Level 11

• State “DONE” from [2025-03-27 Thu 14:45]

http://natas11.natas.labs.overthewire.org/

This level seems to work with an encryption called XOR encryption:

Background

Okay, some theory first: what is an XOR cipher? It is a type of additive cipher that works with the applying the bitwise XOR operator to every character using a given key. To decrypt, applying the XOR function with the key will remove the cipher. To take an example from Wikipedia, let’s take the string ‘Wiki’ and encode it in 8-bit ASCII and encode it with the key 11110011:

Above it says that cookies are encoded with XOR encryption. So let’s use our browser to check the cookies.

As expected, the cookies contains some ciphertext: HmYkBwozJw4WNyAAFyB1VUcqOE1JZjUIBis7ABdmbU1GdGdfVXRnTRg=

Understanding the code

Then we look at the sourcecode:

  <?
$defaultdata = array( "showpassword"=>"no", "bgcolor"=>"#ffffff");

function xor_encrypt($in) {
    $key = '<censored>';
    $text = $in;
    $outText = '';

    // Iterate through each character
    for($i=0;$i<strlen($text);$i++) {
    $outText .= $text[$i] ^ $key[$i % strlen($key)];
    }

    return $outText;
}

function loadData($def) {
    global $_COOKIE;
    $mydata = $def;
    if(array_key_exists("data", $_COOKIE)) {
    $tempdata = json_decode(xor_encrypt(base64_decode($_COOKIE["data"])), true);
    if(is_array($tempdata) && array_key_exists("showpassword", $tempdata) && array_key_exists("bgcolor", $tempdata)) {
        if (preg_match('/^#(?:[a-f\d]{6})$/i', $tempdata['bgcolor'])) {
        $mydata['showpassword'] = $tempdata['showpassword'];
        $mydata['bgcolor'] = $tempdata['bgcolor'];
        }
    }
    }
    return $mydata;
}

function saveData($d) {
    setcookie("data", base64_encode(xor_encrypt(json_encode($d))));
}

$data = loadData($defaultdata);

if(array_key_exists("bgcolor",$_REQUEST)) {
    if (preg_match('/^#(?:[a-f\d]{6})$/i', $_REQUEST['bgcolor'])) {
        $data['bgcolor'] = $_REQUEST['bgcolor'];
    }
}
saveData($data);
?>

<?
if($data["showpassword"] == "yes") {
    print "The password for natas12 is <censored><br>";
}

?>

This is a bit more involved, so let’s look at this step by step.

$defaultdata = array( "showpassword"=>"no", "bgcolor"=>"#ffffff");

This sets the default value for the array $defaultdata. Then we have a function that encrypts using the XOR cipher with a $key.

  function xor_encrypt($in) {
    $key = '<censored>';
    $text = $in;
    $outText = '';

    // Iterate through each character
    for($i=0;$i<strlen($text);$i++) {
        $outText .= $text[$i] ^ $key[$i % strlen($key)];
    }

    return $outText;
}

The following function then reads data from the cookie and decrypts it using base64, xor_encrypt (XOR encryption is reversible) and then parses it as json_decode().

The function then checks if the loaded and decrypted array is valid and contains "showpassword" and "bgcolor". If so, then it updates $mydata.

  function loadData($def) {
    global $_COOKIE;
    $mydata = $def;

    if(array_key_exists("data", $_COOKIE)) {
        $tempdata = json_decode(xor_encrypt(base64_decode($_COOKIE["data"])), true);

        if(is_array($tempdata) && array_key_exists("showpassword", $tempdata) && array_key_exists("bgcolor", $tempdata)) {
            if (preg_match('/^#(?:[a-f\d]{6})$/i', $tempdata['bgcolor'])) {
                $mydata['showpassword'] = $tempdata['showpassword'];
                $mydata['bgcolor'] = $tempdata['bgcolor'];
            }
        }
    }

    return $mydata;
}

Then there is a function that saves the data to cookies:

  function saveData($d) {
    setcookie("data", base64_encode(xor_encrypt(json_encode($d))));
}

So let’s gather what we have:

• We have a reversible cipher (XOR cipher) and both
• cleartext (the value stored in $defaultdata)
• ciphertext (the value stored in the cookie)

Obtaining the key

So if we XOR the ciphertext with the known plaintext, we should get the key. Break this down into several steps:

• First encode the original plaintext to json and then base64

$defaultdata = array( "showpassword"=>"no", "bgcolor"=>"#ffffff");
echo base64_encode(json_encode($defaultdata));

• Then take the plaintext data and encrypt it with the key obtained from the cookie.

  function xor_encrypt($in, $key) {
    $text = $in;
    $outText = '';

    // Iterate through each character
    for($i=0;$i<strlen($text);$i++) {
    $outText .= $text[$i] ^ $key[$i % strlen($key)];
    }

    return $outText;
}

  $plaintext = 'eyJzaG93cGFzc3dvcmQiOiJubyIsImJnY29sb3IiOiIjZmZmZmZmIn0=';
  $decoded_data = base64_decode($plaintext);
  $key = 'HmYkBwozJw4WNyAAFyB1VUcqOE1JZjUIBis7ABdmbU1GdGdfVXRnTRg=';
  $decoded_key = base64_decode($key);
  $encrypted = xor_encrypt($decoded_data, $decoded_key);
  echo $encrypted;

This gives us the string eDWoeDWoeDWoeDWoeDWo on repeat. So it seems that our key might be eDWo.

Modifying the plaintext

Now we have the supposed key for encryption, and we know that we want to set the showpassword bit to yes in the json array. So first change the cleartext, create a JSON text object from it and encode it to base64 (just for readability).

$defaultdata = array( "showpassword"=>"yes", "bgcolor"=>"#ffffff");
echo base64_encode(json_encode($defaultdata));

This gives us:

eyJzaG93cGFzc3dvcmQiOiJ5ZXMiLCJiZ2NvbG9yIjoiI2ZmZmZmZiJ9

Then we use the functions we have already utilized above to encode the new plaintext with the obtained key to get the new cookie value.

  function xor_encrypt($in, $key) {
    $text = $in;
    $outText = '';

    // Iterate through each character
    for($i=0;$i<strlen($text);$i++) {
    $outText .= $text[$i] ^ $key[$i % strlen($key)];
    }

    return $outText;
}

  $plaintext = 'eyJzaG93cGFzc3dvcmQiOiJ5ZXMiLCJiZ2NvbG9yIjoiI2ZmZmZmZiJ9';
  $decoded_data = base64_decode($plaintext);
  $key = 'eDWo';
  $encrypted = xor_encrypt($decoded_data, $key);
  $cookie_value = base64_encode($encrypted);
  echo $cookie_value;

Finally, use the value below and use the developer tab in your browser to replace the cookie value with this. This should get the website to show you the password.

HmYkBwozJw4WNyAAFyB1VUc9MhxHaHUNAic4Awo2dVVHZzEJAyIxCUc5

• yZdkjAYZRd3R7tq7T5kXMjMJlOIkzDeB

Security implications

There are some implications to this exercise:

• XOR encryption is a simple bitwise operation, not a cryptographically secure algorithm. So if an attacker know any part of the plaintext and access to the encrypted data (e.g. via a cookie), the key can easily be derived.
• Storing sensitive information in cookies is risky; cookies can be tampered with. We should instead use secure cookes with HTTP-only & HTTPS or session-based authentication.
• We should use a more secure encryption such as AES.