ITSec HW 1

<?php

// Basic settings
$filename_v1_1 = "LabProfile-v1.1.crypt";
$filename_v1 = "LabProfile-v1.crypt";
$blockSize = 16;

// reading input files as hex strings
$content_v1 = read_file_as_hex($filename_v1);
$content_v1_1 = read_file_as_hex($filename_v1_1);

$result = "";
// We know the real 9th block and we can start from there
$previousR2 = "20202020202020202020202020202020";
for ($i = 9; $i < (strlen($content_v1) / $blockSize / 2); $i++) {
    // first xor the same blocks
    $r1 = xor_blocks($content_v1, $i, $content_v1_1, $i, $blockSize);
    // then xor the result with the previous message
    $r2 = xor_strings($previousR2, $r1);
    // then convert it to readable string
    $result .= hex_to_string($r2);
    $previousR2 = $r2;
}
echo $result;

// XOR two blocks from the encrypted bytes
function xor_blocks(string $hex1, int $blockNum1, string $hex2, int $blockNum2, int $blockSize): string
{
    // Split the string into octets
    $chunk1 = get_block($hex1, $blockNum1, $blockSize);
    $chunk2 = get_block($hex2, $blockNum2, $blockSize);
    return xor_strings($chunk1, $chunk2);
}

// get a block by block number and block size
function get_block(string $hex, int $blockNum, int $blockSize): string
{
    return substr($hex, (2 * ($blockNum - 1) * $blockSize), $blockSize * 2);
}

// xor two hex strings and returns the result as hex string
function xor_strings(string $hex1, string $hex2): string
{
    $octets_1 = str_split($hex1, 2);
    $octets_2 = str_split($hex2, 2);

    $result = '';
    for ($i = 0; $i < count($octets_1); $i++) {
        // Calculate XOR (converting to decimal numbers first then back to hex string)
        $res = strtoupper(dechex(hexdec($octets_1[$i]) ^ hexdec($octets_2[$i])));
        // Pad with 0 if necessary
        $result .= strlen($res) == 1 ? ("0" . $res) : $res;
    }
    return $result;
}

// read a file and convert it to hex strings
function read_file_as_hex(string $filename): string
{
    $handle = fopen($filename, "r");
    $content = strtoupper(unpack("H*", fread($handle, filesize($filename)))[1]);
    fclose($handle);
    return $content;
}

// convert a hex string to ascii characters
function hex_to_string(string $hex): string
{
    $string = '';
    $octets = str_split($hex, 2);
    for ($i = 0; $i < count($octets); $i++) {
        $string .= chr(hexdec($octets[$i]));
    }
    return $string;
}