code: def lpad(s, N, char='\0'):
      
      assert isinstance(char, bytes) and len(char) == 1, 'char should be a string with length 1'
      return s.rjust(N, char)
docs: pads a string to the left 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