Abstract: A method for accessing at least a portion of the data on a microprocessor card, such as a Subscriber Identity Module (SIM) card having at least one freely accessible card number for use with mobile phones. A separate master card uses a self-contained key and encryption algorithm together with the accessible SIM card number to generate a first identification number that is communicated to and stored on the SIM card during initial programming of the SIM card. When access to data stored on the SIM card is thereafter required, a second identification number is calculated in the same manner, is communicated to the SIM card, and is compared on the SIM card with the stored first identification number. Access to the data stored on the SIM card is granted only when the first and second identification match.