Authcode

function authcode($string, $operation = 'DECODE', $key = '', $expiry = 0){
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 if($operation == 'DECODE') {
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  $string = str_replace('[a]','+',$string);
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  $string = str_replace('[b]','&',$string);
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  $string = str_replace('[c]','/',$string);
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 }
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    $ckey_length = 4;
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    $key = md5($key ? $key : 'livcmsencryption ');
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    $keya = md5(substr($key, 0, 16));
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    $keyb = md5(substr($key, 16, 16));
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    $keyc = $ckey_length ? ($operation == 'DECODE' ? substr($string, 0, $ckey_length): substr(md5(microtime()), -$ckey_length)) : '';
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    $cryptkey = $keya.md5($keya.$keyc);
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    $key_length = strlen($cryptkey);
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    $string = $operation == 'DECODE' ? base64_decode(substr($string, $ckey_length)) : sprintf('%010d', $expiry ? $expiry + time() : 0).substr(md5($string.$keyb), 0, 16).$string;
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    $string_length = strlen($string);
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    $result = '';
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    $box = range(0, 255);
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    $rndkey = array();
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    for($i = 0; $i <= 255; $i++) {
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        $rndkey[$i] = ord($cryptkey[$i % $key_length]);
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    }
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    for($j = $i = 0; $i < 256; $i++) {
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        $j = ($j + $box[$i] + $rndkey[$i]) % 256;
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        $tmp = $box[$i];
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        $box[$i] = $box[$j];
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        $box[$j] = $tmp;
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    }
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    for($a = $j = $i = 0; $i < $string_length; $i++) {
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        $a = ($a + 1) % 256;
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        $j = ($j + $box[$a]) % 256;
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        $tmp = $box[$a];
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        $box[$a] = $box[$j];
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        $box[$j] = $tmp;
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        $result .= chr(ord($string[$i]) ^ ($box[($box[$a] + $box[$j]) % 256]));
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    }
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    if($operation == 'DECODE') {
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        if((substr($result, 0, 10) == 0 || substr($result, 0, 10) - time() > 0) && substr($result, 10, 16) == substr(md5(substr($result, 26).$keyb), 0, 16)) {
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            return substr($result, 26);
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        } else {
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            return '';
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        }
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    } else {
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  $ustr = $keyc.str_replace('=', '', base64_encode($result));
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  $ustr = str_replace('+','[a]',$ustr);
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  $ustr = str_replace('&','[b]',$ustr);
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  $ustr = str_replace('/','[c]',$ustr);
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        return $ustr;
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    }
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}