open_nonce_off method
Implementation
Uint8List? open_nonce_off(Uint8List box, final int boxoff, Uint8List theNonce) {
  if (!(box.length > boxoff &&
      theNonce.length == nonceLength)) return null;
  // prepare shared key
  if (_sharedKey == null) before();
  return open_after_len(box, boxoff, box.length - boxoff, theNonce);
}