Using C5's User System to access a subdomain

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Hey there folks :-)

I am facing something with which I don't really know IF it is possible and if so - how.

Scenario:

Customer is running a Concrete5 (5.2.2) site with users and permissions for them, secured with a SSL certificate.

Now he wants to use a python-based software (a Query) which has nothing to do with Concrete5 but wants his Users to only be able to access the Query when being logged into the C5 site.

I was thinking about putting the Query under a subdomain and using the login-cookie or something to verify the login-status of the users.

Goal is to be able to organize user permissions and payment with Concrete5 (because it is so easy) but restrict the use of the Query to the public, even though it is not part of C5 itself.

Has anybody done this before? I do not want to use iFrame or "quick&dirty"-Solutions...

Thank you!