AnyPay is a React Native mobile app that enables instant cryptocurrency payments via NFC. Users tap phones to exchange payment details and complete transactions. Core Technology: Built with React Native and Expo for iOS and Android NFC communication: uses react-native-nfc-manager for reading and react-native-hce (Android Host Card Emulation) for sharing payment data Dark fintech UI with gradient animations and glassmorphism Blockchain Infrastructure: Cross-chain payments via Layer Zero Omnichain Protocol Automatic asset swapping via Uniswap when sender and recipient prefer different assets Supports Ethereum, Polygon, Arbitrum, and Optimism testnets Supports multiple tokens: USDC, USDT, ETH, wBTC User Experience: Send: choose asset/chain, tap recipient’s device, transaction executes automatically Receive: set amount and preferred asset/chain, wait for NFC tap, receive in preferred format Configuration: customize default send/receive chains, assets, and payment preferences Technical Flow: NFC exchange of payment details (address, amount, desired asset/chain) Source chain: sender’s asset locked in bridge contract Layer Zero: secure cross-chain message delivery Asset conversion: Uniswap swaps if needed Target chain: recipient receives preferred asset on preferred chain Key Features: Instant NFC-based payment initiation Cross-chain compatibility (send on one chain, receive on another) Automatic asset conversion (send ETH, receive USDC) Trustless execution via smart contracts Mobile-first design with smooth animations The project includes smart contracts for bridging, swapping, and cross-chain messaging, deployed on testnets for the EthGlobal Buenos Aires Hackathon. It demonstrates how NFC can simplify crypto payments while maintaining the flexibility of cross-chain and cross-asset transactions.

How It's Made - Technical Description

Architecture Overview

This project implements a cross-chain token bridging system that automatically bridges tokens after swaps, using LayerZero V2 for omnichain messaging and Uniswap v4 hooks for swap integration.

Core Technologies

LayerZero V2 OApp Infrastructure

• OApp Contracts: Built custom SourceBridge and TargetBridge contracts inheriting from LayerZero's OApp and OAppOptionsType3
• Cross-Chain Messaging: Uses LayerZero's messaging protocol to send lock/unlock instructions between Arbitrum Sepolia and Optimism Sepolia
• OptionsBuilder: Configured executor options with gas limits (200,000) for reliable message execution
• Endpoint IDs: Arbitrum Sepolia (40231) and Optimism Sepolia (40232)

Smart Contract Stack

• Solidity 0.8.24: All contracts written in modern Solidity with safety features
• OpenZeppelin Contracts v5: Used SafeERC20 for secure token transfers, Ownable for access control
• Hardhat: Development environment with custom network configurations for testnets

Uniswap v4 Integration

• BaseHook: Extended Uniswap's BaseHook contract to create BridgeHook
• Hook Permissions: Configured with afterSwap permission flag (bit 7) for post-swap execution
• CREATE2 Deployment: Implemented deterministic deployment using CREATE2 to meet Uniswap v4's hook address requirements
• BridgeHelper: Helper contract that interfaces between the hook and LayerZero bridge

Technical Implementation Details

Bridge Architecture

1. SourceBridge (Arbitrum Sepolia):

   • Locks tokens when users bridge
   • Sends cross-chain messages via LayerZero with recipient address and amount
   • Uses quoteLockAndBridge to calculate native fees before execution

2. TargetBridge (Optimism Sepolia):

   • Receives messages from source chain
   • Mints bridged tokens to recipients
   • Handles burn operations when users bridge back

Swap-to-Bridge Flow

1. User performs a swap on Uniswap v4 pool
2. BridgeHook's afterSwap callback is triggered automatically
3. BridgeHelper receives swap output and calls lockAndBridge with native ETH for fees
4. SourceBridge locks tokens and sends LayerZero cross-chain message
5. TargetBridge receives message and mints bridged tokens on destination chain

