Back to Home 100% Free

JWT Debugger, Decoder & Encoder Studio

Decode, verify signatures, inspect claims timestamps (exp/iat), and generate signed JSON Web Tokens in real-time. 100% client-side and secure.

Encoded Token String

BASE64URL ENCODED
Token Expiration Status
Valid
Signature Verified

Header: Algorithm & Token Type 

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

Payload: Claims Data 

{
  "sub": "1234567890",
  "name": "Antigravity Dev",
  "iat": 1516239022,
  "exp": 1831523922,
  "role": "admin"
}
Developer Security Guide

Mastering JSON Web Tokens: How to Decode, Inspect & Verify JWT Signatures

In-depth guide on RFC 7519 JWT structure, registered claims parsing, HMAC SHA-256 signature verification, and client-side security best practices.

1. What is a JSON Web Token (JWT)?

A **JSON Web Token (JWT)** is a compact, URL-safe container used for authorization and stateless user session management across modern web applications, microservices, and mobile APIs. RFC 7519 specifies the format of JWTs as 3 Base64Url encoded segments separated by dots:

Header.Payload.Signature

1. Header

Contains token metadata specifying the signing algorithm (e.g. HS256 or RS256) and the type JWT.

2. Payload Claims

Holds user identity data and session timestamps (sub, exp, iat, role).

3. HMAC Signature

Cryptographic hash generated by signing header.payload with your secret key to prevent tampering.

2. Standard RFC 7519 Registered Claims Reference

Registered claims are reserved key names defined by the IANA JSON Web Token Registry to ensure interoperability across OAuth2 and OpenID Connect (OIDC) identity providers:

Claim Key Claim Name Description & Example
iss Issuer Identifies the principal that issued the JWT (e.g. "https://auth.zeeaitools.com").
sub Subject Identifies the user ID or principal subject of the token (e.g. "usr_9842").
aud Audience Identifies the recipient service or API intended to consume the token (e.g. "https://api.zeeaitools.com").
exp Expiration Time Unix timestamp specifying when the token expires and must be rejected by backend servers.
nbf Not Before Unix timestamp specifying the earliest time at which the token becomes valid.
iat Issued At Unix timestamp recording the exact time the token was created.

3. JWT Security Vulnerabilities & Prevention Best Practices

  • Never Store Secrets Client-Side: HMAC secret keys must remain strictly confidential on backend servers.
  • Reject 'alg: none' Attacks: Configure servers to explicitly enforce expected algorithms (e.g. HS256) and reject unsigned alg: none header manipulations.
  • Use Short Expiration Times: Set access token expiry times (exp) between 15 minutes to 1 hour, paired with HTTP-only refresh tokens.
FAQ

Frequently Asked Questions (10 FAQs)

Everything you need to know about JSON Web Token security, verification, and decoding.

1. What is JWT (JSON Web Token)?

JSON Web Token (JWT) is an open RFC 7519 standard for securely transmitting information between client applications and web servers as a compact, self-contained JSON object verified by a digital signature.

2. How is a JWT structured?

A JWT consists of 3 dot-separated Base64Url strings: Header (algorithm & token type), Payload (claims data like sub, exp, iat, role), and Signature (HMAC SHA-256 key hash).

3. Is my secret key or JWT token safe on this tool?

Yes! All JWT decoding, Base64Url parsing, and HMAC-SHA256 signature verification happen 100% inside your browser using JavaScript. Tokens and secret keys are never transmitted to any external server.

4. How does expiration checking work?

The tool reads the 'exp' (Expiration Time) claim Unix timestamp and compares it against the current browser clock, calculating whether the token is currently valid or expired.

5. Can I generate new JWT tokens with custom claims?

Yes! Switch to the Generator mode, customize the JSON header and payload claims, set your secret key, and generate a 100% valid signed JWT string.

6. Which signing algorithms are supported?

The tool supports HMAC algorithms (HS256, HS384, HS512) for signature verification and token encoding, as well as header inspection for RS256/ES256 asymmetric keys.

7. What are registered JWT claims?

Registered claims are standard JWT fields defined in RFC 7519: iss (Issuer), sub (Subject), aud (Audience), exp (Expiration), nbf (Not Before), iat (Issued At), and jti (JWT ID).

8. What happens if a JWT has an invalid signature?

If the signature verification fails against the provided secret key, the tool flags a red 'Invalid Signature' alert, warning that the payload or secret may have been tampered with.

9. Can I copy decoded claims or generated tokens with 1 click?

Yes! Click the Copy buttons to copy decoded Header JSON, Payload JSON, or generated JWT string directly to your clipboard.

10. Is this JWT debugger completely free?

Yes, it is 100% free with unlimited decodes, verifications, and encodings.

Copied to Clipboard!