JWT Signature Verifier & Key Validator

Cryptographically verify JSON Web Token signatures, inspect header claims, and test secret keys in your browser using the Web Crypto API.

At a glance

JWT Signature Verifier & Key Validator is a free web tool that verifies JWT token signatures online using HMAC secrets or public keys. It supports plain text UTF-8, Base64 binary secrets, and Hex-encoded keys. Hardware-accelerated cryptographic verification for HS256, HS384, and HS512. Your inputs never leave your device, and there is no sign-up or install.

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100% In-Browser JWT Signature Verifier & Claims Inspector

Verify JSON Web Token signatures using HMAC (HS256, HS384, HS512) via Web Crypto API. Your auth tokens and secret keys never leave your device.

Zero Server Calls Web Crypto Subtle

Load Presets:

Encoded JWT Token

0 chars

Header Payload Signature

HMAC Secret Key

Shared Secret (HMAC Key)

Leave empty to inspect claims without signature verification, or provide secret to test validity.

Validation Rules

Ignore Expiration (exp)

Clock Drift: sec

Paste a JWT to inspect & verify

Supports HMAC signatures (HS256, HS384, HS512) and standard claim timestamps.

Awaiting Input

Signature: Not Checked

Expiration: Not Checked

Decoded Header

{\n  /* Waiting for token */\n}

Decoded Payload

{\n  /* Waiting for token */\n}

Signature Component

(No signature parsed)

Why Use This JWT Signature Verifier & Key Validator?

100% In-Browser Privacy: Secret keys and sensitive authentication payloads never touch external servers.

Web Crypto HMAC Engine: Hardware-accelerated cryptographic verification for HS256, HS384, and HS512.

Real-Time Claims Inspection: Decodes Unix epoch timestamps (iat, exp, nbf) into human-readable local dates.

Flexible Secret Encodings: Supports plain text UTF-8, Base64 binary secrets, and Hex-encoded keys.

How to Use JWT Signature Verifier & Key Validator

Paste Encoded JWT Token

Paste your 3-segment JSON Web Token (header.payload.signature) into the token input area.

Enter HMAC Secret Key

Provide the secret key used to sign the token and choose the corresponding encoding format (UTF-8, Base64, or Hex).

Inspect & Verify

Review the signature verification status banner, check expiration status, and inspect decoded JSON headers and claims.

Verify HS256 JWT Authentication Token

Cryptographically verifies the HMAC SHA-256 signature against the shared secret key, decodes token header and payload claims, and validates expiration timestamp.

JWT Token & Secret Source

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyLCJyb2xlIjoiYWRtaW4ifQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Verification Result Processed

{
  "valid": true,
  "signatureValid": true,
  "algorithm": "HS256",
  "payload": {
    "sub": "1234567890",
    "name": "John Doe",
    "role": "admin"
  }
}

Technical Specifications & Engine Details

Technical specifications and engine details for JWT Signature Verifier & Key Validator
Specification	Details
Supported Formats	TEXT/PLAIN, APPLICATION/JWT, .JWT, .TXT → APPLICATION/JSON, TEXT/PLAIN, .JSON
Execution Mode	100% Client-Side In-Browser (client)
Browser Requirements	Modern browser with Web Crypto API support.
Performance Constraints	Runs client-side in browser memory using Web Crypto Subtle. Secret keys and token payloads never leave device memory.

Maintained by Pragyan Pahadi

100% Client-Side Privacy Guarantee

Your tokens and secret keys remain strictly on your local device. Zero data is transmitted to external servers.

Frequently Asked Questions

Is it safe to paste production JWT tokens and secret keys into this tool?

Yes! All decoding and cryptographic verification happens 100% locally inside your browser using the Web Crypto API. No data or secrets are ever sent to any backend server.

Which signing algorithms are supported?

The verifier supports HMAC SHA-256 (HS256), HMAC SHA-384 (HS384), and HMAC SHA-512 (HS512) algorithms with UTF-8, Base64, and Hex secret key formats.

Why does the tool show 'Signature Authentic (Token Expired)'?

This means your secret key is correct and matches the signature, but the token's 'exp' timestamp has passed. You can check 'Ignore Expiration' to inspect claims without time gating.

Methodology

Rankings and recommendations here are never paid for.

Last updated October 2026.