The SIM card is a smart chip card with a microprocessor. It consists of the following modules: --- CPU --- program memory (ROM) --- working memory (RAM) --- data memory (EPROM or E2PROM)---The serial communication unit these five modules must be integrated in an integrated circuit, otherwise its security will be threatened. Because the connection between chips may be an important clue for illegal access and misappropriation of SIM cards. In actual use, there are two types of SIM cards with the same functions and different forms: (a) A card type (commonly known as a large card) SIM card. This type of SIM card conforms to the ISO...7816 standard of an IC card, and is similar to an IC card. (b) The embedded (commonly known as small card) SIM card, which is only 25mm x 15mm in size, is a card that is semi-permanently loaded into a mobile station device. Both card exteriors are waterproof, wear-resistant, anti-static, reliable and highly accurate.

Features of SIM Card Function The SIM card mainly performs two functions: storing data (control access to various data) and completing the client identity authentication and client information encryption under security conditions (personal identification number PIN, authentication key Ki is correct). The entire process of the algorithm. This function is mainly accomplished by a microprocessor with an operating system in the SIM card. The chip has eight contacts. Interworking with the mobile station device is completed after the card insertion device is turned on. At this point, the operating system and instruction settings can provide intelligent features for the SIM. The logical structure of the smart features of the SIM card is a tree structure. All characteristic parameter information is expressed in the form of a data field. That is, there are three application directories in the root directory, one is the application directory of the administrative department, and two are the application directories of the technology management, namely, the GSM application directory and the telecommunications application directory. All directories are data fields with binary and formatted data fields. Some of the information in the data field is persistent and cannot be updated, and some are temporary and need to be updated. Each data field must express its purpose, degree of update, characteristics of the data field (such as an identifier), whether the type is binary or formatted. In addition to storing normal data fields, the SIM card also stores non-file fields such as authentication keys, personal identification numbers, and personal unlock codes. The SIM card was developed with the GSM phase to develop its phased functions. The functions proposed in the second stage are usually established on the basis of the first stage, so the second phase of the SIM card also applies to the first stage.

The processing flow of the SIM card The processing of the SIM card generally goes through the following steps: --- Purchase card --- SIM card processing (Write card) --- Data transfer --- Issuing the card in the card manufacturing process, the operating system Has been loaded into the ROM of the chip. Before the card is purchased and the customer is put into normal use, a personal program must be executed on the card. The personalization program can be divided into: (1) Initialization Generally, the card manufacturer puts the operating system code, such as specific data (factory number, shipping number, etc.), A3, A8 algorithm, etc., into the SIM card and completes it once. (2) Pre-personalization Pre-personalization of the SIM card, ie establishing a directory structure, formatting A3, A8, and loading the data required by the network operator, such as personal identification code (PIN), personal identification code The decryption key (PUK code) is also completed once. (3) Personalization Loads the specified information into the SIM card, such as IMSI, Ki, Kc, access control level, etc. When the relevant data is to be transmitted to the authentication center after the card is written, the IMSI and Ki are encrypted and transmitted if a high-level algorithm equation dedicated to the data transmission method is used; the floppy disk containing the data is packaged separately with the SIM card by mail. Mail to prevent loss of confidentiality.