I have web pages on web.mit.edu, and I want to know if a visitor is using certificates. Can I?
If you are using the web.mit.edu server, it is already configured to check certificates. Upon success, it sets environmental variables to reflect certificate information. For illustration, please compare the the "http" and "https" versions of the following pages, particularly the "HTTPS" and "SSL" variables:
Note that you need to use "https" encrypted pages, in order for certificates to work.
If you want to block access if people do not have a certificate, see