JWT Encoder and Signer

How to create a signed JWT

1. Choose the Algorithm. The alg field in Header (JSON) follows the selection automatically.
2. Edit Payload (JSON). + iat now inserts the current time, + exp in 1 h sets an expiry one hour from now; both use seconds, as the JWT spec requires.
3. For HS256, HS384 or HS512, enter the Secret and set Secret is to Text, Base64 or Hex. For the other algorithms, paste a private key or press Generate key pair.
4. Copy token, or press Open in decoder to check it: the Decode & verify tab opens with the token and the verification key filled in.

The sample on load is a live HS256 token signed with the secret base64hub-demo-secret, so you can change one claim and watch the signature change with it.

Choosing an algorithm

HS256 is the simplest choice when one service signs and verifies, such as a session token checked by the same backend. Once a second party has to verify, sharing an HMAC secret with them also lets them mint tokens, so switch to an asymmetric algorithm and publish only the public key. ES256 and EdDSA give much shorter tokens than RSA, which matters for cookies and headers. PS256 is RSA with the PSS padding scheme; pick it only if every verifier supports it.

none is in the list for testing parsers. A token with alg: none has an empty signature and any server that accepts it can be fooled by anyone, so the tool warns about it.

Keys this encoder accepts and generates

For signing it takes a PEM PKCS#8 key (BEGIN PRIVATE KEY), a PKCS#1 RSA PRIVATE KEY, a SEC1 EC PRIVATE KEY or a private JWK. To make test keys with OpenSSL instead of the generator:

openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out rsa.pem
openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out ec.pem
openssl genpkey -algorithm ed25519 -out ed.pem
openssl pkey -in ec.pem -pubout -out ec.pub

Generate key pair creates RSA-2048, P-256, P-384, P-521 or Ed25519 keys in the browser with WebCrypto, depending on the algorithm. You get the private key as PKCS#8 PEM, the public key as SPKI PEM, and a JWKS whose kid is the RFC 7638 thumbprint of the key. The JWKS is what you would serve from a jwks_uri for verifiers to download.

For HS256 the secret is used as bytes. If your server reads the secret from an environment variable and Base64-decodes it first, set Secret is to Base64 here as well, or the signatures will not match. A value from the JWT secret preset of the Base64 key generator is Base64URL and decodes to 32 bytes.

Which claims to put in the payload

A useful access token usually carries iss (who issued it), sub (who it is about), aud (which API it is for), iat and a short exp. Add jti if you need to revoke single tokens. Times are whole seconds since 1970; if you build them in JavaScript, use Math.floor(Date.now() / 1000), as Date.now() returns milliseconds.

The payload is signed, never encrypted. Anyone who sees the token can decode it with a JWT decoder, so leave out passwords, API keys and personal data you would not print in a log.

Signing a JWT in code

The format is simple enough to build by hand, which helps when debugging a library. This Node.js function produces exactly the token PyJWT’s jwt.encode(payload, secret, algorithm="HS256") gives for the same input:

const crypto = require("crypto");

const b64url = (obj) => Buffer.from(JSON.stringify(obj)).toString("base64url");

function signHS256(payload, secret) {
  const input = b64url({ alg: "HS256", typ: "JWT" }) + "." + b64url(payload);
  const sig = crypto.createHmac("sha256", secret).update(input).digest("base64url");
  return input + "." + sig;
}

console.log(signHS256({ sub: "user-42", iat: 1767225600, exp: 1767229200 },
                      "replace-me-with-32-random-bytes!"));

It prints eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJ1c2VyLTQyIiwiaWF0IjoxNzY3MjI1NjAwLCJleHAiOjE3NjcyMjkyMDB9.F3xnXnXW8nUQy-Ay37DF626Lb5lHPKI3IFvqoEf3mTw. Enter the same header, payload and secret on this page and you get the same string. The tool compacts the JSON before signing, so indentation does not matter, but key order does, because the signature covers the exact bytes. In production use a maintained library; see Base64 in Node.js for the encoding side.

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