protected function _prepareInput($data, $isFile = false, $allowEmpty = true)
    {
        if ($isFile) {
            $input = @fopen($data, 'rb');
            if ($input === false) {
                throw new Crypt_GPG_FileException(
                    'Could not open input file "' . $data . '"',
                    0,
                    $data
                );
            }
        } else {
            $input = strval($data);
            if (!$allowEmpty && $input === '') {
                throw new Crypt_GPG_NoDataException(
                    'No valid input data found.',
                    self::ERROR_NO_DATA
                );
            }
        }

        return $input;
    }