SIM Card Internals
Understanding the physical structure and components of SIM cards
Physical Structure
A SIM (Subscriber Identity Module) card is a small, portable memory chip that stores information specific to a mobile subscriber. The physical structure of a SIM card consists of several key components:
Chip
The integrated circuit (IC) chip is the core component of the SIM card, containing the processor, memory, and security modules. This chip stores the IMSI (International Mobile Subscriber Identity), authentication keys, and other subscriber data.
Contacts
The gold-plated contacts on the surface of the SIM card provide the electrical connection between the SIM card and the mobile device. These contacts follow the ISO/IEC 7816 standard and enable communication with the device.
Plastic Casing
The plastic body of the SIM card protects the chip and provides structural support. It's typically made of PVC or other durable plastics and includes embossed information such as the ICCID (Integrated Circuit Card Identifier).
Polarization Mark
As specified in TS 02.07, SIM cards include a polarization mark that indicates how the user should insert the card into the mobile equipment (ME).
SIM Card Types and Form Factors
Standard SIM (Mini-SIM)
The original SIM card format, also known as Mini-SIM or ID-1 SIM, measures 25mm × 15mm × 0.76mm. According to the specifications, "Format and layout of the ID-1 SIM shall be in accordance with ISO/IEC 7816-1,2".
SIM Card Chip Components
The SIM card chip contains several key components that enable its functionality:
CPU (Central Processing Unit)
The microprocessor that executes instructions and manages the SIM card's operations. It handles authentication algorithms, data encryption/decryption, and communication with the mobile device.
Memory
SIM cards contain different types of memory:
- ROM (Read-Only Memory): Contains the operating system and fixed data
- EEPROM (Electrically Erasable Programmable ROM): Stores subscriber data, contacts, and SMS messages
- RAM (Random Access Memory): Used for temporary data processing
Cryptographic Co-processor
Dedicated hardware for performing cryptographic operations, including:
- A3 Algorithm: Used for authenticating the subscriber
- A8 Algorithm: Used to generate the cipher key
- A5 Algorithm: Used for enciphering/deciphering data
I/O Controller
Manages the communication between the SIM card and the mobile device through the physical contacts, following the protocols defined in standards like ISO/IEC 7816.
Role of Each Component
Each component of the SIM card plays a specific role in its overall functionality:
CPU
The CPU executes the SIM card's operating system, processes commands from the mobile device, and manages access to the file system. It enforces security policies and executes the authentication algorithms during network registration.
Memory Components
The memory components store different types of data:
- ROM: Stores the operating system and fixed applications
- EEPROM: Stores the IMSI, authentication key (Ki), phone book entries, SMS messages, and other user data
- RAM: Provides working memory for the CPU during operations
Cryptographic Co-processor
The cryptographic co-processor handles security-related operations:
- Executes authentication algorithms to verify the subscriber's identity to the network
- Generates encryption keys for secure communication
- Protects sensitive data from unauthorized access
I/O Controller
The I/O controller:
- Manages the physical communication between the SIM card and the mobile device
- Processes incoming commands and formats outgoing responses
- Implements the T=0 protocol as specified in the GSM standards