Dev

JWT Builder & Verifier

Q: Which key formats are accepted?

Private keys as PKCS#8 PEM (BEGIN PRIVATE KEY), public keys as SPKI PEM (BEGIN PUBLIC KEY), and JWK or JWKS JSON. Legacy BEGIN RSA PRIVATE KEY / BEGIN EC PRIVATE KEY files are not understood by Web Crypto — convert with openssl pkcs8 -topk8 -nocrypt -in key.pem. X.509 certificates are not parsed.

Q: What do “expired” and “not yet valid” mean?

exp is the moment after which a token must be rejected, nbf the moment before which it must not be accepted, and iat when it was issued — all Unix seconds. The badges compare them with your device clock plus the skew tolerance from Settings. A valid signature on an expired token is still shown as valid signature + expired, because those are separate checks.

Q: Why is alg: none flagged?

A token with alg: none is unsigned; libraries that accept it have been the source of real breaches. The tool decodes it but marks it as unsafe, and it will never report a “valid signature” for it.

Q: How do ES256 signatures compare with openssl output?

JWS uses the raw r || s form (64 bytes for P-256), which is also what Web Crypto produces, so tokens made here verify in jose, PyJWT, jsonwebtoken and friends. openssl prints DER-encoded signatures, which is a different wrapper around the same values.

Sign, decode and verify JSON Web Tokens with HS, RS, PS and ES algorithms — secrets and keys never leave your browser.

Token —

Mode

Algorithm

Key ID (kid)optional

Payload (JSON)click a claim to insert it

Secretshared HMAC secret

Result —

The signed or decoded token appears here

Settings No clock skew · UTF-8 secret

Clock skew tolerance (seconds)

HMAC secret is

Pretty-print JSON in the payload box after inserting a claim

How It Works

How to Use JWT Builder & Verifier in 3 Steps

Build a token

Pick an algorithm, write the payload as JSON (click iat, exp, jti… to insert standard claims), and provide a secret for HS* or a PKCS#8 private key for RS*, PS* and ES*. No key yet? Generate key pair creates one in the browser and shows the matching public key. The token is signed live with Web Crypto.

Verify a token

Switch to Verify & decode and paste a token. Header and payload are decoded immediately, with exp / nbf / iat shown as readable dates and status badges. Add the HMAC secret, a PEM public key or a JWK / JWKS document to check the signature — the right key in a JWKS is picked by kid.

Copy or download

Copy puts the token (or decoded JSON) on the clipboard. Download the token, the decoded JSON, or the generated private / public key as PEM files.

FAQ

JWT Builder & Verifier — Frequently Asked Questions

Are my secrets and keys safe here?

They never leave the tab: signing and verification use the browser’s built-in crypto.subtle, nothing is stored in localStorage, and nothing is sent to Toolity. Still, treat anything you paste into any web page as sensitive — rotate test secrets before production.

Which key formats are accepted?

Private keys as PKCS#8 PEM (BEGIN PRIVATE KEY), public keys as SPKI PEM (BEGIN PUBLIC KEY), and JWK or JWKS JSON. Legacy BEGIN RSA PRIVATE KEY / BEGIN EC PRIVATE KEY files are not understood by Web Crypto — convert with openssl pkcs8 -topk8 -nocrypt -in key.pem. X.509 certificates are not parsed.

What do “expired” and “not yet valid” mean?

exp is the moment after which a token must be rejected, nbf the moment before which it must not be accepted, and iat when it was issued — all Unix seconds. The badges compare them with your device clock plus the skew tolerance from Settings. A valid signature on an expired token is still shown as valid signature + expired, because those are separate checks.

Why is alg: none flagged?

A token with alg: none is unsigned; libraries that accept it have been the source of real breaches. The tool decodes it but marks it as unsafe, and it will never report a “valid signature” for it.

How do ES256 signatures compare with openssl output?

JWS uses the raw r || s form (64 bytes for P-256), which is also what Web Crypto produces, so tokens made here verify in jose, PyJWT, jsonwebtoken and friends. openssl prints DER-encoded signatures, which is a different wrapper around the same values.

Can I decode a token without the key?

Yes — header and payload are only base64url-encoded, not encrypted, so anyone can read them. Verification needs the key; without it the signature badge says “unverified”. Encrypted tokens (JWE, five segments) show their header only.