code: func I64(key []byte, data []byte) ([]byte, []byte) {
  	mac := hmac.New(sha512.New, key)
  	mac.Write(data)
  	I := mac.Sum(nil)
  	return I[:32], I[32:]
  }
docs: // I64 returns the two halfs of the SHA512 HMAC of key and data.