There are some wifi networks (large universities for example) which use HTTPS to get to a login page, and in order to verify that they have not been spoofed, provide the certificate ahead of time (i.e. from the companies official website, using SSL, which you are supposed to have accessed & downloaded ahead-of-time).
Now, assuming I followed their instructions and installed this cert (I can't seem to find a good example, it seems many use an exe tool instead that installs the certs into the system), I can log into their HTTPS captive portal and gain internet access - but would it be secure? how can I know that they aren't using the cert that they asked me to install to snoop on all HTTPS traffic? (aside from manually checking the certs for each website)? can I limit what domains their cert is valid for?
P.S. - it may be possible to access the page without the cert, after ignoring the big red text displayed by a browser, but then the risk is transferred to avoiding spoof networks by manually inspecting.. the same cert - question is more aimed at the vulnerability of those less tech savvy who simply follow the instructions as-is.