function _rsassa_pkcs1_v1_5_sign($m)
    {
        // EMSA-PKCS1-v1_5 encoding

        $em = $this->_emsa_pkcs1_v1_5_encode($m, $this->k);
        if ($em === false) {
            user_error('RSA modulus too short');
            return false;
        }

        // RSA signature

        $m = $this->_os2ip($em);
        $s = $this->_rsasp1($m);
        $s = $this->_i2osp($s, $this->k);

        // Output the signature S

        return $s;
    }