I built a browser-only JWT Creator & Signer — HS256/384/512, verify, expiry check, 77 tests

Debugging JWT authentication usually means copying tokens between tabs and tools. I built a free, browser-only JWT Creator & Signer — create, sign, and verify JWTs entirely in your browser using the Web Crypto API. Live Tool 👉 https://devnestio.pages.dev/jwt-creator/ What it does Create JWTs — edit header (alg, typ) and payload (any JSON) Sign with HMAC — HS256, HS384, or HS512 Quick claim buttons — insert sub , name , exp (+1h), iss with one click Generate random secrets — 256-bit hex secret via crypto.getRandomValues() Verify existing JWTs — paste any token and verify signature + expiry Color-coded output — header in red, payload in green, signature in blue 100% client-side — Web Crypto API, no server, your secrets stay local How signing works (Web Crypto API) const key = await crypto . subtle . importKey ( " raw " , new TextEncoder (). encode ( secret ), { name : " HMAC " , hash : " SHA-256 " }, false , [ " sign " ] ); const sig = await crypto . subtle . sign ( " HMAC " , key , new TextEncoder (). encode ( header + " . " + payload ) ); The output is base64url-encoded (replacing + → - , / → _ , stripping = padding) to form the final JWT. Why browser-only matters for a JWT tool JWT secrets are sensitive. Any tool that sends your signing secret to a server is a liability. This tool never sends anything — the Web Crypto API runs entirely inside your browser tab. Testing 77 tests, all passing ✅ Tests cover: Base64url encoding edge cases JWT structure (3-part dot-separated) HMAC algorithm mapping (HS256 → SHA-256 etc.) Expiry check (expired vs. valid tokens) Error states: invalid JSON payload, malformed JWT UI: claim insertion, secret toggle, copy, clear Web Crypto API usage verification All tools at devnestio.pages.dev — free browser-only developer utilities. Feedback welcome!