mcrypt long time to initialize module

Question

I have a class which has following __construct.

final class mydecoder {
    private $td;
    public function __construct($key){
        /* Open the cipher */
        $this->td = mcrypt_module_open(MCRYPT_BLOWFISH, '', 'ecb', '');
        /* Create the IV and determine the keysize length, use MCRYPT_RAND
        * on Windows instead */
        $iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($this->td), MCRYPT_DEV_RANDOM);
        $ks = mcrypt_enc_get_key_size($this->td);
        /* Create key */
        $key = substr(md5($key), 0, $ks);
        /* Intialize encryption */
        mcrypt_generic_init($this->td, $key, $iv);
    }
}

When I call it like:

$encoder = new myfish('mykey1');
$encoder = new myfish('mykey2');

I have following problems.

Opening page for first time

construct #1 execution time: 5s
construct #2 execution time: 0s

Pressing F5

construct #1 execution time: 0s
construct #2 execution time: 14s

Pressing F5

construct #1 execution time: 5s
construct #2 execution time: 14s

Mashing F5 for 3 times (to queue)

construct #1 execution time: 15s
construct #2 execution time: 45s

It looks like it queues? How does it work? Maybe I'm not using it properly? I don't have much experience with cryptography since I never needed it before :)

Answer 1

The problem is caused by the initialising vector (IV) genertor, i.e. this line:

$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($this->td), MCRYPT_DEV_RANDOM);

If you use MCRYPT_DEV_RANDOM mode, PHP will wait until there is enough entropy to be secure.

However, if you change the mode to MCRYPT_DEV_URANDOM (it will be less secure) but it wont wait if entropy gets too low, thus improving the speed.