Function signEIP712

src/services/x402/client.ts:241–289

* Sign an EIP-712 typed data hash with a secp256k1 private key. * Returns the signature in compact format (r + s + v).

(
  domainSeparator: Buffer,
  structHash: Buffer,
  privateKeyHex: string,
)

 * Returns the signature in compact format (r + s + v).
 */
function signEIP712(
  domainSeparator: Buffer,
  structHash: Buffer,
  privateKeyHex: string,
): string {
  const { sign } = require('crypto') as typeof import('crypto')

  // EIP-712 signing hash: keccak256("\x19\x01" + domainSeparator + structHash)
  const prefix = Buffer.from('1901', 'hex')
  const message = createHash('sha3-256')
    .update(Buffer.concat([prefix, domainSeparator, structHash]))
    .digest()

  const keyHex = privateKeyHex.startsWith('0x')
    ? privateKeyHex.slice(2)
    : privateKeyHex
  const keyBuf = Buffer.from(keyHex, 'hex')

  // Use secp256k1 ECDSA signing
  // Node.js crypto sign with EC key
  const { createPrivateKey } = require('crypto') as typeof import('crypto')

  // DER prefix for secp256k1 private key
  const derPrefix = Buffer.from('30740201010420', 'hex')
  const derMiddle = Buffer.from('a00706052b8104000aa144034200', 'hex')

  const ecPrivateKey = createPrivateKey({
    key: Buffer.concat([derPrefix, keyBuf, derMiddle]),
    format: 'der',
    type: 'sec1',
  })

  const signature = sign(null, message, {
    key: ecPrivateKey,
    dsaEncoding: 'ieee-p1363',
  })

  // Extract r and s (each 32 bytes for secp256k1)
  const r = signature.subarray(0, 32)
  const s = signature.subarray(32, 64)

  // Recovery ID (v) — for Ethereum it's 27 or 28
  // We try v=27 first; the facilitator will handle recovery
  const v = 27

  return (
    '0x' + r.toString('hex') + s.toString('hex') + v.toString(16)
  )
}

/**
 * Create a signed x402 payment payload for a given requirement.