JWT Decoder, Encoder & Signature Debugger
Encode, decode, parse, and verify JSON Web Tokens (JWT) directly in your browser. Inspect encoded headers, verify payload claims and expiration timestamps, and sign tokens securely using the browser Web Crypto API.
What is a JSON Web Token (JWT)?
A JSON Web Token (JWT) is an open, industry-standard (RFC 7519) compact and self-contained URL-safe string used for securely transmitting claims between two parties—most commonly an authentication server and a client application. A JWT consists of three parts separated by dots (.): a Base64URL-encoded Header (defining the hashing algorithm and token type), an encoded Payload (holding user identity claims and expiration dates), and a cryptographic Signature that verifies the authenticity of the message.
NexTools JWT Decoder & Encoder executes entirely inside your client browser using native crypto.subtle APIs. This guarantees that your proprietary secrets, API payloads, and tokens never touch an intermediary server, solving security vulnerabilities present in standard online debuggers.
Key Capabilities & Features
Live Token Decomposition
Automatically parses tokens into syntax-highlighted Header, Payload, and Signature components with live character counters and error detection.
Real-Time Cryptographic Signing
Construct custom payloads and immediately sign tokens with HMAC-SHA256, HMAC-SHA384, or HMAC-SHA512 algorithms using browser WebCrypto.
Timestamp & Expiration Inspector
Translates UNIX epoch timestamps (exp, iat, nbf) into human-readable local time with active/expired relative status banners.
Air-Gapped Client-Side Security
Zero backend network requests. Private keys, company authorization tokens, and user PII never leave your browser sandbox.
How to Use the JWT Tool
To Decode an Existing Token:
Paste your Bearer token or raw JWT into the left textarea. The header and claims will automatically format into editable JSON on the right.
To Verify Signature Authenticity:
Enter your shared HMAC secret key in the cyan box. The validator immediately recalculates the cryptographic digest and confirms whether the signature matches.
To Generate & Sign a New JWT:
Switch to "Build & Sign Token" mode, customize claims (e.g. sub, roles, exp), select your algorithm, and copy the instantly generated token.
Frequently Asked Questions
Is it safe to paste production JWTs into this tool?
Yes. Unlike many online tools that send tokens to a backend server for parsing, NexTools executes all parsing, JSON decoding, and WebCrypto operations locally in your browser. You can inspect the network tab to verify zero external calls.
What algorithms are supported for signature verification?
The tool supports HMAC algorithms (HS256, HS384, HS512) and unsigned tokens (none). Cryptographic hashing uses the browser's hardware-accelerated window.crypto.subtle engine.
Why does my signature say "Invalid Signature"?
An invalid signature occurs if the payload or header was altered after generation, if an incorrect secret key was provided, or if the secret was base64-encoded on the issuing server but entered as plain text here.
What are standard JWT reserved claims?
Standard registered claims include iss (Issuer), sub (Subject), aud (Audience), exp (Expiration Time in seconds), nbf (Not Before), iat (Issued At), and jti (JWT ID).