Sign HS256 JWT

Paste a header and payload, provide a secret, copy the signed JWT. Useful for fixture generation in tests. Don't sign production tokens here.

What this tool does

Given:

1. A header object (alg is forced to HS256)
2. A payload object (your claims)
3. A secret (UTF-8 string)

The tool produces a JWT in the form base64url(header).base64url(payload).base64url(HMAC-SHA256(...)). The signature is computed via the browser's Web Crypto API.

When to use the Sign tab

• Unit test fixtures. Generate a known JWT with specific claims for a test that mocks a real auth response.
• Integration test setup. Pre-generate a valid token to seed your test database or HTTP test client.
• Debugging your auth service. Compare a token your service produces to one this tool produces with the same input — if they differ, your service is doing something unexpected.
• Learning. Watching the JWT structure update as you tweak claims is the fastest way to internalize what each part does.

When NOT to use the Sign tab

• Real production signing. Don't paste a real secret here. Even though everything stays in your browser, copy-paste habits leak secrets to clipboard managers, AI chat assistants, or screenshots.
• RSA / ECDSA tokens. Use a server-side library; this tool is HMAC-only.
• Tokens for systems you don't control. If you're crafting a token to send to someone else's API, you'd need their secret — which they won't share. Their service signs the tokens.

What "alg=HS256 is forced" means

Even if you change "alg" in the header textarea, the tool overwrites it to "HS256" before signing. This is a safety measure: it prevents you from accidentally generating a "none" token (which would bypass verification on misconfigured servers) or a header that doesn't match the actual signing operation.

Common claim shapes

Standard auth token

{
  "iss": "https://auth.example.com",
  "sub": "user-12345",
  "aud": "api.example.com",
  "exp": 1735689600,
  "iat": 1735603200,
  "jti": "0e6f1b8c-2c33-4f1f-9c0b-2a3d4e5f6a7b"
}

Refresh token

{
  "iss": "https://auth.example.com",
  "sub": "user-12345",
  "exp": 1740960000,
  "iat": 1735603200,
  "type": "refresh"
}

OIDC ID token

{
  "iss": "https://accounts.google.com",
  "sub": "1234567890",
  "aud": "your-client-id",
  "exp": 1735689600,
  "iat": 1735603200,
  "email": "alice@example.com",
  "email_verified": true,
  "name": "Alice Example",
  "nonce": "abc123"
}

Programmatic equivalents

// Node.js with jose
import { SignJWT } from "jose";
const secretKey = new TextEncoder().encode(secret);
const jwt = await new SignJWT(payload)
  .setProtectedHeader({ alg: "HS256" })
  .sign(secretKey);

// Python with PyJWT
import jwt
token = jwt.encode(payload, secret, algorithm="HS256")

// Go with golang-jwt
import "github.com/golang-jwt/jwt/v5"
token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
signed, _ := token.SignedString([]byte(secret))