def _toBlock(self, bs):
        """ Convert binary string to array of bytes, state[col][row]"""
        assert ( len(bs) == 4*self.Nb ), 'Rijndarl blocks must be of size blockSize'
        return [[ord(bs[4*i]),ord(bs[4*i+1]),ord(bs[4*i+2]),ord(bs[4*i+3])] for i in range(self.Nb)]