JWT sign
Build and sign a JSON Web Token from a payload (and optional header) — HS256/384/512, RS256/384/512 or ES256/384. Runs in your browser; your secret and private key never leave your device.
About this tool
JWT sign builds a JSON Web Token (JWT — the JWS compact serialization)
from your payload (the claims set) and an optional header, then signs it
with the algorithm you choose. The result is the familiar
header.payload.signature string that you can hand to an API or store in a
session.
- HMAC (shared secret):
HS256,HS384,HS512— sign with any secret string. The same secret verifies the token. - RSA (private key):
RS256,RS384,RS512— RSASSA-PKCS#1 v1.5. Paste a PEM private key (PKCS#8-----BEGIN PRIVATE KEY-----or PKCS#1-----BEGIN RSA PRIVATE KEY-----); verify with the matching public key. - ECDSA (private key):
ES256(P-256),ES384(P-384) — JWS rawr‖ssignatures. Paste a PEM PKCS#8 EC private key.
The alg header is always set from the algorithm you pick, and typ defaults to
JWT unless your header overrides it.
Privacy
Everything runs in your browser via WebAssembly — your secret, private key, and claims are never uploaded to a server. You can also run it from the gizza CLI or inside a gizza chat.
Notes
- The payload and header must each be a JSON object. Standard registered
claims include
iss,sub,aud,exp(expiry, seconds since epoch),nbf,iat, andjti— but any keys are allowed. - For HS* the secret is taken as raw UTF-8 bytes (the same way most JWT libraries treat a string secret).
- Verify the token with any standard JWT library using the same algorithm and the secret (HS*) or the matching public key (RS*/ES*). Need a key pair? See the RSA / ECDSA key-pair generator tools.
FAQ
Can I add extra header fields like "kid"?
Yes — paste a JSON object into the header field (e.g. {"kid":"2024-key-1"}) and it's merged in. Two fields are managed for you: alg is always overwritten with the algorithm you selected, and typ defaults to JWT unless your header sets its own value.
What key format do RS256 and ES256 expect?
A PEM private key pasted into the secret field. RSA accepts both PKCS#8 (BEGIN PRIVATE KEY) and PKCS#1 (BEGIN RSA PRIVATE KEY); ECDSA requires PKCS#8 with a P-256 key for ES256 or P-384 for ES384. Verification is done elsewhere with the matching public key.
Does the tool add exp or iat claims automatically?
No — the payload is signed exactly as you wrote it. If you want an expiry, add "exp" yourself as seconds since the Unix epoch (e.g. {"sub":"alice","exp":1767225600}). That keeps the tool predictable for testing tokens with any claim combination.
Why does my token verify differently than in some other library?
Check three things: the algorithm must match exactly (HS256 vs HS512 tokens are incompatible), the HS* secret is taken as raw UTF-8 bytes (not base64-decoded — some libraries offer a "secret is base64" toggle), and ES* signatures use the JWS raw r‖s format, not DER.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool jwt-sign '{"sub":"1234567890","name":"Ada Lovelace","iat":1893456000}' 'secret=your-256-bit-secret (or paste a PEM private key for RS*/ES*)'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/jwt-sign/?payload=%7B%22sub%22%3A%221234567890%22%2C%22name%22%3A%22Ada%20Lovelace%22%2C%22iat%22%3A1893456000%7D&secret=your-256-bit-secret%20%20%20%28or%20paste%20a%20PEM%20private%20key%20for%20RS%2A%2FES%2A%29&algorithm=HS256&header=%7B%22kid%22%3A%22key-1%22%7DMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.