SIMkaardi

SIMkaardi, or SIM card, is a small smart card used in mobile devices to identify a subscriber on a cellular network. It holds identifiers such as the ICCID (Integrated Circuit Card Identifier) and the IMSI (International Mobile Subscriber Identity), as well as a secret authentication key (Ki). The card also stores the operator profile and service settings that allow access to voice, data, and messaging services.

Physically, SIM cards have evolved from the standard/mini to micro and nano sizes. In modern devices, many phones use nano-SIM, while some wearables and tablets use embedded SIMs (eSIM). An eSIM is a soldered chip or an integrated circuit on the device that can be remotely provisioned with operator profiles via remote SIM provisioning (RSP). The newer embedded Universal Integrated Circuit Card (eUICC) enables multiple profiles and switching without exchanging cards.

Functions: The SIM authenticates the device to the network and authorizes use of subscribed services. It stores authentication credentials and network operator data, and may contain a local address book.

Management and security: A PIN code can lock the SIM; if entered incorrectly, a PUK code unlocks it. Carriers can lock a device with a SIM lock. Remote provisioning of eSIM profiles allows changing operators or plans without physical card changes, while also introducing new security considerations such as profile management and potential attacks if the device is compromised.

Lifecycle and usage: SIM cards are issued by mobile operators and linked to a subscriber agreement. They enable roaming and device authentication across networks and are widely used in mobile phones, tablets, and IoT devices. As eSIM adoption grows, physical SIMs remain common but are gradually supplemented or replaced in many devices.