function get_prefix_len($key)
{
$mult = 3;
$bsize = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_ECB);
$enc = encrypt_aes_ecb(str_repeat('A', $bsize * $mult), $key);
$blocks = str_split($enc, $bsize);
$index = $prefix_len = 0;
for ($i = 1, $c = count($blocks); $i < $c; $i++) {
if ($blocks[$i] == $blocks[$i - 1]) {
$index = $i;
} else {
if ($index != 0) {
break;
}
}
}
for ($i = $bsize * $mult - 1; $i >= 0; $i--, $prefix_len++) {
$enc = encrypt_aes_ecb(str_repeat('A', $i), $key);
$blocks = str_split($enc, $bsize);
if ($blocks[$index] != $blocks[$index - 1]) {
$prefix_len += $bsize * ($index - $mult + 1);
break;
}
}
return $prefix_len;
}
// Attempts to determine block cipher mode based on duplicate blocks (or lack of) - (requires repitition).
function detect_aes_mode($data)
{
$blocks = str_split($data, mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_ECB));
return count(array_unique($blocks)) < count($blocks) ? 'ecb' : 'cbc';
}
$key = random_aes_key(16);
// Determines block-size of padded ciphertext by invoking the pad function and monitoring the result.
for ($s = 'A', $j = strlen(encrypt_aes_ecb($s, $key)); ($k = strlen(encrypt_aes_ecb($s, $key))) == $j; $s .= 'A') {
}
$bsize = $k - $j;
$mode = detect_aes_mode(encrypt_aes_ecb(str_repeat('A', $bsize * 2), $key));
$chars = range(chr(0), chr(255));
echo "Block size: {$bsize}\nCipher mode: {$mode}\n\n";
for ($i = 0, $j = 1, $end = $bsize, $dec = ''; $i < $end; $i++, $j++) {
$str = str_repeat('A', $bsize - $j);
$block = substr(encrypt_aes_ecb($str, $key), 0, $end);
$str2 = $str . $dec;
foreach ($chars as $char) {
$dict[$char] = substr(encrypt_aes_ecb($str2 . $char, $key), 0, $end);
}
if (in_array($block, $dict)) {
$char = array_search($block, $dict);
$dec .= $char;
echo $char;
}
if (strlen($dec) % $bsize == 0) {
$j = 0;
$end += $bsize;
}
}
