docs: add privacy policy for Chrome Web Store (#7)

Author: 0xneves

PRIVACY.md

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+# Privacy Policy
+
+**Quack - Universal Web Encryption**
+
+*Last updated: February 7, 2026*
+
+## Summary
+
+**Quack does not collect, transmit, or share any user data.** Everything stays on your device.
+
+## Data Storage
+
+Quack stores the following data **locally on your device** using Chrome's storage API:
+
+- **Cryptographic keys** — Your personal keys, contact public keys, and group keys
+- **Vault data** — Encrypted with your master password using AES-256-GCM
+- **Settings** — Your preferences (auto-lock timer, stealth mode toggle, etc.)
+
+This data is:
+- ✅ Encrypted at rest with your master password
+- ✅ Stored only in your browser's local storage
+- ✅ Never transmitted to any server
+- ✅ Never accessible to us or any third party
+
+## Data Collection
+
+**We collect nothing.** Specifically:
+
+- ❌ No personal information
+- ❌ No usage analytics
+- ❌ No browsing history
+- ❌ No message content (encrypted or decrypted)
+- ❌ No IP addresses
+- ❌ No cookies or tracking
+- ❌ No telemetry
+
+## Network Activity
+
+Quack makes **zero network requests**. All cryptographic operations happen entirely on your device. Your encrypted messages travel only through whatever platform you paste them into (YouTube, Twitter, etc.) — we never see them.
+
+## Permissions Explained
+
+| Permission | Why We Need It |
+|------------|----------------|
+| `storage` | Store your encrypted vault locally |
+| `clipboardWrite` | Copy encrypted messages for you to paste |
+| `activeTab` | Detect trigger phrase and scan for encrypted messages |
+| `sidePanel` | Provide Side Panel UI option |
+| `<all_urls>` | Enable encryption on any website you visit |
+
+## Third-Party Services
+
+Quack uses **no third-party services**. No analytics, no crash reporting, no external APIs.
+
+## Data Sharing
+
+We do not share any data because we do not have any data. Your keys and messages exist only on your device.
+
+## Data Retention
+
+All data remains on your device until you:
+- Uninstall the extension
+- Clear browser data
+- Manually delete your vault
+
+## Security
+
+- **Encryption:** AES-256-GCM for vault and messages
+- **Key Exchange:** ML-KEM-768 (post-quantum, NIST FIPS 203)
+- **Key Derivation:** PBKDF2 with 100,000 iterations
+- **Session Security:** Keys held in memory only while unlocked
+
+## Children's Privacy
+
+Quack does not knowingly collect any information from anyone, including children under 13.
+
+## Changes to This Policy
+
+If we ever change this policy, we will update the "Last updated" date above. Any changes will be reflected in the extension's GitHub repository.
+
+## Open Source
+
+Quack is open source. You can verify our privacy practices by reviewing the code:
+https://github.com/0xneves/quack
+
+## Contact
+
+For privacy concerns or questions:
+- GitHub Issues: https://github.com/0xneves/quack/issues
+- Author: [@0xneves](https://github.com/0xneves)
+
+---
+
+**The short version:** We built Quack for privacy. We don't want your data. We can't see your data. Your secrets are yours.