Zorrito
Prize-linked savings on Stellar. Nobody loses. One person wins the yield.
You put money into a shared pool. The pool lends it on Blend and earns
interest. Once a week, one participant wins the interest everyone generated.
Everybody else keeps exactly what they put in, and can withdraw at any time.
Built for the Argentina Builder Challenge (BAF × Stellar), Genesis track.
Live on Stellar mainnet with real money since September 2026.
What it does
- Deposit USDC. It goes straight into Blend as a Supply position and
starts earning at once. No minimum. You can also pay with XLM or
USDT0: the app quotes Soroswap and the classic Stellar DEX at the same
time and swaps through whichever pays more, in your wallet, before
depositing. The pool itself only ever holds USDC.
- The round runs. One week on mainnet. Every second your money is in
adds to your weight:
deposit × time. Marking "I saved today" seven days
in a row doubles your odds. Friends who join through your link add 10 %
of their capital to your weight.
- The round closes. Anyone can close it once it expires. The prize
(everything the pool earned) and everyone's weight are frozen. The
contract picks a future drand round that will decide the winner.
- drand publishes. Anyone brings the signature. The contract verifies
it on-chain with BLS12-381 and picks the winner proportionally to weight.
- One person gets paid. The prize goes to the winner's wallet in the
same transaction. Everyone else still has exactly what they deposited.
The next round has already started.
Withdrawing always works: no penalty, no waiting for the draw, no permission.
What makes it different
- No roles. No admin, no pause, no key that can touch funds or influence
the draw. Closing a round and running the draw are permissionless.
- Randomness nobody can bias. Nothing from the ledger is used as a seed.
The winner comes from a drand quicknet signature for
a round that did not exist when the pool closed, verified inside the
contract with the BLS12-381 host functions of Protocol 22. Biasing it would
take a majority of the ~20 organisations that run drand.
- No keeper dependency. A serverless keeper closes rounds and runs draws,
and so does every visit to the app that finds work to do. If the keeper
disappears, the first person to open the app unblocks the pool.
- Built for a million accounts from day one. Weights live in a Fenwick
tree over contract storage with capacity 2^20. Deposit, withdraw and draw
touch ~21 storage entries whether there are 2 participants or 1,000,000.
- Fair by construction. Weight is money × time, the same proportion in
which each participant generated the prize. Jumping in with a lot of money
a minute before the close barely counts.
Architecture
contracts/pozo/ the pool: deposits, weights, close, drand-verified draw, streak, referrals, cap
contracts/blend_adapter/ Supply position on Blend v2 behind a 3-function interface
contracts/mock_rendimiento a yield source for tests and local demos
web/ Next.js app, serverless keeper, scripts
scripts/ deploy scripts (testnet and mainnet)
Pool contract (contracts/pozo, Rust / Soroban SDK 27). Entry points:
| Function | Who | What |
|---|
depositar(user, amount) / depositar_con_referente(user, amount, referrer) | the user | join the pool and the draw |
retirar(user, amount) | the user | take capital out, always, even mid-draw |
ahorrar_hoy(user) | the user | mark the daily streak |
cerrar_ronda() | anyone | freeze weights and prize; pin a drand round ≥ 10 min in the future |
ejecutar_sorteo(signature) | anyone | verify the drand BLS signature on-chain, pick the winner, pay |
estado(), cuenta_de(user), chances_bps(user) | read-only | pool state, account state, "your odds" |
The weight deposit × time enters the tree by linearity (a·T − b, two
coefficients that sum over prefixes). Closing copies nothing: the new round
starts at the close and the closed round's weights stay frozen in the tree by
lazy versioning. Measured: a deposit's footprint is 23–30 writes with 1 or
with 1,001 accounts, and a draw with 1,001 accounts costs 45M instructions
against a 100M limit.
Blend adapter (contracts/blend_adapter). A single Supply (non-collateral)
position on a Blend v2 pool: it earns interest and cannot be liquidated.
Tested against Blend's real bytecode. Blend rounds b-tokens down on deposit,
so a withdrawal of the exact amount could come up one stroop short; the
adapter asks the pool for one stroop extra and pays the exact amount from its
own balance, keeping the remainder as a dust fund.
Web app (web/). Next.js 16, Tailwind 4, @stellar/stellar-sdk 17,
Stellar Wallets Kit (Freighter, xBull, Lobstr) plus a custom module for
Cosmos Wallet. Prize growing live, countdown
to the second, Blend APY read from the pool, streak, referral link, swap-in
from XLM or USDT0, human-readable status messages with a link to every
transaction. Deployed on Vercel.
Keeper (web/src/app/api/keeper/route.ts). The same logic as the CLI
script, as a serverless function triggered by cron and by page visits.
Addresses
Mainnet (the home page): USDC, weekly rounds, 5,000 USDC cap.
Every address links to its page on stellar.expert.
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