php error The Encrypt library requires the Mcrypt extension in codeigniter

Question

I have a  login and sign-up form and use the encrypt library to encrypt the password. login and sign up form and use the encrypt library to encrypt the password. 

Code to encrypt the password:

$this->encrypt->encode('my password'); 

Add encrypt library

 $autoload['libraries'] = array('encrypt');

Setting the secret key in config:

$config['encryption_key'] = 'nmsc encrypt secret key';

 when I try to upload it to my website to the ubuntu server I've got an error that says

The Encrypt library requires the Mcrypt extension

How can I fix this?