一个php加密方法

// 加密解密
function authcode($string, $operation, $key = '') {
global $_MCFG;
$key = md5($key ? $key : $_MCFG['auth_key']);
$key_length = strlen($key);
$string = $operation == 'DECODE' ? base64_decode($string) : substr(md5($string.$key), 0, 8).$string;
$string_length = strlen($string);
$rndkey = $box = array();
$result = '';
for($i = 0; $i <= 255; $i++) {
$rndkey[$i] = ord($key[$i % $key_length]);
$box[$i] = $i;
}
for($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($operation == 'DECODE') {
if(substr($result, 0, 8) == substr(md5(substr($result, 8).$key), 0, 8)) {
return substr($result, 8);
} else {
return '';
}
} else {
return str_replace('=', '', base64_encode($result));
}
}