JWT Signature Verifier
Verify JSON Web Token signatures securely in your browser. Powered entirely by the native Web Crypto API — your sensitive tokens and secrets never leave your device.
How ZeroData protects your privacy
- ✓ No Uploads: Processing happens entirely via client-side JavaScript.
- ✓ No Storage: We do not have a database. We physically cannot save your data.
- ✓ No Tracking: We don't log what you process or track your inputs.
- ✓ Verifiable: Check your DevTools Network tab. You will see 0 outbound requests.
100% Local JWT Signature Verification
JSON Web Tokens (JWT) are widely used to secure modern web applications and microservice APIs. However, because JWTs are base64-encoded rather than encrypted, anyone who intercepts a token can read its payload. The security of a JWT relies entirely on its cryptographic signature. If the signature matches the secret or public key of the issuer, the recipient can be sure that the claims in the token have not been tampered with.
Most online JWT tools require you to paste your secret key or public key into a web page that transmits that data to a remote backend server. If the tool is compromised, or its owner logs inputs, your application security is completely breached. ZeroData Tools solves this by using the browser's native Web Crypto API (crypto.subtle) to perform all cryptographic operations locally. Your keys are processed inside your browser sandbox and are never sent over the network.
Symmetric and Asymmetric Signature Support
This verifier supports both symmetric signing algorithms (HMAC) and asymmetric signing algorithms (RSA and ECDSA):
- HMAC (HS256, HS384, HS512): A single symmetric secret key is shared between the party generating the token and the party verifying it. Simply enter your secret key to check the signature.
- RSA (RS256, RS384, RS512): Uses a public/private key pair. Paste the public key in standard PEM format (starting with
-----BEGIN PUBLIC KEY-----) or in standard JWK (JSON Web Key) format to verify signatures. - ECDSA (ES256, ES384, ES512): Elliptic Curve Digital Signature Algorithm. Paste your Elliptic Curve public key (PEM or JWK) to verify signatures using curves like P-256, P-384, or P-521.
Interlinked Security Tools
Need to generate test tokens locally? Try our JWT Generator to create HS256 signed tokens securely. Or inspect expiration timestamps in real time with our JWT Expiry Checker.
How to Use the JWT Signature Verifier
- Paste your complete JSON Web Token (header.payload.signature) into the input box.
- The tool will automatically parse the token parts and detect the signature algorithm (e.g. HS256, RS256).
- Enter the symmetric HMAC secret or paste the public key (PEM or JWK) in the appropriate field.
- The native Web Crypto engine will instantly verify the signature and display the cryptographic status.
Common Use Cases
- Cryptographically verifying JWTs generated during local development or integration tests.
- Checking if a suspicious or expired token has been tampered with without exposing it to third-party servers.
- Validating public key pairings (PEM or JWK) against asymmetric tokens locally.
- Inspecting JWT payload claims while simultaneously verifying their integrity in a high-privacy environment.
Frequently Asked Questions
How do I verify a JWT signature?
Paste your JSON Web Token (JWT) into the input area. The tool automatically decodes the Header and Payload, parses the 'alg' header claim, and displays input fields. For HS256/384/512 tokens, enter your symmetric HMAC secret. For RS256/384/512 or ES256/384/512 tokens, paste your public key in PEM (spki) or JSON Web Key (JWK) format. The verification status updates instantly.
Is it safe to paste my JWT key or secret online?
Usually, NO. Most online JWT tools upload your sensitive tokens, private keys, or symmetric secrets to their backend servers, exposing them to logging or interception. ZeroData Tools is different: all cryptographic parsing and verification are performed 100% locally in your browser using the native Web Crypto API. Your keys, tokens, and secrets never leave your device.
Can I verify an RS256 or ES256 JWT signature offline?
Yes, absolutely! Since our tools run entirely client-side using standard Web APIs, you can save this page or disconnect from the internet and continue to cryptographically verify signatures securely in a completely offline environment.
What is the difference between a JWT signature and the payload?
The payload contains the actual claims and data (such as user ID, name, permissions, and expiration) encoded in base64url format, which anyone can read. The signature is created by signing the header and payload together with a secret or private key. Verifying the signature ensures the payload has not been tampered with or altered since it was issued.
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