3GPP43.318Generic Access Network (GAN)Release 16Stage 2TS
The message flow for security mode control is shown in the following figure.
Figure 46: Security Mode Control
1. The CN sends the RANAP Security Mode Command message to the GANC. This message contains the integrity key (IK) and allowed algorithms, and optionally the cipher key (CK) and allowed algorithms.
2. The GANC selects the integrity algorithm and (optionally) the ciphering algorithm based on the permitted algorithms received from the CN and the MS security capabilities indicated in the IE "3G Security Capability" received from the MS in the GA-RC REGISTER REQUEST message. The GANC sends the GA-RRC SECURITY MODE COMMAND message to the MS. This message indicates the selected integrity protection algorithm and ciphering algorithm (i.e., that are applicable after handover to UTRAN), and a random number. The MS stores the information for possible future use after a handover to UTRAN.
3. The MS computes a MAC based on the random number, the MS IMSI and the integrity key. The MS then sends the GA-RRC SECURITY MODE COMPLETE message including the computed MAC.
4. The GANC verifies the MAC. If the GANC verifies the MAC to be correct it sends the Security Mode Complete message to the CN.
NOTE: The MAC proves that the identity that is authenticated to the GANC is the same as the identity authenticated to the core network.