A8 (algorithm)

Relationship between A3, A5 and A8

In GSM networks, the key generation algorithm A8 calculates the key Kc (64 bit) from the secret subscriber key Ki (128 bit) and a random number RAND (128 bit). The Kc is used for the encryption algorithm A5 for encrypting the air interface. The A8 is implemented both in the Authentication Center (AuC) and on the subscriber's SIM . The algorithm is not standardized in GSM and can be selected in a suitable manner by the network operator. The interface for the algorithm, however, is standardized. Nevertheless, there are example implementations from the mobile network operator association GSMA , which are known as COMP128are known. Since the authentication algorithm A3 and the A8 have the same input parameters, they are often implemented together and referred to as A3 / A8.