JWT Encoder / Decoder
Encode and decode JSON Web Tokens (JWT) in your browser.
Header
Payload
Signature
Encode and decode JSON Web Tokens (JWT) in your browser.
This free JWT Encoder / Decoder from KX Toolkit is part of our all-in-one online toolkit. It runs entirely in your browser, so your data never leaves your device for client-side operations. 100% free, forever - no paywall, no credit card, no trial.
How to use the JWT Encoder / Decoder
- Paste your input - JSON, regex pattern, JWT, URL etc.
- Pick any flags or options the tool supports.
- Click the action button (Format, Test, Decode).
- Copy the result or download it as a file.
What you can do with the JWT Encoder / Decoder
- Format and validate API responses while debugging.
- Test regex patterns against real input before deploying.
- Decode JWTs to inspect claims and expiry.
- Generate UUIDs for migrations, tests and seeders.
Why use KX Toolkit's JWT Encoder / Decoder
- Browser-based: Works on Windows, macOS, Linux, iOS and Android - no install, no extension.
- Privacy-first: Client-side tools never upload your data; server-side tools delete files right after processing.
- Mobile-friendly: Full feature parity on phones and tablets - not a stripped-down view.
- Fast: Optimised for instant feedback. No artificial waiting screens, no email-gated downloads.
- One hub for everything: 300+ tools across SEO, text, image, PDF, code, color, calculators and more - skip switching between sites.
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What does a JWT actually contain?
A JWT has three base64url-encoded parts separated by dots: a header that describes the algorithm and token type, a payload that holds the claims like sub, iss, exp, and any custom data, and a signature that proves the first two parts have not been altered. The payload is not encrypted - anyone with the token can read it.
Is decoding a JWT the same as verifying it?
No. Decoding just splits the token and base64url-decodes the parts. Verification recomputes the signature using the secret or public key and checks it matches, then confirms claims like exp, nbf, and aud. A decoded but unverified JWT is just user-supplied data - always verify on the server before trusting any claim it carries.
Should I store JWTs in localStorage or cookies?
HttpOnly secure cookies are safer because JavaScript cannot read them, blocking token theft via XSS. localStorage is simpler but exposes tokens to any script on the page, which is dangerous if you load third-party code. If you must use localStorage, pair it with strict CSP and short-lived tokens. Cookies need CSRF protection in return.
What is the alg none vulnerability?
Some libraries accept tokens whose header alg is set to none and treat them as valid without checking the signature. Attackers exploit this by stripping the signature and sending a forged payload. Always whitelist accepted algorithms on the server side and reject none. Modern libraries do this by default, but legacy code should be audited for the issue.
Why is my JWT failing to verify even though the secret is correct?
Common causes are mismatched algorithms (HS256 vs RS256), copy-paste whitespace in the secret or token, expired tokens (exp claim in the past), and clock skew between issuer and verifier. Double-check the alg header matches what your library expects, trim the values, and allow a small leeway (typically 30 seconds) for clock differences across servers.
Can I put sensitive data in a JWT?
Avoid it. The payload is encoded but not encrypted, so anyone who intercepts the token can read every claim. Put only identifiers and authorization context. If you must transmit sensitive data, use a JWE (JSON Web Encryption) token, which encrypts the payload, or keep the data server-side and reference it by id from the JWT.
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