getUpdateOutputSize method
Returns the amount of bytes being outputted
by the next processBytes for len bytes input.
Implementation
@override
int getUpdateOutputSize(int len) {
  if (_forEncryption) {
    return len;
  } else if (_bufFull) {
    return len;
  } else {
    return max(0, len + _bufOff - _macSize);
  }
}