How does VBScript communicate with IE popup dialog box?

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I am very new to VBScript. I am working on a web application that
requires user authentication through digital certificates.

When the user attempts to log into his account from a web form, IE
pops up a dialog box with a list of digital certificates available to
this user. The dialog box looks exactly like this one:

http://farm1.static.flickr.com/167/442962234_ab4a756b8c_o.png

The user can select an appropriate certificate from that list. Now, I
would like to know in my VBScript which one has been selected by the
user, so that I can retrieve the selected certificate (and thus the
private key) and sign a token, which will be sent to the web server
for verification.

I have no idea how VBScript communicates with such kind of IE popups.

Any hint? Thanks a lot!

.