测试了下，在没有原$ikey以及$key的情况下是无法对加密后的字符串进行逆运算的。

 /*
     * 加密，可逆
     * 可接受任何字符
     * 安全度非常高
     */
    function encrypt($txt, $key = 'anihc ctI')
    {
        $chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_.";
        $ikey ="-x6g6ZWm2G9g_vr0Bo.pOq3kRIxsZ6rm";
        $nh1 = rand(0,64);
        $nh2 = rand(0,64);
        $nh3 = rand(0,64);
        $ch1 = $chars{$nh1};
        $ch2 = $chars{$nh2};
        $ch3 = $chars{$nh3};
        $nhnum = $nh1 + $nh2 + $nh3;
        $knum = 0;$i = 0;
        while(isset($key{$i})) $knum +=ord($key{$i++});
        $mdKey = substr(md5(md5(md5($key.$ch1).$ch2.$ikey).$ch3),$nhnum%8,$knum%8 + 16);
        $txt = base64_encode($txt);
        $txt = str_replace(array('+','/','='),array('-','_','.'),$txt);
        $tmp = '';
        $j=0;$k = 0;
        $tlen = strlen($txt);
        $klen = strlen($mdKey);
        for ($i=0; $i<$tlen; $i++) {
            $k = $k == $klen ? 0 : $k;
            $j = ($nhnum+strpos($chars,$txt{$i})+ord($mdKey{$k++}))%64;
            $tmp .= $chars{$j};
        }
        $tmplen = strlen($tmp);
        $tmp = substr_replace($tmp,$ch3,$nh2 % ++$tmplen,0);
        $tmp = substr_replace($tmp,$ch2,$nh1 % ++$tmplen,0);
        $tmp = substr_replace($tmp,$ch1,$knum % ++$tmplen,0);
        return $tmp;
    }
    
    /*
     * 解密
     * 
     */
     function decrypt($txt, $key = 'anihc ctI')
     {
        $chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_.";
        $ikey ="-x6g6ZWm2G9g_vr0Bo.pOq3kRIxsZ6rm";
        $knum = 0;$i = 0;
        $tlen = strlen($txt);
        while(isset($key{$i})) $knum +=ord($key{$i++});
        $ch1 = $txt{$knum % $tlen};
        $nh1 = strpos($chars,$ch1); 
        $txt = substr_replace($txt,'',$knum % $tlen--,1);
        $ch2 = $txt{$nh1 % $tlen};
        $nh2 = strpos($chars,$ch2);
        $txt = substr_replace($txt,'',$nh1 % $tlen--,1);
        $ch3 = $txt{$nh2 % $tlen};
        $nh3 = strpos($chars,$ch3);
        $txt = substr_replace($txt,'',$nh2 % $tlen--,1);
        $nhnum = $nh1 + $nh2 + $nh3;
        $mdKey = substr(md5(md5(md5($key.$ch1).$ch2.$ikey).$ch3),$nhnum % 8,$knum % 8 + 16);
        $tmp = '';
        $j=0; $k = 0;
        $tlen = strlen($txt);
        $klen = strlen($mdKey);
        for ($i=0; $i<$tlen; $i++) {
            $k = $k == $klen ? 0 : $k;
            $j = strpos($chars,$txt{$i})-$nhnum - ord($mdKey{$k++});
            while ($j<0) $j+=64;
            $tmp .= $chars{$j};
        }
        $tmp = str_replace(array('-','_','.'),array('+','/','='),$tmp);
        return trim(base64_decode($tmp));
    }

原地址：http://bbs.phpchina.com/thread-37376-1-1.html