Add README with architecture overview

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

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+# Cipher
+
+End-to-end encrypted messenger — a WhatsApp/Signal-style app running entirely on Cloudflare.
+
+**Live:** https://cipher.theradicalparty.com
+
+## Design
+
+The server is a **dumb key directory + encrypted relay**. It never sees message
+plaintext or private keys — only public keys and ciphertext. Real-time delivery
+plus offline store-and-forward, so messages arrive even when the recipient is away.
+
+| Concern | How |
+|---|---|
+| Encryption | **Signal Protocol** (X3DH + Double Ratchet) via libsignal, in the browser |
+| Real-time + offline queue | Per-user **Mailbox Durable Object** (WebSocket + SQLite queue) |
+| Key directory | **D1** — users, devices, prekey bundles (public keys only) |
+| Media | Client-side AES-GCM, ciphertext stored in **R2**; key travels inside the E2E message |
+| API | **Hono** on Workers |
+| Client | Installable **PWA**, IndexedDB key store, Web Push wake-up |
+| Identity | Username + password; QR device-linking planned |
+
+Private keys are generated on the device and stored only in IndexedDB. They are
+never transmitted. See `docs/` notes inline in each source file.
+
+## Architecture
+
+```
+ Alice (PWA)                    Cloudflare Worker                 Bob (PWA)
+ ┌─────────────┐   publish pubkeys   ┌──────────────┐
+ │ libsignal   │ ──────────────────► │ D1 key dir   │
+ │ IndexedDB   │ ◄── prekey bundle ─ │              │
+ └─────┬───────┘                     └──────────────┘
+       │ encrypt (X3DH/ratchet)
+       │ POST ciphertext             ┌──────────────┐   WS push    ┌──────────┐
+       └───────────────────────────► │ Mailbox DO   │ ───────────► │ decrypt  │
+                                      │ (queue+WS)   │   or queue   │ locally  │
+       encrypt media (AES-GCM) ─► R2 └──────────────┘              └──────────┘
+```
+
+## Develop
+
+```bash
+npm install
+npm run dev                 # local Worker + assets
+npm run db:migrate:local    # apply schema to local D1
+```
+
+## Deploy
+
+Push to `main` → Forgejo post-receive hook runs `wrangler deploy`.
+
+```bash
+git pushall main            # pushes to Forgejo (origin) + GitHub (backup)
+```
+
+## Vendored crypto
+
+`public/js/vendor/libsignal.js` is a self-contained browser bundle of
+`@privacyresearch/libsignal-protocol-typescript` (built with esbuild + a Buffer
+shim). Rebuild with the esbuild command noted in the commit history if bumping
+the library — loading crypto from a CDN at runtime is intentionally avoided.
+
+## Status
+
+Working: registration, login, key directory, E2E text + media, real-time and
+offline delivery (both verified live). Planned: QR multi-device linking, group
+chats (sender keys / MLS), WebRTC voice/video, Web Push fan-out, delivery/read
+receipts, disappearing messages.