Hello, i have got several questions, please help me... it's really urgent. First question. i guess that there are several authentication cerificates such as server certificate, client certificate and user certificate on android OS. Is that right ?? If it is right, could you expain that what difference is between? i mean in aspect of concept. Second question also i'm not sure it is right that there are OS free installed and Add in in CA. if it is right, please expain me. Third question if the above facts are right , how it is applied to mail, browser, wireless LAN and VPN on android OS? Thank you so much for reading me and i will really appeciate it if someone answer about it.