How I built a zero-dependency, 100% client-side JWT Verifier using the Web Crypto API
The article describes the development of a client-side JWT verifier that operates entirely in the browser without relying on external dependencies or backend services. It leverages the Web Crypto API to decode and verify JWTs locally, ensuring sensitive token data is not exposed to third parties. The tool supports both HMAC and RSA signature verification, providing a secure alternative to online JWT inspection services.
- ▪The JWT verifier is built using only Vanilla JavaScript and the Web Crypto API.
- ▪It performs all operations client-side, eliminating the risk of leaking tokens to external servers.
- ▪The solution supports HS256 (HMAC) and RS256 (RSA) signature verification methods.
- ▪Base64URL decoding is implemented manually to handle JWT format without libraries.
- ▪The tool is part of ToolsMatic, aimed at providing privacy-first developer utilities.
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try { if(localStorage) { let currentUser = localStorage.getItem('current_user'); if (currentUser) { currentUser = JSON.parse(currentUser); if (currentUser.id === 3908449) { document.getElementById('article-show-container').classList.add('current-user-is-article-author'); } } } } catch (e) { console.error(e); } ToolsMatic Posted on May 2 How I built a zero-dependency, 100% client-side JWT Verifier using the Web Crypto API #security #webdev #javascript #tutorial JSON Web Tokens (JWTs) are everywhere. Whether you're debugging an OAuth flow, a rogue microservice, or a broken single-page application, inspecting a JWT is a daily task for most developers. But there's a massive, glaring problem with how we usually do it: We paste production tokens into random third-party websites.
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