CrossyFi

CrossyFi Payment, is a cross-chain payment app. Senders can transfer any form of local currency on chain from one chain to another recipient on another chain (e.g.Eth to Bitcoin, Solana etc) and the Sender automatically receives a unique, generative on-chain NFT "postcard" as a receipt/memento of the transfer. Built with Foundry for smart contracts (PostcardNFT, PostcardRouter) and Next.js + wagmi + viem for the frontend, using Across Protocol for cross-chain bridging.

CrossyFi Payment is built with a Foundry-based smart contract suite and a Next.js frontend. The core contracts are PostcardRouter.sol, which handles initiating a cross-chain USDC transfer, and PostcardNFT.sol, an ERC-721 contract that mints a generative on-chain "postcard" NFT to the recipient as a receipt of the payment. We used OpenZeppelin contracts as the base for security and standards compliance.

For cross-chain bridging, we integrated Across Protocol — when a user sends USDC from Base Sepolia to a recipient on Arbitrum Sepolia, the router relays the transfer through Across's SpokePool contracts, letting funds move across chains without a custom bridge. Across's speed and existing testnet liquidity meant we didn't have to build bridging infrastructure from scratch, which was a huge time-saver in a hackathon setting.

The frontend uses Next.js, wagmi, and viem for wallet connection and contract interaction across both Base Sepolia and Arbitrum Sepolia. One hacky bit: the NFT's artwork/metadata is generated entirely on-chain (no IPFS/off-chain server), so the "postcard" design is derived deterministically from transfer data like sender, recipient, and amount.