Creditz

Creditz is a World Mini App for reloadable, non-transferable private credits: think multi-day ski passes, college cafeteria meal cards, conference vouchers, or restricted wellness benefits that should belong to one verified human, not whoever gets a screenshot or secret key. A user registers with World ID, an issuer reloads Credits into a private note, and an approved merchant creates a short-lived in-person QR invoice. To spend, the user proves they are the registered human and generates a Noir/ProveKit zk balance-transition proof showing the hidden balance is sufficient, the merchant policy is valid, and the new hidden balance was computed correctly. The ledger records commitments and spent nullifiers to prevent replay and double-spend, while the user’s remaining balance stays private. Creditz turns gift cards from transferable bearer instruments into programmable human-bound benefits with privacy by default.

Creditz is built as a Worldcoin Mini App using a Next.js/React/TypeScript frontend with MiniKit-style mobile flows for registration, issuer reloads, merchant invoices, spending, and debug views. World ID is used server-side to bind accounts and spends to a verified human. The private balance transition is modeled in Noir and proved through ProveKit: the proof checks the old commitment, nullifier, hidden balance, spend amount, policy, merchant, invoice expiry, and new commitment. The backend verifies the proof envelope, enforces nullifier uniqueness, and coordinates the spend flow. On-chain state is handled by an EVM Solidity contract on World Chain Sepolia, storing note commitments, spent nullifiers, registered humans, merchant policy approvals, and settlement records. The frontend is served from GitHub Pages, while a local/tunneled backend handles API routes and proof/ledger operations for the hackathon demo.