FadeKey: zero-knowledge, self-destructing secret sharing engine

The Comeback Story

We resurrected an unfinished prototype that had major security flaws, incomplete features, and a leaking state. Here is our before-and-after journey:

1. Active Revocation (SDK & API Core)

• Before: Secrets could only expire via TTL or view limits. The backend lacked a deletion endpoint, and the @fadekey/sdk had an empty placeholder where revoke() should have been.


• After: We built a secure DELETE /api/items/:id route in the Fastify API (with owner verification checks) and implemented the corresponding revoke(id) method in the SDK package.
  ### 2. Zero-Knowledge Compliance for QR Codes (UX/Security)


• Before: The prototype suggested using external public APIs to generate QR codes. However, sending the full secret URL (containing the #key hash) to a third-party server violates zero-knowledge compliance.


• After: We moved 100% of the QR code generation to the browser using local client-side rendering with the qrcode library, ensuring the decryption key never leaves the client.
  ### 3. Local-to-Cloud Preference Syncing


• Before: Users lost their theme (dark/light) and locale (en/es/pt-BR) settings upon page reload, and there was no way to persist these preferences for registered users.


• After: We built a dual-persistence system. Anonymous users store preferences in cookies/localStorage, while authenticated users sync their choices automatically to a PostgreSQL database via a new PATCH /api/auth/preferences endpoint.
  ### 4. Critical Nuxt SSR Session Leaks (Major Bugfix)


• Before: The authentication state was stored in a global reactive singleton. During Server-Side Rendering (SSR), this leaked the session/token of one user to subsequent requests from completely different visitors, causing security risks and hydration errors.


• After: We refactored the auth composable to store sessions in request-isolated Nuxt useState wrappers, guaranteeing sandboxed memory for every client.
  ### 5. Transition Polish & Redirection Guarding


• Before: Logging out caused dashboard pages to immediately display blank values and zeroes before redirecting. Redirections also dropped the active language prefix (e.g. redirecting /pt-BR/app/dashboard to / instead of /pt-BR).

* After: We deferred state destruction in signOut() until after the router navigation completes, backed by a nextTick + 150ms delay in router.afterEach to allow the DOM to mount. We wrapped all redirects in localePath() to ensure language consistency.

My Experience with GitHub Copilot

GitHub Copilot acted as a core partner during this sprint, particularly in:

1. Refactoring Auth for SSR Safety: Copilot recognized the risks of our initial module-level singleton state in Nuxt and drafted the migration to request-scoped useState hooks, pointing out how to avoid server-side data pollution.


2. Writing Vitest Suites with Testcontainers: It accelerated the creation of our backend integration tests, generating mock Postgres databases and Redis instances using Docker Testcontainers to test rate-limiting and quota rollback scenarios.


3. Client-side Encryption Pipelines: Copilot assisted in structuring the Web Crypto API calls in our SDK to correctly derive password hashes using PBKDF2 without introducing synchronous blocking.