Do you mean SIM as in the SIM cards you get out of phones, or a similar technology called SMART. SIMs are based on SMART but have more than normal encryption, without knowing any of the algorythums it is very difficult. Unencrypted SMART is quite simple, but with SIM and "CHIP and PIN" and "phone cards", all manor of tricks and steadfast security measures are used to prevent the data from being comprimised. They do things like rewireing them, using different memory protocols and so on as well as encryption in many different shapes and sizes. SMART is a great thing to play with, but SIMs are not worth the effort unless you have inside knowledge of how they work, or the equipment to start with to take apart. Hope this is not too pesimistic for you!