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) [^1].

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" [^1].

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 [^1]
  • A8 Algorithm: Used to generate the cipher key [^1]
  • A5 Algorithm: Used for enciphering/deciphering data [^1]

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
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