CastPay

CastPay enables seamless, ETH-free USDC transfers via social usernames, reducing friction in crypto payments on social platforms.

Overview

CastPay allows Farcaster users to send USDC payments using simple commands like @username /pay 5 USDC without needing to hold ETH for gas fees. The system uses a Stylus-based Paymaster contract on Arbitrum to sponsor gas fees, making crypto payments as easy as sending a message.

Problem and Solution

The Problem

Traditional crypto payments on social platforms face several barriers:

• Gas Fee Friction: Users need to hold native tokens (ETH) to pay gas fees, creating a poor UX
• Complex Addresses: Wallet addresses are hard to remember and type, especially on mobile
• Multiple Steps: Current payment flows require multiple app switches and confirmations
• Technical Barriers: Users need deep crypto knowledge to navigate gas prices and network fees

Our Solution

CastPay eliminates these barriers through:

• Account Abstraction: ERC-4337 compatible smart accounts remove gas fee requirements
• Social Identity: Pay using familiar @usernames instead of complex wallet addresses
• Gasless Transactions: Stylus Paymaster sponsors all gas fees from USDC balance
• Native Integration: Seamless Farcaster Mini App experience without leaving the social platform
• Instant Settlement: Sub-5 second transaction completion with immediate confirmation

This creates a Web2-like payment experience while maintaining full crypto ownership and transparency.

Features

• Gasless Payments: Send USDC without holding ETH
• Username-based Transfers: Pay using @usernames instead of wallet addresses
• Instant Transactions: Complete payments in under 5 seconds
• Farcaster Integration: Native mini app integration with Warpcast
• ERC-4337 Compatible: Built on Account Abstraction standards
• Stylus Paymaster: Efficient Rust-based smart contracts on Arbitrum

Quick Start

Prerequisites

• Node.js 16+ and pnpm
• Warpcast account with linked wallet
• Arbitrum testnet ETH for initial setup

Installation

git clone https://github.com/your-org/castpay
cd castpay
pnpm install

Development

1. Start the frontend:

   cd frontend
   pnpm dev

2. Start the relayer service:

   cd backend
   pnpm start

3. Deploy contracts:

   cd contracts/template
   cargo build --release
   # Deploy using your preferred method

Usage

Sending a Payment

1. Enable CastPay Mini App in Warpcast
2. Type @username /pay [amount] USDC in a cast or DM
3. Confirm the payment in the popup modal
4. Payment completes instantly without ETH fees

Example

@alice /pay 10 USDC

This sends 10 USDC to the user @alice.

Architecture

  Farcaster               CastPay              Arbitrum     
  Mini App                 Relayer          Blockchain    
  (Frontend)              (Backend)            (Contracts)    

Components

• Frontend: React-based Farcaster Mini App for user interface
• Backend: Node.js relayer service for transaction bundling
• Contracts: Stylus Paymaster contract written in Rust
• Integrations: Farcaster API for username resolution

Project Structure

castpay/
    frontend/           # Farcaster Mini App UI
        src/
            App.tsx
            components/
        package.json
    backend/            # Relayer service
        src/
            relayer.js
        package.json
    contracts/          # Smart contracts
        template/
            src/
                lib.rs
            Cargo.toml
    scripts/           # Deployment scripts

Configuration

Environment Variables

Create .env files in the respective directories:

Frontend (.env):

VITE_RELAYER_URL=http://localhost:3001
VITE_CHAIN_ID=421614

Backend (.env):

PRIVATE_KEY=your_private_key
ARBITRUM_RPC_URL=https://sepolia-rollup.arbitrum.io/rpc
FARCASTER_API_KEY=your_api_key

Smart Contracts

The Paymaster contract is built using Arbitrum Stylus (Rust/WASM) and implements ERC-4337 standards:

• Validates user operations
• Sponsors gas fees by deducting from USDC balance
• Includes ~10% surcharge for sustainability
• Deployed on Arbitrum testnet

API Reference

Relayer Endpoints

• POST /send-payment - Submit a payment operation
• GET /payment-status/:txHash - Check payment status
• GET /user-balance/:address - Get user USDC balance

Payment Format

{
  "recipient": "@username",
  "amount": "10.00",
  "currency": "USDC",
  "signature": "0x..."
}

Security

• No private keys stored by the application
• Rate limiting to prevent abuse
• Signature validation for all transactions
• Paymaster validates operations before sponsoring

Contributors

• @emmaglorypraise
• @ajtech001
• @nnennaokoye

Contributing

1. Fork the repository
2. Create a feature branch
3. Make your changes
4. Add tests
5. Submit a pull request

License

MIT License - see LICENSE for details