SIM card (Subscriber Identity Module) is a type of integrated circuit, typically in the form of a smart card, designed to securely store the International Mobile Subscriber Identity (IMSI) number and its related key, used to identify and authenticate subscribers on mobile telephone devices such as mobile phones, tablets, laptops, smartwatches, and satellite phones.
Overview
A SIM card is a removable smart card that contains a unique serial number (ICCID), an International Mobile Subscriber Identity (IMSI), security authentication and ciphering information, temporary network-related data, a list of accessible services, and two pairs of passwords: a PIN (Personal Identification Number) for ordinary use and a PUK (Personal Unblocking Key) for PIN unlocking. The card enables a mobile device to connect to a cellular network and is transferable between different devices.
Technically, the physical card is known as a Universal Integrated Circuit Card (UICC), with the SIM being the application running on it. In common usage, "SIM card" refers to the entire unit.
History
The first SIM card was manufactured in 1991 by the Munich-based smart-card maker Giesecke+Devrient, which sold the first 300 SIM cards to the Finnish wireless network operator Radiolinja. Radiolinja subsequently launched the world's first commercial 2G GSM cellular network that same year. The SIM was initially specified by the European Telecommunications Standards Institute (ETSI) in specification TS 11.11.
Data Stored
SIM cards store network-specific information used for authentication and identification:
- ICCID (Integrated Circuit Card Identifier): A unique serial number of up to 19 or 20 digits, defined by ITU-T recommendation E.118.
- IMSI (International Mobile Subscriber Identity): A unique identifier for the subscriber on the operator's network, typically up to 15 digits.
- Ki (Authentication Key): A 128-bit value used to authenticate the SIM on the GSM network.
- LAI (Location Area Identity): Network state information used for location tracking.
- SMS messages and contacts: Most SIM cards can store SMS messages and phone book contacts (name and number pairs), with modern cards typically storing over 250 contacts.
Form Factors
SIM cards have been produced in several physical formats:
| Format | Introduced | Dimensions |
|---|---|---|
| Full-size (1FF) | 1991 | 85.6 × 53.98 mm |
| Mini-SIM (2FF) | 1996 | 25 × 15 mm |
| Micro-SIM (3FF) | 2003 | 15 × 12 mm |
| Nano-SIM (4FF) | 2012 | 12.3 × 8.8 mm |
| Embedded-SIM (eSIM) | 2016 | 6 × 5 mm (soldered) |
All non-embedded formats share the same ISO/IEC 7816 pin arrangement and contact area.
Security
SIM cards use cryptographic algorithms to authenticate subscribers. The authentication key (Ki) is stored on the card and in the carrier's database (Authentication Center). During authentication, the network sends a random number (RAND) to the device, which passes it to the SIM. The SIM signs it with the Ki and returns a signed response. If the response matches the network's calculation, the subscriber is authenticated.
Notable security incidents include the 2013 discovery of vulnerabilities in SIM cards using the outdated DES encryption, and the 2015 revelation that the NSA and GCHQ had allegedly stolen encryption keys from Gemalto (now Thales DIS), a major SIM card manufacturer.
Developments
- USIM (Universal Subscriber Identity Module): Introduced with UMTS/3G networks, providing mutual authentication and longer encryption keys.
- eSIM (Embedded SIM): A programmable SIM soldered directly into a device, allowing remote provisioning and operator switching without physical card replacement. Began superseding physical SIM cards in the late 2010s.
- iSIM (Integrated SIM): SIM functionality integrated directly into the device's modem chip or main processor, offering smaller size and lower cost for IoT applications.
- Dual SIM: Devices with two SIM card slots, allowing use of two separate subscriptions on one device, common in developing markets.
Carrier Locking
On some networks, mobile phones are locked to a specific carrier's SIM card, meaning the device will only function with SIM cards from that carrier. This practice is common in markets where carriers subsidize the cost of handsets. In countries where phones are not subsidized (e.g., India, Israel, Belgium), devices are typically sold unlocked.
