Other meanings of Subscriber identity module
Telecommunications
A subscriber identity module (SIM) is an integrated circuit that securely stores the international mobile subscriber identity (IMSI) and related keys used to identify and authenticate a subscriber on a mobile network. It is typically a removable card inserted into a mobile device, enabling the device to connect to the network and access services such as voice, data, and messaging. The SIM also stores contacts and SMS messages, and can be programmed with carrier-specific applications.
The SIM card was developed in 1991 by Munich smart-card maker Giesecke & Devrient, which sold the first 300 cards to Finnish wireless operator Radiolinja. The concept was standardized by the European Telecommunications Standards Institute (ETSI) as part of the GSM specification, which defined the SIM as a mandatory component for mobile devices.1 Early SIMs were the size of a credit card, but the form factor shrank over time to mini, micro, and nano sizes to fit increasingly compact phones. The introduction of the embedded SIM (eSIM) in 2016 allowed the SIM function to be soldered directly onto a device's motherboard, enabling remote provisioning and eliminating the need for a physical card.2
The SIM stores the IMSI, a unique 15-digit number that identifies the subscriber, along with a 128-bit authentication key (Ki) and a cryptographic algorithm (e.g., COMP128 or MILENAGE) used to authenticate the device to the network.3 When a device powers on, it reads the IMSI and sends it to the network, which challenges the SIM with a random number; the SIM computes a response using the Ki and algorithm, and the network verifies it to grant access. The SIM also stores an integrated circuit card identifier (ICCID), a unique serial number printed on the card, and a location area identity (LAI) to track the device's current network region. Additionally, the SIM can store contacts, SMS messages, and carrier-specific applications such as mobile payment or roaming profiles.
SIM cards have evolved through several physical sizes: the full-size (85.6×53.98 mm), mini-SIM (25×15 mm), micro-SIM (15×12 mm), and nano-SIM (12.3×8.8 mm), with the nano-SIM being the current standard for most smartphones.4 The eSIM, standardized by the GSMA, is a chip embedded in the device that can be programmed with multiple profiles, allowing users to switch carriers without swapping cards. A related variant is the virtual SIM, which is software-based and used in some IoT devices. Additionally, the SIM can be implemented as a UICC (universal integrated circuit card) that supports multiple applications, such as USIM for 3G/4G/5G networks and CSIM for CDMA networks.
The SIM is designed to be tamper-resistant, with the Ki never leaving the card; attempts to extract it require physical decapsulation and are difficult due to hardware and software protections.5 SIM cloning, once a concern with older COMP128 algorithms, has been mitigated by stronger algorithms and mutual authentication. Beyond phones, SIMs are used in machine-to-machine (M2M) devices such as smart meters, vehicle tracking systems, and industrial sensors, where they enable cellular connectivity. The eSIM has also enabled new use cases like smartwatches and laptops with built-in cellular capabilities. In addition, the SIM can store a personal identification number (PIN) that locks the card, and a personal unblocking key (PUK) to reset the PIN if forgotten.
One lesser-known fact is that the SIM card was originally conceived as a way to allow subscribers to move their identity between phones, a feature that was not initially supported by the GSM standard but became a key selling point. Another is that the SIM can store a short message service (SMS) center number, which is used to route text messages. The SIM also contains a file system with a hierarchical structure, including files for network parameters, service provider name, and emergency call codes. In some countries, SIM cards are registered to the user's identity for security and law enforcement purposes, a practice known as SIM registration. Additionally, the SIM can be used for non-telecom applications, such as storing digital certificates for secure transactions or as a form of identification in some government programs.
The SIM card is a cornerstone of mobile communications, enabling secure identification and authentication across billions of devices worldwide.
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