Kavach

Protection for those who cannot verify. Proof for those who can. Kavach is a zero-knowledge shield protecting seniors from fraud with pre-ring call drops & a RevenueCat multi-seat model.

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Inspiration

I am a 13-year-old student builder entering the RevenueCat Shipaton 2026 — Next Gen Award.

Every year, predatory fraudsters steal over $10 Billion from seniors worldwide. But the financial theft is only half the tragedy. The psychological shame is devastating: elderly parents stop trusting their phones, their banks, and even their own families.

One morning, watching news of "Digital Arrest" scams and bank-freeze frauds targeting elderly grandparents, I imagined an all-too-common nightmare: 11:40 AM. My grandmother's phone lights up. "Bank account FROZEN. Police warrant issued. Share OTP now or police will arrive in 30 minutes." In that moment of intense psychological panic, seniors freeze.

Most anti-scam tools are reactive chatbots that tell you what you did wrong after your savings are stolen. Others demand invasive permissions to spy on confidential calls and private messages.

As a student passionate about applied cryptography and systems programming, I decided to build Kavach (कवच — Shield) to solve this crisis with dignity: protection for those who cannot verify, and cryptographic proof for the family members who can — without ever uploading call audio or violating privacy.

What it does

Built specifically as a Next Gen project, Kavach is a two-sided, zero-knowledge fraud defense operating layer split into two dedicated Android applications built from a single codebase:

  1. 🧓 Kavach Senior (.senior): A calm, distraction-free sanctuary designed for elderly parents. Featuring 64dp WCAG AAA one-thumb touch targets, a 1-tap Hindi/English toggle, and zero technical jargon. When a scam SMS arrives, it never buzzes. Instead, Kavach silently quarantines the threat on-device or triggers an emergency bilingual voice siren: "रुको! OTP मत दो" (Stop! Do not give the OTP).
  2. 🏠 Kavach Family (.manager): A dark Guardian Command Center for adult children. Displays a real-time Household Safety Score (0–100), evidence-chained threat cards with masked OTPs (******), and a 1-tap Pre-Ring Call Killer that terminates repeat scam callers via Android's native CallScreeningService before ring #1.
  3. 💎 RevenueCat Multi-Seat Monetization: Powered by the modern RevenueCat SDK 10.23.2. Free for seniors; financed by worrying adult children through a multi-seat household subscription ($79.99/yr Family Fortress, protecting up to 3 parent devices). Includes dynamic remote paywalls, entitlement-gated access (sheild_protection), and Customer Center support.

How I built it

As a 13-year-old solo student developer, I architected and coded the entire system end-to-end across mobile and cloud:

  • Native Android (Kotlin & Jetpack): Two dedicated flavor APKs (senior and manager). Integrated Android's native CallScreeningService for 0-ring call rejection and background SmsReceiver with asynchronous triage.
  • Hardware-Sealed Cryptography (Google Tink): End-to-end encrypted alerts using ECIES_P256_HKDF_HMAC_SHA256_AES128_GCM backed by Android Keystore. Devices verify mutual trust via a visual 6-Emoji Short Authentication String (SAS) derived from public key fingerprints.
  • Zero-Knowledge Cloud Relay (FastAPI & Python): Hosted live on Render. The server holds nothing but noise and salted SHA-256 hashes (SHA-256("kavach|household|number")). Plaintext phone numbers and OTPs never touch the database.
  • RevenueCat In-App Purchases: Dynamic offerings fetched via Purchases.sharedInstance.getOfferings() with remote package display, paywallActivityLauncher.launchIfNeeded(requiredEntitlementIdentifier = "sheild_protection"), server-side signed webhook reconciliation (POST /api/v1/billing/webhook), and a zero-card sandbox unlock code (SHIPATON-JUDGE) for competition judges.
  • Self-Updating Signed Rules: Deterministic regex rulepacks signed with Ed25519 (GET /api/v1/rules/pack) that auto-update daily. If 3 independent households report the same scammer hash, it automatically federates to the community pre-ring blocklist.

Challenges I faced

  1. Protecting Telecom Call Screening Latency: Android's CallScreeningService requires decision-making within milliseconds. Calling external cloud APIs during an incoming phone call is impossible. I engineered an O(1) in-memory hash set cache backed by a thread-safe LocalStore and background ExecutorService so call termination decisions execute in under 5 milliseconds.
  2. True Zero-Knowledge vs. Usable Evidence: Guardians need to know why a message was flagged, but the relay server must never see the message. By implementing Google Tink ECIES asymmetric encryption directly between the parent's and child's device public keys, the relay server acts as a blind mailbox passing opaque ciphertext.
  3. SMS Multipart Reassembly in Android Background: In modern Android, long scam messages arrive across multiple PDUs. Handling them without ANR required implementing goAsync() on the broadcast receiver with an atomic chunk assembly cache.

Accomplishments that I'm proud of

  • Building a Complete Production System as a Solo 13-Year-Old Student: Taking an idea from scratch to full-stack implementation across native Kotlin, Android Telecom, Python, cryptography, Docker, and RevenueCat SDK.
  • 61 Automated Tests Passing 100%: Complete test coverage across race conditions, cryptographic tampering, rate limiters, and corrupt-state degradation.
  • 4-Job GitHub Actions CI: Every push builds release APKs for Senior and Manager, runs Python linting, executes the full test suite, and builds Docker containers.
  • Production-Grade RevenueCat Architecture: Built a zero-marginal-cost business model where 95% of inference runs locally on-device for free, while RevenueCat subscriptions sustainably fund cloud relays and multi-parent device seats.
  • Dignity-Centered Design: Built with genuine empathy for elderly victims — zero victim-blaming copy, zero intrusive GPS tracking, and complete offline autonomy.

What I learned

  • How to design and structure in-app subscription architectures using RevenueCat's modern SDK, remote offerings, and webhook lifecycles.
  • Real-world mobile cryptography using Google Tink and hardware-backed Android Keystores.
  • Deep Android Telecom internals, telecom call screening, and broadcast receiver lifecycle constraints.

What's next for Kavach

  • Android Jetpack Room & WorkManager: Migrating local caching to Room DB and background sync to WorkManager.
  • iOS Companion App in SwiftUI: Building a native iOS Guardian console so iPhone-using adult children can protect Android-using parents.
  • Expanded Telecom Partnerships: Working with carrier spam registries to feed anonymous community hashes back into telecom-level filtering.

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