Need to get signature/certificate on web page

I haven't seen this any where but maybe I missed it, this seems like it should be simple but again maybe I am just missing something.

I have a web application (.Net C#) that requires a certificate to login/access. On a particular page I need to have the user "approve" the data. ie they click something to confirm/approve and I save that user's info to the database with a timestamp to indicate who approved it and when. What we want in the database it the name that goes with the digital certificate of the user that is logged in (FirstName.LastName.MiddleName.123456790). How do I access this information? Can I somehow ask them to select a certificate again or just access the information on the one they have already selected to log into the application?