Browser Proxy — Nym Wasm Client

BlindHop includes a browser-based proxy that runs the Nym mixnet client directly in your browser using WebAssembly. No local software installation needed — just open a URL.

How It Works

┌─────────────────────────────────────────────────────┐
│ Browser                                             │
│  ┌──────────┐    ┌──────────────────────────────┐   │
│  │ Demo UI  │───▶│ Nym Wasm Client (Web Worker) │   │
│  │ (main)   │◀───│ @nymproject/sdk-full-fat      │   │
│  └──────────┘    └──────────┬───────────────────┘   │
└─────────────────────────────┼───────────────────────┘
                              │ Sphinx packets
                              ▼
                    ┌───────────────────┐
                    │ Nym Mixnet (500+) │
                    └────────┬──────────┘
                             ▼
                    ┌───────────────────┐
                    │ blindhop-exit     │──▶ Substrate Node
                    │ (Oracle Cloud)    │
                    └───────────────────┘

The @nymproject/sdk-full-fat npm package bundles:

• A Nym mixnet client compiled to WebAssembly
• Sphinx packet creation and parsing
• Gateway connection management
• SURB (Single Use Reply Block) handling for responses

All of this runs in a Web Worker to keep the main UI thread responsive.

Connecting to the Mixnet

When you open the demo, it connects directly through the mixnet without requiring any local software installation:

1. In-Browser Client (Default): Runs the Nym Wasm client directly inside your browser. By default, the demo never touches localhost and never reveals your IP address to full nodes.
2. Local Proxy (Advanced Opt-in): If you tick the "Use my local BlindHop proxy" checkbox, the demo will attempt to connect to ws://127.0.0.1:9500. (Note: because browsers attach Origin headers, your local proxy must be launched with --allowed-origin https://demo.blindhop.wtf).

The connection mode badge in the header shows which mode is active:

• 🌐 Browser (Nym Wasm) — purple badge (default)
• 🖥️ Native Proxy — blue badge (when local proxy is opted in and connected)

Privacy Protections in the Browser Demo

• No Silent Fallback (Fail-Closed): If the Nym client fails to initialize or connect, querying stops with an error. It never silently falls back to a direct connection that would reveal your IP address.
• Opt-in Direct Comparison: The "Compare with direct" toggle (for measuring mixnet latency overhead vs a direct RPC query) is off by default and clearly flagged, as sending comparison queries connects directly to the Substrate full node from your IP.
• Decompression Support: When the browser supports DecompressionStream('deflate-raw'), the browser client signals 0x40 (accepts compression) and transparently decompresses large responses (e.g. state_getMetadata), bounded by an 8 MiB decompression limit.

Limitations vs Native Proxy

Feature	Native Proxy	Browser Proxy
None (direct)	✅	✅
Fast (2-hop dVPN)	✅	❌
Full (5-hop mixnet)	✅	✅
Cover traffic	✅	✅
Cold start time	~2s	~5-10s (Wasm init)
Memory usage	~30 MB	~50-80 MB
Works offline	After initial Nym connect	After initial Nym connect

The Fast (2-hop) mode is not available in the browser because the Nym Wasm SDK doesn't expose low-level hop control. When in browser mode, the Fast option is greyed out with a tooltip explaining this.

Self-Hosting the Demo

cd demo/
npm install
npm run build    # Vite builds to dist/

# Deploy dist/ to any static host:
# Vercel, Netlify, GitHub Pages, etc.

Important: COOP/COEP Headers

The Nym Wasm SDK uses SharedArrayBuffer, which requires these HTTP headers:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

These are configured in vercel.json and vite.config.js for local dev and Vercel deployment. If self-hosting elsewhere, ensure your server sets these headers.

Security Considerations

• Exit address is public — it's like a server address, not a secret
• Your Nym client address is ephemeral — a new address is generated each session
• Wasm execution — the Nym client runs sandboxed in a Web Worker
• No local storage of keys — keys are ephemeral and discarded on page close
• CSP compatibility — the sdk-full-fat package inlines workers and Wasm, so it works with strict CSP policies

Performance Characteristics

Metric	Typical Value
Wasm initialization	3-8 seconds
Nym gateway connection	2-5 seconds
First message round-trip	5-15 seconds (includes setup)
Steady-state RTT (Full mode)	1-3 seconds
Browser memory usage	50-80 MB
Wasm binary size	~2-4 MB (gzipped)