JWT Debugger & Signature Verifier
Decode, inspect, and verify JSON Web Tokens (JWT) in real time with HMAC-SHA256 signature verification and complete browser privacy.
Browser onlyYour data never leaves this device.
100% private in-browser decoding. Your JWTs, private claims, authorization tokens, and HMAC secrets are processed strictly on your device and are never transmitted to any server.
Details: Privacy policy
Paste your Bearer token or ID token. Formatting: Header.Payload.Signature
Verify Signature (HS256)
Test signature validity by entering your HMAC secret key. Computed 100% locally via Web Crypto API.
Token Claims & Expiry
Active / Valid ✓Header: Algorithm & Token Type
{
"alg": "HS256",
"typ": "JWT"
}Payload: Data & Claims
{
"sub": "usr_9081123",
"name": "Alex Chen",
"email": "alex@example.com",
"role": "developer",
"iat": 1516239022,
"exp": 2500000000
}JWT Debugger & Signature Verifier is an interactive developer environment for inspecting JSON Web Tokens and verifying cryptographic signatures directly in your browser. Unlike basic decoders that only read raw claims, this tool combines Base64Url header and payload inspection with active HMAC-SHA256 (HS256) signature verification using the native Web Crypto API. Paste your token, review color-coded segments, inspect expiration and subject claims, and enter your secret key to verify whether the signature matches the token data with zero server transmission.
How to use JWT Debugger & Signature Verifier
- Paste your encoded JSON Web Token (JWT) string into the left input textarea or click a sample preset button.
- Review the automatically color-coded token breakdown highlighting the Header (red), Payload (purple), and Signature (blue).
- Inspect the decoded JSON panels on the right, noting formatted claims, user identities, and token expiration status.
- Optionally enter your HMAC secret key into the verification box to validate whether the HS256 signature matches the data.
- Copy individual decoded JSON objects or the verified token with a single click for your test scripts and documentation.
What you get
- Decoded JSON Header object displaying signing algorithm (alg) and token type (typ).
- Decoded JSON Payload object displaying all standard and custom registered claims.
- Computed token lifecycle status (Active / Expired) with UTC and relative time indicators.
- Client-side cryptographic HMAC-SHA256 signature verification status.
When to use JWT Debugger & Signature Verifier
- Web & API Development — Inspect Authorization Bearer headers to troubleshoot 401 Unauthorized and 403 Forbidden errors.
- Session Expiration Auditing — Confirm whether user session expiration (exp) and issued-at (iat) claims are configured correctly.
- Role-Based Access Control (RBAC) — Verify user permission scopes, tenant identifiers, and custom claims embedded in access tokens.
- Secret & Signature Validation — Test whether an API gateway and backend service share the exact same HMAC-SHA256 signing secret.
Frequently asked questions about JWT Debugger & Signature Verifier
Is it safe to paste private JWT tokens into this debugger?
Yes, 100%. All Base64Url decoding and HMAC signature checks execute entirely inside your local browser sandbox using standard JavaScript and the Web Crypto API. No token or secret is ever uploaded to a server.
Can anyone read the contents of a JWT token?
Yes. Standard JWTs are signed, not encrypted. The header and payload are simply Base64Url-encoded JSON strings that anyone can decode without knowing the secret key.
What does an expired JWT token mean?
When the 'exp' (expiration) Unix timestamp in the payload is in the past, API servers and gateways will reject requests with an HTTP 401 Unauthorized error until a new token is obtained via refresh token flows.
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