Question

Lately, I have been doing some research into cryptography.
To get a better understanding of all of it, I have been trying to write a more advanced version of the XOR cypher in PHP.
I got the encryption function to work just fine, but the decryption function output is quite strange and totally different from the message inputted.

The idea of the algorithm is to run a XOR operation first on the first and last character, then on the second and one but last character, and so on.
After that, it runs a XOR operation on the first two characters and the last two characters, then the third and fourth character and the 2th and 3th to last, and so on once again.
This goes on with blocks of 3, 4, 5 and more characters.

The code I have right now:

<?php
function encrypt($message, $key) {
    $output_text = '';

    // Add zeros at the end until the length of the message corresponds with the length of
the key
    $message = str_pad($message,strlen($key),0);

    if((strlen($message) % 2)) {
        // The lenght of the message is odd, add a zero
        $message = $message . 0;
    }

    // Define the final length of the message
    $length = strlen($message);

    // Firstly, take 1 character, then 2, then 3, etc. until you reach half the length of the message
    for($characters=1; $characters<=($length/2); $characters++) {
        // Loop from i til half the length of the message
        for($i=0; $i<=(($length/2)-1); $i += $characters) {
            // Take the first and last character, the the first two and the last two, etc.

            // Stop when it crosses half the length
            if( ($i + $characters ) >= ( $length / 2 ) ) break;

            // firstly, the characters at the beginning
            $beginning = substr($message, $i, $characters);
            for($j=0; $j<$characters; $j++) {
                $position = ( $i + 1 ) + $j;
                $output_text .= chr(ord($beginning{$j}) ^ ord($key{$position}));
            }

            // Then those at the end
            $ending = substr($message, $length-(($i+1) * $characters), $characters);
            for($j=0; $j<$characters; $j++) {
                $position = ( $length - ( ( $i + 1 )* $characters) ) + $j;
                $output_text .= chr(ord($ending{$j}) ^ ord($key{$position}));
            }
        }
    }

    return $output_text;
}
function decrypt($message, $key) {
    $output_text = null;

    // Define the final length of the message
    $length = strlen($message);

    // Firstly, take 1 character, then 2, then 3, etc. until you reach half the length of the message
    for($characters=1; $characters<=($length/2); $characters++) {

        // Loop from i til half the length of the message
        for($i=0; $i<=(($length/2)-1); $i += $characters) {
            // Take the first and last character, the the first two and the last two, etc.

            // Stop when it crosses half the length
            if( ($i + $characters ) >= ( $length / 2 ) ) break;

            // firstly, the characters at the beginning
            $beginning = substr($message, $i, $characters);
            for($j=0; $j<$characters; $j++) {
                $position = ( $i + 1 ) + $j;
                $output_text .= chr(ord($key{$position}) ^ ord($beginning{$j}));
            }

            // The those at the end
            $ending = substr($message, $length-(($i+1) * $characters), $characters);
            for($j=0; $j<$characters; $j++) {
                $position = ( $length - ( ( $i + 1 )* $characters) ) + $j;
                $output_text .= chr(ord($key{$position}) ^ ord($ending{$j}));
            }
        }
    }

    return $output_text;
}

$message = 'sampletextjusttotrythisoutcreatedin2012';
$key = '123';
$output_text = encrypt($message, $key);
echo $output_text . '<br /><hr />';
echo decrypt($output_text, $key);

Thanks in advance for trying to help me!

Answer 1

Right now, the hardest part of “decrypting” a string is figuring out how long the input was. If we take that as an additional parameter, we can nearly decrypt it like this:

function decrypt($cipher, $messagelen, $key) {
    if($messagelen % 2) { $messagelen++; }
    $x = substr($cipher, -$messagelen + 2);
    $y = substr($x, 0, strlen($key) - 1) ^ substr($key, 1);
    $z = substr($x, strlen($key) - 1);
    return $y . $z; 
}

This is made much easier because most of the message appears in the clear at the end of the ciphertext. Oops. The only characters in that repetition which are “encrypted” are the first few, which are just XORed with the key.

The middle two characters are irretrievably lost due to an off-by-one error in encryption. Notes on how to fix this are in my comments below.