KitVela

JWT Decoder & Inspector (Parse JSON Web Tokens)

Inspect and decode JSON Web Tokens (JWT) client-side.

Decode and inspect JSON Web Tokens (JWT) client-side. Paste any encoded Bearer token to inspect header algorithms, payload claims, user roles, and expiration dates with 100% in-browser security.
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* Decoding happens entirely in your browser. No tokens are sent to the server. Note: This tool only decodes the token, it does not verify the signature.

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How It Works & Calculation Method

JSON Web Tokens (JWT) are a compact, URL-safe standard (RFC 7519) used for securely transmitting claims between parties in modern authentication systems (OAuth 2.0, OpenID Connect).

A standard JWT consists of three parts separated by dots: Header, Payload, and Signature. Because tokens often contain sensitive claims and user metadata, our tool decodes tokens entirely inside your local browser without transmitting tokens to external servers.

The decoder splits the token string by '.' delimiters, extracts the Base64Url-encoded header and payload segments, and translates them back into standard UTF-8 JSON.

It formats and pretty-prints both JSON objects in interactive, syntax-highlighted blocks for immediate inspection.

Mathematical Formula

JWT Structure: Base64Url(Header) . Base64Url(Payload) . HMAC-SHA256(Header.Payload, secret)

Worked Example: Decoding an Authorization Token

A frontend engineer inspects a JWT token received from an OAuth endpoint.

•Paste token string into the decoder.
•Header segment decoded: {"alg":"HS256","typ":"JWT"}.
•Payload segment decoded: {"sub":"1234567890","name":"John Doe","iat":1516239022}.
•Inspect expiration timestamp (exp) to verify token validity.
Final Result: Header and Payload successfully decoded into clean JSON.

Common Pitfalls to Avoid

  • •Assuming a JWT is encrypted (JWTs are merely signed and base64-encoded; anyone can read the payload).
  • •Storing confidential passwords or private encryption keys inside a public client-side JWT payload.
  • •Confusing token decoding with cryptographic signature verification (verification requires the server secret).

Who This Tool Is For

  • •Frontend developers debugging authentication flows and user session claims.
  • •Backend engineers verifying token payload claims and expiration timestamps.
  • •Security researchers auditing OAuth and OpenID Connect token implementations.

How to Use the JWT Decoder

1

Paste JWT String

Paste your encoded token beginning with 'ey...'.

2

Inspect Header

View the signing algorithm and token type.

3

Audit Payload

Examine user claims, permissions, and expiration timestamps.

Frequently Asked Questions

Is my JWT token sent to your server?
Never. All parsing and base64 decoding runs 100% locally on your machine in the browser sandbox.
Can this tool verify the JWT signature?
This is purely a decoding tool; validating cryptographic signatures requires your private secret key, which should never be pasted into any web browser.
Are JWT tokens encrypted?
Standard JWTs (JWS) are signed but not encrypted; anyone with access to the token string can decode and read all payload claims.
What does the 'exp' claim mean in a JWT?
The 'exp' claim is a Unix epoch timestamp indicating when the token expires and is no longer valid for authentication.
What is Base64Url encoding?
Base64Url is a modified version of Base64 that replaces '+' with '-' and '/' with '_' so tokens can be transmitted in URLs and headers without escaping.
What are common algorithms used in JWT headers?
Common signing algorithms include HMAC with SHA-256 (HS256), RSA signatures (RS256), and ECDSA signatures (ES256).

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Last updated: 10/9/2026