Methods and systems are provided that use smartcards, such as subscriber identity module (SIM) cards to provide secure functions for a mobile client. One embodiment of the invention provides a mobile communication network system that includes a mobile network, a mobile terminal, a server coupled to the mobile terminal via the mobile network, and a subscriber identity module (SIM) card coupled to the mobile terminal. The SIM card includes a first key and a second key. The first key is used to authenticate an intended user of the mobile terminal to the mobile network. Upon successful authentication of the intended user to the mobile network, the mobile terminal downloads a function offered from the server through the mobile network. The second key is then used by the mobile terminal to authenticate the intended user to the downloaded function so that the intended user can utilize the function.