F.5 Encryption of voice group calls

3GPP43.020Release 17Security related network functionsTS

For the encryption of a voice group call the same encryption algorithms are used as for a normal GSM speech call. Which algorithm out of the algorithm suite A5/x is used is determined by the identifier A5_Id, which is stored on the USIM (together with the group key V_Ki(Group_Id, VK_Id, Service_type)). The algorithm A5/X is used in the same way as in the GSM (see clause C.1) using the key V_Kc as encryption/decryption key Kc as input to A5/x.

If the key length KL of the encryption algorithm A5/X is shorter than the length of V_Kc (128 bit) then only bits [0] to [KL-1] of V_Kc are used.

Figure F.4