XOR string in PHP with key

Question

I need to XOR a string/text in PHP the base64 encode it, but something goes wrong:

<?php

$mustget = 'Kw4SCQ==';
$string = 'Josh';

echo("Must get: " . $mustget . "\n");
echo("We got: " . base64_encode(xor_this($string)) . "\n");

function xor_this($text) {
    $key = 'frtkj';
    $i = 0;
    $encrypted = '';
    foreach (str_split($text) as $char) {
        $encrypted .= chr(ord($char) ^ ord($key{$i++ % strlen($key)}));
    }
    return $encrypted;
}

?>

I get the following result, but I need to get the "$mustget" one:

Must get: Kw4SCQ==
We got: LB0HAw==

What do I do wrong?

I'm working on decrypting a malware in the wild that uses that "encryption" to communicate with its PHP admin panel: blog.spiderlabs.com/2012/12/… — bsteo
– bsteo, Commented Dec 14, 2012 at 9:35
Question is, how did you get that result $mustget = 'Kw4SCQ==' ? — Touki
– Touki, Commented Dec 14, 2012 at 9:36
@xtmtrx: "Each character in the decoded string is xored sequentially against each character of the key we previously identified". So you should xor each data character with each key character — zerkms
– zerkms, Commented Dec 14, 2012 at 9:36
1.The data is Base64 decoded 2.Each character in the decoded string is xored sequentially against each character of the key we previously identified. In Ruby, it looks something like this: "A".xor("f").xor("r").xor("t").xor("k").xor("j") — bsteo
– bsteo, Commented Dec 14, 2012 at 9:37
@xtmtrx: right "A".xor("f").xor("r").xor("t").xor("k").xor("j") --- you xor data character with every key character, not with one — zerkms
– zerkms, Commented Dec 14, 2012 at 9:37

zerkms · Accepted Answer · 2012-12-14 09:43:56Z

9

$mustget = 'Kw4SCQ==';

$key = 'frtkj';
$key_length = strlen($key);

$encoded_data = base64_decode($mustget);

$result = '';

$length = strlen($encoded_data);
for ($i = 0; $i < $length; $i++) {
    $tmp = $encoded_data[$i];

    for ($j = 0; $j < $key_length; $j++) {
        $tmp = chr(ord($tmp) ^ ord($key[$j]));
    }

    $result .= $tmp;
}

echo $result; // Josh

http://ideone.com/NSIe7K

I'm sure you can reverse it and create a function, that "crypts" the data ;-)

answered Dec 14, 2012 at 9:43

zerkms

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1 Comment

Drasill Over a year ago

Modulo is your friend !

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