Is it possible for Drupal to receive a secure cookie for login?

I have just completed a client site.

Part of the site has a 'partner' section that is only accessible by loggin in with a username and password.

The client also has another site (done by another company in .net) which allows online donations and other private login information.

The issue is that from a partner point of view going from one site to the other means having to login twice.

The .net people say that can get their site to send Drupal a secure cookie to avoid this.

So a partner will login to the .net site and then when they go to the Drupal site they will automatically have access to the secure section (because the secure cookie will have logged them into the Drupal site).

Does anyone know how to set this up on a Drupal site? (Receive a secure cookie and use it to log the user in.)

Thanks.