docs: def rpad(s, N, char='\0'):
      """pads a string to the right with null-bytes or any other given character.
  
      ..note:: This is used by the :py:func:`xor` function.
  
      :param s: the string
      :param N: an integer of how much padding should be done
      :returns: the original bytes
      """
      assert isinstance(char, bytes) and len(char) == 1, 'char should be a string with length 1'
      return s.ljust(N, char)