Looks like natas8 is another "secret input" challenge. We'll start by jumping right into the source code.

<html>
<head>
<!-- This stuff in the header has nothing to do with the level -->
<link rel="stylesheet" type="text/css" href="http://natas.labs.overthewire.org/css/level.css">
<link rel="stylesheet" href="http://natas.labs.overthewire.org/css/jquery-ui.css" />
<link rel="stylesheet" href="http://natas.labs.overthewire.org/css/wechall.css" />
<script src="http://natas.labs.overthewire.org/js/jquery-1.9.1.js"></script>
<script src="http://natas.labs.overthewire.org/js/jquery-ui.js"></script>
<script src=http://natas.labs.overthewire.org/js/wechall-data.js></script><script src="http://natas.labs.overthewire.org/js/wechall.js"></script>
<script>var wechallinfo = { "level": "natas8", "pass": "<censored>" };</script></head>
<body>
<h1>natas8</h1>
<div id="content">

<?

$encodedSecret = "3d3d516343746d4d6d6c315669563362";

function encodeSecret($secret) {
    return bin2hex(strrev(base64_encode($secret)));
}

if(array_key_exists("submit", $_POST)) {
    if(encodeSecret($_POST['secret']) == $encodedSecret) {
    print "Access granted. The password for natas9 is <censored>";
    } else {
    print "Wrong secret";
    }
}
?>

<form method=post>
Input secret: <input name=secret><br>
<input type=submit name=submit>
</form>

<div id="viewsource"><a href="index-source.html">View sourcecode</a></div>
</div>
</body>
</html>

So it looks like they tried to hide the secret by encoding the input in the following order:

1) base64_encode - Regular Base64 encoding
2) strrev - reverses a string
3) bin2hex - Convert binary data into hexadecimal representation

They also gave us the final output which is "3d3d516343746d4d6d6c315669563362". We simply need to run this string through the following proccess:

1) hex2bin
2) strrev
3) base64_decode

An easy way of testing this is by running:

php -a

This will open and interactive php command line when you don't want to create php files just to run a quick test. Screenshot1

This gives us the input string of: oubWYf2kBq

After that, we get our answer: Screenshot2

W0mMhUcRRnG8dcghE4qvk3JA9lGt8nDl