You can override default library with your own library.

You can try below code:

Create a file: /codeigniter/application/libraries/MY_Encrypt.php

And have below code to get started:

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class MY_Encrypt extends CI_Encrypt {

    public function __construct()
    {
        //parent::__construct();
    }

    /*
    Create your custom encryption and decryption logic by
    overriding function
    */

}