As a developer and frequent traveler, I was frustrated with how cloud-based storage apps handle sensitive documents. Even “secure” apps can leak metadata, require accounts, or store your data on servers you don’t control.
That’s why I built Proof Pocket — a mobile app for storing photos and documents completely offline. Everything stays on your device, and users don’t have to trust any server.
🔒 Encryption & Security
To secure data on-device, I used ChaCha20-Poly1305, which provides both confidentiality and integrity. Here’s why I chose it:
Fast and lightweight — perfect for mobile devices
Authenticated encryption — protects both data and integrity
Widely regarded as secure — used in TLS, WireGuard, and other modern protocols
All files are encrypted locally, stored in the phone’s secure storage, and never leave the device. There’s no analytics, no tracking, and no hidden cloud sync.
💡 Offline-First Design
The app is designed to work entirely without internet access:
Users can add, view, and organize documents anytime
Everything is persisted securely on-device
No network dependency means true privacy
💰 Freemium Model
Proof Pocket is free to try:
Store up to 2 documents/photos for free
All features unlock via a yearly subscription or a one-time lifetime unlock
📱 Available On
Android: [Play Store Link]
iOS: [App Store Link]
📝 Lessons Learned
Offline-first apps force you to carefully manage local storage and encryption performance.
Choosing a fast, mobile-friendly cipher like ChaCha20-Poly1305 is crucial to avoid lag on lower-end devices.
AES is also fine as most mobile devices got AES hardware acceleration already, but as both of those were pretty fast I decided to go with the ChaCha having similar level of secureness.
I’d love to hear feedback from fellow developers and privacy enthusiasts — especially on UI, encryption approach, or potential improvements.
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