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:]
}
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:]
}
// I64 returns the two halfs of the SHA512 HMAC of key and data.