Cross-Site Authorization

Question

The Scenario:
1. We have a site running .Net 4.0 who's manage the users, logins, data etc...;
2. We have another site running just client side script (javascript);

We need:
A. Log from client side site on .Net site;
B. For the time of .Net session(15min) access data in Database ,
implementing some authorization, or save some info on client on a secure way ;

Any clues how to accomplish or the better way to implement this??
Thx