A3 (algorithm) - A3 (Algorithmus)
In GSM networks, the algorithm uses the secret subscriber key Ki (128 bit) and a random number RAND (128 bit) to calculate what is known as a signed response SRES (32 bit, "signed response" in German). The A3 is implemented both in the Authentication Center (AuC) of the home network and on the subscriber's SIM card . The algorithm is not standardized in GSM and can be selected in a suitable manner by the network operator. Nevertheless, there are example implementations from the mobile network operator association GSMA .