PHP AES 128 bit CBC Encryption gives a size warning

Question

i am using the AES ALgo with 128 bit with CBC cipher mode encryption, below is the code:

$cc = 'my secret text';
$key = '3aa22e01c04c7059778c54d122b0273689fba00f4a166a66d15f7ba6a8ba8743';
$iv =  '1234567890123456';
$length = strlen($cc);

$cipher = mcrypt_module_open(MCRYPT_RIJNDAEL_128,'','cbc','');

mcrypt_generic_init($cipher, $key, $iv);
$encrypted = base64_encode(mcrypt_generic($cipher,$cc));
mcrypt_generic_deinit($cipher);

mcrypt_generic_init($cipher, $key, $iv);
$decrypted = mdecrypt_generic($cipher,base64_decode($encrypted));
mcrypt_generic_deinit($cipher);

echo "encrypted: " . $encrypted;
echo "\n";
echo "decrypted: " . substr($decrypted, 0, $length) . "\n";

But this gives me a warning with output as :

Warning: mcrypt_generic_init(): Key size too large; supplied length: 64, max: 32 in /var/www/cipher.php on line 10 Warning: mcrypt_generic_init(): Key size too large; supplied length: 64, max: 32 in /var/www/cipher.php on line 14 encrypted: vM/XVYSjs/QApdCUEQ8bdQ== decrypted: my secret text

Now can someone guide me on why I am facing the size issue and how I can eradicate the issue of size without changing the key...

Could you please indicate if using the first 16 bytes of the key value helped? — Maarten Bodewes, Jun 17 '13 at 08:08

score 0 · Answer 1 · answered Jun 13 '13 at 13:33

0

You are doing 128-bit encryption but your key is 64 hex characters long, which is 265 bits.

Either you need to be doing 256 bit encryption or your key needs to half the size.

answered Jun 13 '13 at 13:33

James Grant

217
1
11

is there no way you can use a smaller key... as trying to use that key for some other online AES encryptors is not allowed. – James Grant Jun 13 '13 at 14:32
It seems that it does decrypt, so using the first 32 characters (16 hexadecimal bytes) should fix the issue. – Maarten Bodewes Jun 14 '13 at 01:01

score 0 · Answer 2 · answered Jul 30 '13 at 14:25

I modify your code and it runs fine, Please, check my changes, ok

$data_to_encrypt = "2~1~000024~0910~20130723092446~T~00002000~USD~F~375019001012120~0~0~00000000000~";
$key128 = "abcdef0123456789abcdef0123456789";
$iv = "0000000000000000";

$cc = $data_to_encrypt;
$key = $key128;
$iv =  $iv;
$length = strlen($cc);

$cipher = mcrypt_module_open(MCRYPT_RIJNDAEL_128,'','cbc','');

mcrypt_generic_init($cipher, $key, $iv);
$encrypted = base64_encode(mcrypt_generic($cipher,$cc));
mcrypt_generic_deinit($cipher);

mcrypt_generic_init($cipher, $key, $iv);
$decrypted = mdecrypt_generic($cipher,base64_decode($encrypted));
mcrypt_generic_deinit($cipher);

echo "encrypted: " . $encrypted;
echo "<br/>";
echo "length:".strlen($encrypted);
echo "<br/>";
echo "decrypted: " . substr($decrypted, 0, $length);

PHP AES 128 bit CBC Encryption gives a size warning

2 Answers2