decrypt.php

<?php
// with permutations of 'matrix' the encrypted text couldn't be decrypted
// there are more 6 char long names, see the following list:
// http://de.wikipedia.org/wiki/Matrix_%28Film%29#Synchronisation
// !! these are the German names !! 
$initial_guess = "matrix";

function permutate($input) {

    $guesses = array();

    if (strlen($input) > 1) {
        $first_char = $input[0];
        $permutations = permutate(substr($input, 1, strlen($input)));

        foreach($permutations as $permutation) {
            $guesses[] = strtolower($first_char) . $permutation;
            $guesses[] = strtoupper($first_char) . $permutation;
        }
    } else {
        $guesses[] = strtolower($input);
        $guesses[] = strtoupper($input);
    }

    return $guesses;
}

function replace_chars_on_position(array $permutations, $substitutes) {
    $arr = array();

    foreach ($permutations as $permutation) {
        foreach ($substitutes as $substitute) {
            foreach ($substitute as $sub) {
                $permutation_to_be_changed = $permutation;
                $permutation_to_be_changed[$sub->position] = $sub->symbol;
                $arr[] = $permutation_to_be_changed;
            }
        }
    }

    return $arr;
}

$l33t_substitutions = <<<JSON
[
    [
        { "position": 1, "symbol": "4" }
    ],
    [
        { "position": 4, "symbol": "1" }
    ],
    [
        { "position": 1, "symbol": "4" },
        { "position": 4, "symbol": "1" }
    ]
]
JSON;

$permutations = permutate($initial_guess);
$permutations = array_merge(
    $permutations,
    replace_chars_on_position($permutations, json_decode($l33t_substitutions))
);


$encrypted = "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";
$encrypted = base64_decode($encrypted);

$iv = "DkBbcmQo1QH+ed1wTyBynA==";
$iv = base64_decode($iv);

// no we walk through all the permutations of 'matrix' (with l33t included)
foreach ($permutations as $key) {

    // extend the key by the 26 known bytes
    for ($j=0; $j<26; $j++) {
        $key .= "\0";
    }

    echo $decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $encrypted, MCRYPT_MODE_CBC, $iv);
}