flash.moe/public/key.php

<?php
define('KEY_CHARS', 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz0123456789');
define('KEY_CHARS_LENGTH', strlen(KEY_CHARS));

function generateKey(int $length): string {
    $bytes = str_repeat("\0", $length);
    for($i = 0; $i < $length; ++$i)
        $bytes[$i] = KEY_CHARS[random_int(0, KEY_CHARS_LENGTH - 1)];
    return $bytes;
}

header('Content-Type: text/plain');

if(isset($_GET['length'])) {
    $length = (int)filter_input(INPUT_GET, 'length', FILTER_SANITIZE_NUMBER_INT);
    if($length > 0 && $length < 1000) {
        echo generateKey($length);
        return;
    }
}

echo '8:   ' . generateKey(8) . PHP_EOL;
echo '16:  ' . generateKey(16) . PHP_EOL;
echo '32:  ' . generateKey(32) . PHP_EOL;
echo '64:  ' . generateKey(64) . PHP_EOL;
echo '128: ' . generateKey(128) . PHP_EOL;