C.3 Algorithm A8

3GPP43.020Release 17Security related network functionsTS

Algorithm A8 is considered as a matter for GSM PLMN operators as is Algorithm A3.

A proposal for a possible Algorithm A8 is managed by GSM/MoU and available upon appropriate request.

C.3.1 Purpose

As defined in 3GPP TS 43.020, Algorithm A8 must compute the ciphering key Kc from the random challenge RAND sent during the authentication procedure, using the authentication key Ki.

C.3.2 Implementation and operational requirements

On the MS side, Algorithm A8 is contained in the SIM, as specified in 3GPP TS 42.017.

On the network side, Algorithm A8 is co-located with Algorithm A3.

The two input parameters (RAND and Ki) and the output parameter (Kc) of Algorithm A8 shall follow the following formats:

– length of Ki: 128 bits;

– length of RAND: 128 bits;

– length of Kc: 64 bits.

Since the maximum length of the actual ciphering key is fixed by GSM/MoU, Algorithm A8 shall produce this actual ciphering key and extend it (if necessary) into a 64 bit word where the non-significant bits are forced to zero. It is assumed that any non-significant bits are the least significant bits and that, the actual ciphering key is contained in the most significant bits. For signalling and testing purposes the ciphering key Kc has to considered to be 64 unstructured bits.

Annex D (normative):
Security related network functions for General Packet Radio Service

This annex is only applicable if GPRS is supported.