Lukas' Notes

Subscriber Identity Module

1 min read

mobile-communication

Definition

Subscriber Identity Module

The Subscriber Identity Module (SIM) in a phone contains a symmetric key, which is only known by the SIM and the mobile communication network.

Even the mobile phone does not know the symmetric key.

Authentication in Mobile Communication Network

  1. The network’s access control (AC) generates a random number and sends it to the mobile station
  2. The AC of the network and the SIM encrypts the random number using the symmetric key, which is only known by the two parties. The encrypted result is called signed response (SRES).
  3. The signed response (SRES) generated by AC and SIM is sent to the MSC.
  4. The MSC compares the two signed responses. If they equal, the mobile station is authenticated.

Encryption

  1. The network’s access control (AC) generates a random number and sends it to the mobile station.
  2. The AC of the network and the SIM encrypt the random number using the symmetric key to generate a new cipher key.
  3. The AC sends the cipher key to the BTS.
  4. Now, data blocks can be encrypted between the BTS and the mobile station.

Graph View