PHP RC 4アルゴリズム

 1 function rc4 ($pwd, $data)//$pwd   $data      
 2 {
 3     $key[] ="";
 4     $box[] ="";
 5 
 6     $pwd_length = strlen($pwd);
 7     $data_length = strlen($data);
 8 
 9     for ($i = 0; $i < 256; $i++)
10     {
11         $key[$i] = ord($pwd[$i % $pwd_length]);
12         $box[$i] = $i;
13     }
14 
15     for ($j = $i = 0; $i < 256; $i++)
16     {
17         $j = ($j + $box[$i] + $key[$i]) % 256;
18         $tmp = $box[$i];
19         $box[$i] = $box[$j];
20         $box[$j] = $tmp;
21     }
22 
23     for ($a = $j = $i = 0; $i < $data_length; $i++)
24     {
25         $a = ($a + 1) % 256;
26         $j = ($j + $box[$a]) % 256;
27 
28         $tmp = $box[$a];
29         $box[$a] = $box[$j];
30         $box[$j] = $tmp;
31 
32         $k = $box[(($box[$a] + $box[$j]) % 256)];
33         $cipher .= chr(ord($data[$i]) ^ $k);
34     }
35 
36     return $cipher;
37 }