高效的加密解密函数

核心代码来自discuz，可加密，可解密，是双向的
可定义失效时间

function authcode($string, $operation = 'DECODE', $key = '', $expiry = 0) {    
    $ckey_length = 4;
    $key = md5($key != '' ? $key : C('AUTH_KEY'));
     
    $keya = md5(substr($key, 0, 16));
    $keyb = md5(substr($key, 16, 16));
    $keyc = $ckey_length ? ($operation == 'DECODE' ? substr($string, 0, $ckey_length): substr(md5(microtime()), -$ckey_length)) : '';
 
    $cryptkey = $keya.md5($keya.$keyc);
    $key_length = strlen($cryptkey);
 
    $string = $operation == 'DECODE' ? base64_decode(substr($string, $ckey_length)) : sprintf('%010d', $expiry ? $expiry + time() : 0).substr(md5($string.$keyb), 0, 16).$string;
    $string_length = strlen($string);
    $result = '';
    $box = range(0, 255);
 
    $rndkey = array();
    for($i = 0; $i <= 255; $i++) {
            $rndkey[$i] = ord($cryptkey[$i % $key_length]);
    }
 
    for($j = $i = 0; $i < 256; $i++) {
            $j = ($j + $box[$i] + $rndkey[$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($operation == &#39;DECODE&#39;) {
            if((substr($result, 0, 10) == 0 || substr($result, 0, 10) - time() > 0) && substr($result, 10, 16) == substr(md5(substr($result, 26).$keyb), 0, 16)) {
                    return substr($result, 26);
            } else {
                    return &#39;&#39;;
            }
    } else {
            return $keyc.str_replace(&#39;=&#39;, &#39;&#39;, base64_encode($result));
    }
 
}