I'm currently working as a temp at a well-known institution. (I'm getting paid a healthy hourly rate for what would easily be a salaried position if I was on s

I'm currently working as a temp at a well-known institution. (I'm getting paid a healthy hourly rate for what would easily be a salaried position if I was on staff.) It's effectively freelancing, but I had to join an agency to satisfy this corporation's bureaucracy. It's in my contract with the agency that the rate they pay me is confidential, to be discussed with no one. Is that also illegal?