Returns one byte of CTR keystream

def _nextCTRByte(self):
        """Returns one byte of CTR keystream"""
        b = ord(self.ctr_cks[self.ctr_pos])
        self.ctr_pos += 1
        if self.ctr_pos >= len(self.ctr_cks):
            self._calcCTRBUF()
        return b