Make an external Page- Check user pass

Hi,
i'm building a external php page where there is a login.
I want use the user already registered in table tl_member

All work fine at the home of the file i wrote

define('TL_MODE', 'FE');
require 'system/initialize.php';

To get the cifrate key and other globals.
Now i'm checking only username because i had a big problem with password.
How can i check a password stored in mysql database ? Off corse this password is encrypted.
I tried the passowrd decrypt mode using mccrypt but the result is not the same.

I view in database the default password is in SHA1 but when i create a new member from panel i view a strange encode.
Contao 3.xx
Thank's
Sorry if i'm not more clear, but my goal is make a simple check in my page html is user & password is correct using tl_member field.