NFC Crypto Payment Integration

• Contactless Payment Interface: NFC-enabled devices (smartphones, payment terminals) can trigger the swap and bridge flow with a single tap
• Direct Smart Contract Connection: NFC tap events connect directly to smart contracts, bypassing traditional payment processors
• Automated Transaction Execution: Upon NFC tap, the system automatically:
  • Initiates the swap on Uniswap v4 pool
  • Triggers the bridge hook to lock tokens
  • Sends cross-chain message via LayerZero
  • Completes the entire payment flow in a single transaction
• Seamless User Experience: Users can make cross-chain crypto payments without manual transaction signing or wallet interactions
• Payment Terminal Integration: NFC payment terminals can be configured to accept crypto payments that automatically bridge to recipient's preferred chain
• Multi-Chain Payment Support: Recipients can receive payments on their preferred chain, regardless of which chain the payment was initiated from

Notable Technical Challenges & Solutions

Challenge 1: Uniswap v4 Hook Deployment

• Uniswap v4 requires hooks to be deployed at specific CREATE2 addresses based on permission bits
• Solution: Implemented a CREATE2 factory pattern with salt calculation based on hook permissions (AFTER_SWAP_FLAG = 0x40)
• Used iterative salt generation to find valid addresses matching permission requirements

Challenge 2: LayerZero Fee Handling

• Bridge operations require native ETH for LayerZero messaging fees
• Solution: Made Pool.swap payable and forwarded msg.value through the call chain: Pool → BridgeHelper → SourceBridge
• Implemented fee quoting before execution to ensure sufficient funds

Challenge 3: Cross-Chain State Synchronization

• Ensuring locked tokens on source chain match minted tokens on destination
• Solution: Used LayerZero's reliable messaging with enforced options and DVN (Decentralized Verifier Network) configuration
• Implemented event emissions for tracking lock/unlock operations

Partner Technologies

LayerZero V2

• Benefit: Provides secure, reliable cross-chain messaging without requiring custom validators
• Usage: OApp pattern allows for composable cross-chain applications
• Configuration: Set up default pathways, send/receive libraries, and DVNs for message verification

Uniswap v4 SDK

• Benefit: Enables hook-based customization of swap behavior
• Usage: Integrated afterSwap hook to trigger bridging automatically post-swap
• Implementation: BridgeHook extends BaseHook and implements afterSwap callback for automatic bridging

NFC Technology

• Benefit: Enables contactless crypto payments with the same convenience as traditional NFC payments (Apple Pay, Google Pay)
• Usage: NFC tap triggers the swap-to-bridge flow automatically, eliminating the need for manual wallet interactions
• Integration: Connects NFC-enabled devices directly to smart contract execution via Web3 wallet integration
• Payment Flow:
  • User taps NFC-enabled device on payment terminal
  • Terminal reads payment amount and recipient address
  • Smart contract executes swap (e.g., WETH → USDC) on source chain
  • Bridge hook automatically bridges tokens to recipient's preferred chain
  • Payment completes on destination chain without recipient needing to be on source chain
• Use Cases:
  • Point-of-sale crypto payments
  • Cross-chain merchant payments
  • Contactless donations and tips
  • Automated payroll and vendor payments

Development Workflow

1. Contract Development: Wrote contracts in Solidity with comprehensive error handling
2. Testing: Used Hardhat for local testing and testnet deployments
3. Deployment: Automated deployment scripts with address verification and balance checks
4. Wiring: Configured LayerZero pathways using custom wiring scripts that set peers, libraries, and DVNs

Key Features

• NFC Crypto Payments: Contactless payment system that enables tap-to-pay functionality for cryptocurrency transactions
• Automatic Bridging: Tokens bridge automatically after swaps without user intervention
• Fixed Swap Flow: Deterministic swap amounts (0.1 WETH → 280.9 USDC) for predictable behavior
• Cross-Chain Recipient: Users can specify different recipient addresses on destination chain
• Fee Optimization: Quotes fees before execution to prevent failed transactions
• One-Tap Payments: Complete cross-chain payment flow executed with a single NFC tap
• Merchant-Friendly: Payment terminals can accept crypto payments that automatically bridge to merchant's preferred chain