Solution for Natas web security wargame with by XORing the plaintext with the ciphertext...

Currently I am playing some wargames on

The first 10 levels were very easy and everyone with some technical knowledge and programming experience should be able to solve them. But somehow I got stuck for a few hours on level 11. The task is to modify a XOR encrypted cookie. For some reason I couldn't figure out how to obtain the xor key that was used.

The challenge was to reverse engineer the key by having the plaintext and the ciphertext. Of course I should have realized very quickly that xoring the plaintext with the ciphertext yields us back the key. But why is this so? Consider the following math:

plaintext xor ciphertext == key <=> plaintext xor (plaintext xor key) <=> plaintext xor plaintext xor key <=> 00000... xor key == key

As you can see, the plaintext cancels out. If the plaintext would be a single byte, say, 1100 1101, then XORing this byte with itself yields:
1100 1101 XOR 1100 1101 -------- 0000 0000

To finally get to solution of the wargame, you can safe the following file as a PHP file and run it:


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

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

    return $outText;

function decodeData($data, $key) {
    return json_decode(xor_encrypt(base64_decode($data), $key), true); 

function encodeData($data, $key) {
    return base64_encode(xor_encrypt(json_encode($data), $key));

function encodeData2($data, $key) {
    return base64_encode(xor_encrypt($data, $key));

function xstrings($s1, $s2) {
    if (strlen($s1) != strlen($s2)) {
        print 'Strings must be equal in length!';
    $res = '';
    for($i = 0; $i < strlen($s1); $i++) {
        $res .= $s1[$i] ^ $s2[$i];
    echo $res."\n";
    echo bin2hex($res)."\n";

// finding out the key
                     json_encode(array( "showpassword"=>"no", "bgcolor"=>"#ffffff")));
// the above function outputs
// qw8Jqw8Jqw8Jqw8Jqw8Jqw8Jqw8Jqw8Jqw8Jqw8Jq
// 7177384a7177384a7177384a7177384a7177384a7177384a7177384a7177384a7177384a7177384a71
// we can easily see that the xor key must be 'qw8J'

$key = 'qw8J';
// generate the new data with the key
echo 'Submit the following as the "data" cookie to gain access: '.encodeData(array("showpassword"=>"yes", "bgcolor"=>"#ffffff"), $key)."\n";


Here a screenshot of the message you get when submitting the generated cookie:

Screenshot - 10.09.2015 -16:32:49