Using Captive Portal with wireless

[bilbus]

I want to use Captive portal to control access to the internet from my guest lan.

I have a Cisco ASA with 3 lan ports and one wan port.

Lan 1 Secure corp lan
Lan 2 Server LAN
Lan 3 Guest Lan

I am looking at using Captive portal so i dont have to maintain and change WPA keys. I was hoping that using Captive portal would make it so i could use a open access point. Is that safe … i would think that somone could still sniff the wireless and see the connected user's data.

Is there a way to run a secure access point without the use having to deal with knowing a wpa key .. or other secure way.

My users are guests so i have no access to their computers.

Anyone have any ideas?