jwtforge — offline JWT workbench

Encoded token

Paste a JWT, an Authorization header, or Bearer <token>

Waiting for input Paste a token to decode it. Decoding never proves authenticity.

Signature & verification

alg —

Not verified Decoded content is untrusted until a signature check succeeds.

Algorithm (writes alg into the header)

Browser support checking…

Shared secret (HMAC)

Secret is Base64URL-encoded (decode before use)

Secrets are held in memory only — never stored, never sent.

How this signature is computed

Auto-regenerate on edit

Header

—

Payload

—

A JWT payload is encoded, not encrypted. Anyone holding this token can read everything above.

What a JWT actually guarantees

JWTs are normally signed, not encrypted. The compact JWS form used here protects integrity and origin, not confidentiality. Encryption is a different construction (JWE) and looks different — five segments instead of three.
Anyone holding a token can read its header and payload. Base64URL is an encoding, not a cipher. Never put passwords, card data, secrets or unnecessary personal data in a payload.
Only signature verification establishes authenticity. Decoding proves nothing at all. jwtforge therefore keeps “decoded” and “verified” strictly separate and will never call a token verified because it parsed cleanly.
Expiry checks alone prove nothing. exp, nbf and iat live inside the payload, so an attacker can set them to anything they like. They are only meaningful after the signature has been verified with a key you trust.
Pin the algorithm on the server. Never let the token's own alg field pick the verification method: that is how alg: none and RSA→HMAC confusion attacks work. Decide the expected algorithm from your configuration, then verify.
Handle keys carefully. Private keys and HMAC secrets belong in a secrets manager. Avoid pasting production private keys into any tool — including this one. Use throw-away test keys for debugging.
Short secrets are weak secrets. An HMAC secret should be at least as long as the hash output (32 bytes for HS256) and generated from a CSPRNG, never a memorable phrase — offline brute-forcing of HS256 tokens is cheap.
Validate more than the signature. Check iss, aud, expiry and, where relevant, kid against your own allow-lists, and reject tokens that fetch keys from attacker-controlled URLs (jku, x5u).

How this tool handles your data

Every operation — decode, verify, sign, key generation — happens in this page using the browser's built-in SubtleCrypto implementation.
There is no backend, no analytics, no CDN, no font fetch and no build step. The whole application is one HTML file that works from file:// with networking disabled.
Tokens, secrets and keys are never written to localStorage; only UI preferences (theme, pane width, auto-regenerate, active tab) are remembered. Reloading the page discards all cryptographic material.
Exported reports deliberately omit secrets and key material.