT Parser - PKTools might seem simple on the surface, but it's one of those tools that just works. No complicated setup, no confusing interfaces - just pure functionality.

Give it a try, and I'm pretty confident you'll find it as useful as I do. The fact that it's completely free makes it even better!

Ready to boost your productivity? Check out JWT Parser - PKTools at https://pktools.tech/tools/jwt-parser.html and see the difference for yourself.

This guide was created based on real user experience and extensive testing. Your results may vary, but the tool consistently delivers reliable performance.