A subscriber identity module or subscriber identification module, widely known as a SIM card, is an integrated circuit that is intended to securely store the international mobile subscriber identity number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices.

What information does a SIM card hold? The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. The carrier provides this key.

SIM authentication goes like this:

On startup, the phone obtains the IMSI from the SIM card and relays it to the network. Think of this as the “request for access.”

The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key.

The network generates a random number, A, and signs it with the authentication key to create a new number, B. This is the response it would expect if the SIM card is legitimate.

The phone receives A from the network and forwards it to the SIM card, which signs it with its own authentication key to create a new number, C. This number is relayed back to the network.

If the network’s number A matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted.

Long story short: this data not only determines which network to connect to but also acts as the “login credentials” which allow a phone to use said network.