mcrypt_encrypt fails to initialise

Question

I am trying to encrypt some data in PHP using the Rijndael cipher in CBC mode with a 256bit key but for some reason I get the following error message:

mcrypt_encrypt() Module initialization failed

My code:

    $hashKey = hash('sha256',$key);
    $iv = hash('sha256',$hashKey);

    //                                                 ------Cipher-------------key-------------Data-------------Mode---------IV--
    $encryptedQuestion = base64_encode(mcrypt_encrypt('MCRYPT_RIJNDAEL_256', $hashKey , $_POST['question'], MCRYPT_MODE_CBC, $iv));

Can anyone see whats wrong with this?

Answer 1

There are a few issues with the code that I can spot:

1. Your $iv should not be dependent on $hashKey; rather, you should create it separately using mcrypt_create_iv().

2. Your $hashKey should be binary rather than textual.

3. MCRYPT_RIJNDAEL_256 is a constant, it should not be passed as a string.

The following code is more verbose than yours, but it should give you an insight in the steps required to encrypt something:

$crypto = mcrypt_module_open(MCRYPT_RIJNDAEL_256, '', MCRYPT_MODE_CBC, '');
$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($crypto), MCRYPT_DEV_URANDOM);

$hkey = hash('sha256', $key, true);

mcrypt_generic_init($handle, $hkey, $iv);

$enc_question = mcrypt_generic($handle, $_POST['question']);

mcrypt_generic_deinit($handle);
mcrypt_module_close($handle);

I've also left out any error checks.