encryption - Encrypt/decrypt with XOR in PHP

Question

I am studying encryption. And I got a problem like this:

After I XOR plaintext with a key, I get a crypt, "010e010c15061b4117030f54060e54040e0642181b17", as hex type. If I want to get plaintext from this crypt, what should I do in PHP?

I tried convert it to string/int and after that take them to XOR with the key (three letters). But it doesn't work.

This is the code:

function xor_this($string) {

    // Let's define our key here
    $key = 'fpt';

    // Our plaintext/ciphertext
    $text = $string;

    // Our output text
    $outText = '';

    // Iterate through each character
    for($i=0; $i<strlen($text); )
    {
        for($j=0; $j<strlen($key); $j++,$i++)
        {
            $outText .= ($text[$i] ^ $key[$j]);
            //echo 'i=' . $i . ', ' . 'j=' . $j . ', ' . $outText{$i} . '<br />'; // For debugging
        }
    }
    return $outText;
}

function strToHex($string)
{
    $hex = '';
    for ($i=0; $i < strlen($string); $i++)
    {
        $hex .= dechex(ord($string[$i]));
    }
    return $hex;
}

function hexToStr($hex)
{
    $string = '';
    for ($i=0; $i < strlen($hex)-1; $i+=2)
    {
        $string .= chr(hexdec($hex[$i].$hex[$i+1]));
    }
    return $string;
}

$a = "This is the test";
$b = xor_this($a);
echo xor_this($b), '-------------';
//
$c = strToHex($b);
$e = xor_this($c);
echo $e, '++++++++';
//
$d = hexToStr($c);
$f = xor_this($d);
echo $f, '=================';

And this is the result:

This is the test-------------

PHP Notice: Uninitialized string offset: 29 in C: UsersAdministratorDesktopest.php on line 210 PHP Stack trace: PHP 1. {main}() C:UsersAdministratorDesktopest.php:0 PHP 2. xor_this() C:UsersAdministratorDesktopest.php:239

Notice: Uninitialized string offset: 29 in C:UsersAdministratorDesktopest.p hp on line 210

Call Stack: 0.0005 674280 1. {main}() C:UsersAdministratorDesktopest.php:0 0.0022 674848 2. xor_this() C:UsersAdministratorDesktopest.php:23 9

UBE^A?WEAVA?WEAV@?WEARAFWECWB++++++++

This is zs$fs?=================

Why? The "UBE^A?WEAVA?WEAV@?WEARAFWECWB++++++++" is the result, which I got trouble in my real work.

Answer 1

Try this:

function xor_this($string) {

    // Let's define our key here
    $key = ('magic_key');

    // Our plaintext/ciphertext
    $text = $string;

    // Our output text
    $outText = '';

    // Iterate through each character
    for($i=0; $i<strlen($text); )
    {
        for($j=0; ($j<strlen($key) && $i<strlen($text)); $j++,$i++)
        {
            $outText .= $text{$i} ^ $key{$j};
            //echo 'i=' . $i . ', ' . 'j=' . $j . ', ' . $outText{$i} . '<br />'; // For debugging
        }
    }
    return $outText;
}

Basically to revert text back (even numbers are in) you can use the same function:

$textToObfuscate = "Some Text 12345";
$obfuscatedText = xor_this($textToObfuscate);
$restoredText = xor_this($obfuscatedText);