Build a login page for an existing website which provides a secured link to a script/programme that retrieves data from that website

Build a login page for an existing website which then provides a SECURED LINK to a script/programme that retrieves data from that website. The existing website provides data in JSON format, and I need the new login page to establish a SECURED LINK to existing script/programme (in Google Chrome extension format). There needs to be some sort of "handshake" process between the script/programme and the website (with login page) such that only users who have logged in are able to retrieve that data through the use of the script that is installed in his/her PC/Mac. I will share more details with shortlisted candidates who shows me that they have experience doing such work, and for a reasonable fee.