使用PHP或Laravel解密Zend 2 Block Cipher加密

I was asked to create an API using the Laravel framework on an existing Web application. The web application uses the Zend 2 Block Cipher encryption to encrypt user password. The previous developer who developed the site has already moved on and I need to create a service to verify the user's password in the API. The problem is I can't crack Zend's encryption. I even tried using the PHP mcrypt function but I'm getting this error.

mcrypt_decrypt(): Size of key is too large for this algorithm

This is a bit odd but is there anyway I can decrypt Zend's encryption using just plain PHP? Or is there a work around here? I can't change the current web application as it is being used.

Here is the sample code of the encryption:

use Zend\Crypt\BlockCipher;

class Encryption {
    private $_cipher = null;

    public function __construct(){
        $this->_cipher = BlockCipher::factory('mcrypt',array('algo'=>'aes'));
        $this->_cipher->setKey('fltjXW05820D[1(Z5SknJBZ12goBbyK<*271biqT5"j$WvA2JCycgA"{UIe6qJ2');
    }

    public function encrypt($plainText){
        return $this->_cipher->encrypt($plainText);
    }

    public function decrypt($enctyptedData){
        return $this->_cipher->decrypt($enctyptedData);
    }
}

Any advice by the experts?

Thank you in advance. Sorry I'm a beginner in Laravel and Zend.