Question

I have an encrypted bit of text that I need to decrypt. It’s encrypted with AES-256-CBC. I have the encrypted text, key, and iv. However, no matter what I try I just can’t seem to get it to work.

The internet has suggested that mcrypt’s Rijndael cypher should be able to do this, so here’s what I have now:

function decrypt_data($data, $iv, $key) {
    $cypher = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');

    // initialize encryption handle
    if (mcrypt_generic_init($cypher, $key, $iv) != -1) {
        // decrypt
        $decrypted = mdecrypt_generic($cypher, $data);

        // clean up
        mcrypt_generic_deinit($cypher);
        mcrypt_module_close($cypher);

        return $decrypted;
    }

    return false;
}

As it stands now I get 2 warnings and the output is gibberish:

Warning: mcrypt_generic_init() [function.mcrypt-generic-init]: Key size too large; supplied length: 64, max: 32 in /var/www/includes/function.decrypt_data.php on line 8
Warning: mcrypt_generic_init() [function.mcrypt-generic-init]: Iv size incorrect; supplied length: 32, needed: 16 in /var/www/includes/function.decrypt_data.php on line 8

Any help would be appreciated.

Answer 1

I’m not terribly familiar with this stuff, but it seems like trying MCRYPT_RIJNDAEL_256 in place of MCRYPT_RIJNDAEL_128 would be an obvious next step…

Edit: You’re right — this isn’t what you need. MCRYPT_RIJNDAEL_128 is in fact the right choice. According to the link you provided, your key and IV are twice as long as they should be:

// How do you do 256-bit AES encryption in PHP vs. 128-bit AES encryption???
// The answer is:  Give it a key that's 32 bytes long as opposed to 16 bytes long.
// For example:
$key256 = '12345678901234561234567890123456';
$key128 = '1234567890123456';

// Here's our 128-bit IV which is used for both 256-bit and 128-bit keys.
$iv =  '1234567890123456';