*
* One of them has been encrypted with ECB.
*
* Detect it.
*
* Remember that the problem with ECB is that it is stateless and deterministic; the same 16 byte plaintext block will always produce the same 16 byte ciphertext.
*/
function repeatedBlockCount($data)
{
$dataLen = strlen($data);
$repetitions = 0;
for ($i = 0; $i < $dataLen; $i += 16) {
$block = substr($data, $i, 16);
$repetition = strpos($data, $block, $i + 16);
if ($repetition && $repetition % 16 === 0) {
$repetitions++;
}
}
return $repetitions;
}
// don't output if we're included into another script.
if (!debug_backtrace()) {
$data = array_map('hex2bin', file('08-data.txt', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES));
foreach ($data as $k => $encrypted) {
$repetitions = repeatedBlockCount($encrypted);
if ($repetitions) {
$line = $k + 1;
print "String at on line {$line} has repeated blocks (probable ECB)\n";
}
}
}
*/
require_once '../utils/random-bytes.php';
require_once '../01-basics/07-aes-in-ecb-mode.php';
require_once '10-implement-cbc-mode.php';
function randomlyEncryptECBorCBC($data)
{
$key = getRandomBytes(16);
$pad1 = getRandomBytes(rand(5, 10));
$pad2 = getRandomBytes(rand(5, 10));
if (rand(0, 1)) {
return encryptAES128CBC("{$pad1}{$data}{$pad2}", $key, getRandomBytes(16));
}
return encryptAES128ECB("{$pad1}{$data}{$pad2}", $key);
}
// don't output if we're included into another script.
if (!debug_backtrace()) {
require_once '../01-basics/08-detect-aes-in-ecb-mode.php';
// so the trick is to feed the black box something that will trigger ECBs weakness regardless of padding
// this means we need at least 3 blocks of repeated data (with padding this reduces to 2 blocks)
$plaintext = str_repeat('a', 48);
print "Running 5000 samples\n";
$ecb = 0;
for ($i = 0; $i < 5000; $i++) {
$ciphertext = randomlyEncryptECBorCBC($plaintext);
if (repeatedBlockCount($ciphertext)) {
$ecb++;
}
}
print "{$ecb} samples detected as ECB mode\n";
print $ecb && round(5000 / $ecb) === 2.0 ? "Success!\n\n" : "Failure :(\n\n";
}