*
* Food for thought.
* A folkloric supposed benefit of CTR mode is the ability to easily "seek forward" into the ciphertext; to access byte N of the ciphertext, all you need to be able to do is generate byte N of the keystream. Imagine if you'd relied on that advice to, say, encrypt a disk.
*/
require_once '../utils/random-bytes.php';
require_once '../03-block-and-stream-crypto/18-implement-ctr-the-stream-cipher-mode.php';
function editAES128CTR($ciphertext, $key, $nonce = "", $offset, $newtext)
{
$newLen = strlen($newtext);
// really dirty - I could calculate block specific keys, but I don't really have to for this.
$plaintext = encryptAES128CTR($ciphertext, $key, $nonce);
$plainLen = strlen($plaintext);
$newPlaintext = '';
if ($offset > 0) {
$newPlaintext = substr($plaintext, 0, $offset);
}
$newPlaintext .= $newtext;
if ($newLen + $offset < $plainLen) {
$newPlaintext .= substr($plaintext, $newLen + $offset);
}
return encryptAES128CTR($newPlaintext, $key, $nonce);
}
// don't output if we're included into another script.
if (!debug_backtrace()) {
$key = getRandomBytes(16);
$nonce = getRandomBytes(16);
$ciphertext = encryptAES128CTR(decryptAES128ECB(base64_decode(file_get_contents('25-data.txt')), 'YELLOW SUBMARINE'), $key, $nonce);
$editedCiphertext = editAES128CTR($ciphertext, $key, $nonce, 0, str_repeat("", strlen($ciphertext)));
print "Recovered plaintext:\n";
print $ciphertext ^ $editedCiphertext . "\n\n";
}
function decryptedProfile($ciphertext, $key)
{
parse_str(decryptAES128ECB($ciphertext, $key), $profile);
return $profile;
}
{
$blocks = str_split($data, 16);
foreach ($blocks as &$block) {
$block = _decryptAES128ECB($block, $key);
}
$plaintext = implode($blocks);
try {
return removePKCS7Padding($plaintext);
} catch (Exception $e) {
if ($strictPadding) {
throw $e;
}
return $plaintext;
}
}
// don't output if we're included into another script.
if (!debug_backtrace()) {
$encrypted = base64_decode(file_get_contents('07-data.txt'));
$key = 'YELLOW SUBMARINE';
$decryptedSane = removePKCS7Padding(_decryptAES128ECB($encrypted, $key));
$decrypted = decryptAES128ECB($encrypted, $key);
print "Sanity check:\n";
$sanity = $decryptedSane === $decrypted;
print $sanity ? "Success!\n\n" : "Failure :(\n\n";
print "Homebrew sanity check:\n";
$test = '0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef';
$sanity = decryptAES128ECB(encryptAES128ECB($test, $key), $key) === $test;
print $sanity ? "Success!\n\n" : "Failure :(\n\n";
print "Decrypted data:\n";
print "{$decrypted}\n";
}
