PHP对称加密auth_code优化版

/**
 * @param string $string  加密字符串内容
 * @param string $operation 加密或者解密
 * @param string $key 私钥
 * @param int $expiry 过期时间/秒 0为永不过期
 * @return false|string
 */
function auth_code(string $string, $operation = 'DECODE', $key = '', $expiry = 0)
{
    $cakey_length = 4;
    $key = md5($key);
    $keys = md5(substr($key, 0, 16));
    $keytab = md5(substr($key, 16, 16));
    if ($operation === 'DECODE') {
        $key_str = substr($string, 0, $cakey_length);
    } else {
        $key_str = substr(md5(microtime()), -$cakey_length);
    }
    $keycode = $cakey_length ? $key_str : '';
    $cryptic = $keys . md5($keys . $keycode);
    $key_length = strlen($cryptic);
    $string = $operation === 'DECODE' ? base64_decode(substr($string, $cakey_length)) : sprintf('%010d', $expiry ? $expiry + time() : 0) . substr(md5($string . $keytab), 0, 16) . $string;
    $string_length = strlen($string);
    $result = '';
    $box = range(0, 255);
    $render = array();
    for ($i = 0; $i <= 255; $i++) {
        $render[$i] = ord($cryptic[$i % $key_length]);
    }
    for ($j = $i = 0; $i < 256; $i++) {
        $j = ($j + $box[$i] + $render[$i]) % 256;
        $tmp = $box[$i];
        $box[$i] = $box[$j];
        $box[$j] = $tmp;
    }
    for ($a = $j = $i = 0; $i < $string_length; $i++) {
        $a = ($a + 1) % 256;
        $j = ($j + $box[$a]) % 256;
        $tmp = $box[$a];
        $box[$a] = $box[$j];
        $box[$j] = $tmp;
        $result .= chr(ord($string[$i]) ^ ($box[($box[$a] + $box[$j]) % 256]));
    }
    if ($operation === 'DECODE') {
        if ((strpos($result, 0) === 0 || substr($result, 0, 10) - time() > 0) && strpos(md5(substr($result, 26) . $keytab), substr($result, 10, 16)) === 0) {
            return substr($result, 26);
        }
        return '';
    }
    return $keycode . str_replace('=', '', base64_encode($result));
}