Authentication key (Ki)

The Ki is a secret key that is used to authenticate the subscriber on the network. It is stored securely in the SIM card's memory and is never transmitted over the network.

When the subscriber tries to access the network, the SIM card uses the Ki to generate a response that is sent to the network for authentication.

In addition to these key components, SIM cards may also include other features such as security mechanisms to protect against unauthorized access, storage for contacts and text messages, and even a small amount of memory for storing music or other media.