def _pad(
        s: str,
        bs: int,
) -> str:
    """Pads a string so its length is a multiple of a specified block size.

    :param s: The string that is to be padded
    :type s: str
    :param bs: The block size
    :type bs: int
    :returns: The initial string, padded to have a length that is a multiple of the specified block size
    :rtype: str
    """
    number_of_bytes_to_pad = bs - len(s) % bs
    ascii_string = chr(number_of_bytes_to_pad)
    padding_str = number_of_bytes_to_pad * ascii_string
    return s + padding_str