How to Implement
Cross-Chain Payments

// Complete X402 Cross-Chain Payment Flow
// Access protected endpoints on Base using USDO from Polygon

// STEP 1: Request Payment Requirements
const endpoint = encodeURIComponent('https://api.example.com/protected');
const response = await fetch(
 `https://www.apifortest.shop/api/endpoint?endpoint=${endpoint}&network=polygon`
);
const requirements = await response.json();
// Response (402): { accepts: [{ network, payTo, maxAmountRequired, data }] }

// STEP 2: Build EIP-3009 Authorization
const accept = requirements.accepts[0];
const nonce = ethers.utils.hexlify(ethers.utils.randomBytes(32));
const validAfter = Math.floor(Date.now() / 1000) - 60;
const validBefore = validAfter + 3600;

// Get USDO contract constants for EIP-712
const usdoContract = new ethers.Contract(accept.payTo, USDO_ABI, provider);
const TYPEHASH = await usdoContract.TRANSFER_WITH_AUTHORIZATION_EXTENDED_TYPEHASH();
const DOMAIN_SEPARATOR = await usdoContract.DOMAIN_SEPARATOR();

// Build EIP-712 struct hash
const structHash = ethers.utils.keccak256(
 ethers.utils.defaultAbiCoder.encode(
 ['bytes32', 'address', 'address', 'uint256', 'uint256', 'uint256', 'bytes32', 'bytes'],
 [TYPEHASH, wallet.address, accept.payTo, accept.maxAmountRequired,
 validAfter, validBefore, nonce, accept.data || '0x']
 )
);

// Sign EIP-712 digest
const digest = ethers.utils.keccak256(
 ethers.utils.solidityPack(['string', 'bytes32', 'bytes32'],
 ['\\x19\\x01', DOMAIN_SEPARATOR, structHash])
);
const signingKey = new ethers.utils.SigningKey(wallet.privateKey);
const signature = ethers.utils.joinSignature(signingKey.signDigest(digest));

// STEP 3: Build X402 Payload
const x402Payload = {
 x402Version: 1, scheme: 'exact', network: 'polygon',
 payload: {
 signature,
 authorization: {
 from: wallet.address, to: accept.payTo, value: accept.maxAmountRequired,
 validAfter, validBefore, nonce, data: accept.data
 }
 }
};

// STEP 4: Execute Payment (bridge happens automatically)
const result = await fetch(
 `https://www.apifortest.shop/api/endpoint?endpoint=${endpoint}`,
 { method: 'POST', headers: { 'X-Payment': btoa(JSON.stringify(x402Payload)) } }
);

// STEP 5: Receive Protected Content
const content = await result.json();
const paymentProof = result.headers.get('X-Payment-Response');
console.log('Access granted!', content);