SunuSàv — Our Savings, Our Future

Digitizes tontines using Bitcoin’s Lightning Network multisig wallets to automate secure, transparent group savings via mobile money, enabling fast, low-cost financial inclusion for Senegalese people

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🧩 SunuSàv — Our Savings, Our Future

Powered by Bitcoin. Driven by Community. A decentralized digital tontine system built on the Bitcoin Lightning Network.

🌍 Inspiration

In Senegal and across West Africa, millions rely on tontines — traditional community savings circles — as their primary financial safety net. These systems are rooted in trust, accountability, and solidarity, yet remain vulnerable to human error, cash dependency, and inflation.

Our inspiration came from one simple observation during the Dakar Bitcoin Hackathon:

People already know how to save together — they just need a trustless digital system that respects that tradition.

SunuSàv (from “Sunu Saaf” meaning Our Savings) bridges this gap. It reimagines the tontine for the Bitcoin era — preserving its community-driven spirit while using Bitcoin and the Lightning Network to bring transparency, automation, and financial sovereignty.

⚡ What It Does

SunuSàv is a mobile-first decentralized tontine app built on Bitcoin and Lightning. It allows small groups to pool savings, verify contributions, and distribute payouts automatically — without banks, custodians, or middlemen.

Core Functions

Feature Description
💸 Decentralized Tontine Cycles Members contribute sats via Lightning invoices; pooled funds are distributed automatically to one participant each round.
🔐 Multi-Signature Escrow Funds are locked in 2-of-3 multi-sig wallets, preventing fraud or single-point control.
⚡ Instant, Low-Fee Transactions Lightning Network ensures contributions and payouts settle instantly with near-zero fees.
📱 Inclusive Mobile Access Works on Android (React Native) and via USSD/SMS for feature phones.
🌐 Transparency by Design Every payment is cryptographically verifiable and logged to a shared ledger viewable by all members.
📣 Nostr Notifications Members receive decentralized notifications for contributions, payouts, and reminders.

By blending traditional cooperation with programmable finance, SunuSàv transforms community savings into a trustless, borderless protocol for collective wealth.

🏗️ How We Built It

🧠 System Architecture

SunuSàv’s architecture merges:

  • Bitcoin Base Layer → Immutable settlement and security
  • Lightning Network → Instant microtransactions
Mobile App (React Native)
   ↓ HTTPS/WebSocket
Backend (FastAPI / Node.js)
   ↓ gRPC / LNURL
Lightning Node (LND / Core-Lightning)
   ↓
Bitcoin Base Layer

🔧 Key Components

Frontend

  • React Native app (multilingual: French + Wolof)
  • Offline-first design using WatermelonDB
  • Biometric authentication (react-native-biometrics)
  • QR / LNURL payments
  • USSD integration for low-bandwidth phones

Backend

  • FastAPI (Python) for API and contract orchestration
  • PostgreSQL for off-chain state and audit logs
  • Celery + Redis for scheduled payouts
  • BTCPayServer + LND gRPC for Lightning operations
  • Nostr relay integration for event broadcasting

Smart Contract Logic (off-chain)

  • 2-of-3 multisig scheme: organizer + rotating member + system key
  • Deterministic cycle management: verifies all contributions before triggering payouts
  • Automated audit trail: each event hashed and recorded to an immutable log

Sample Flow

  1. Group is created with 5 members and 10,000 sats weekly target.
  2. Each member pays via LNURL.
  3. System aggregates proofs-of-payment (preimage hashes).
  4. Smart contract releases pooled sats to this round’s winner via Lightning invoice.
  5. Ledger is updated and next cycle starts automatically.

🚧 Challenges We Ran Into

Challenge Solution
⚡ Lightning Node Stability Implemented retry logic and fallback nodes via BTCPayServer.
🔐 Multi-Signature Complexity Used BDK + LND for simplified 2-of-3 multisig with server-assisted signing.
📶 Offline & Low-Bandwidth Users Added local caching, SMS confirmations, and USSD fallback.
🌍 Language & Accessibility Full interface localized to French and Wolof; minimal text; icon-based UI.
💰 Testnet Liquidity Management Automated channel balancing via Python scripts using LND gRPC.

These challenges helped refine our approach toward real-world usability rather than theoretical blockchain design.

🏅 Accomplishments That We're Proud Of

  • Built a working MVP that completes a full tontine cycle (contribution → verification → payout) using Bitcoin Testnet.
  • Integrated Lightning payments and Nostr messaging in a single mobile experience.
  • Achieved full offline mode — users can still track and queue payments without connectivity.
  • Deployed an open, auditable backend that can be run with one command (docker-compose up).
  • Localized UX in Wolof and French, making the app accessible to the real communities we’re building for.
  • Bridged tradition and technology, showing that decentralized finance can grow from existing community culture, not replace it.

📚 What We Learned

  1. Bitcoin can power social finance — not just speculation.
  2. Cultural design matters as much as code — building for real Senegalese users means thinking about trust, not yield.
  3. Lightning is production-ready for micro-savings and group coordination when paired with careful UX.
  4. Open source is the new inclusion — by documenting and publishing our stack, we empower local developers to extend SunuSàv for their own cooperatives.
  5. Community-led governance is possible when rules are encoded as shared, transparent smart contracts.

🚀 What’s Next for SunuSàv — Our Savings, Our Future

SunuSàv is just getting started. Our next phase focuses on scaling beyond the hackathon toward real-world deployment in Senegal and the broader ECOWAS region.

Next Steps

  • 🌍 Pilot Deployment in Dakar: onboard first 5–10 real tontine groups.
  • 🏦 Liquidity Pool Federation: create a shared Lightning liquidity hub for all groups.
  • 🪪 Decentralized Identity (DID) Layer: allow users to build on-chain reputation.
  • 🔒 ZK-enabled Privacy Pools: optional privacy-preserving tontine cycles.
  • 📡 Partnerships: collaborate with Bitcoin Senegal, Btrust Builders, and local women’s cooperatives.
  • 📘 Education Program: integrate Bitcoin literacy modules directly into the app experience.

Ultimately, SunuSàv aims to become the first open-source decentralized savings protocol for Africa — a network of cooperatives powered by math, community, and Bitcoin.

🧠 Team

Developed for: Dakar Bitcoin Hack 2025 Team: Lucy Low, Zhang Low Open Source License: MIT Repo: github.com/lucylow/sunu-sav

“SunuSàv isn’t replacing the tontine — it’s immortalizing it on Bitcoin.” — Dakar Bitcoin Hack 2025 Team

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