13.3 State Transitions and Mobility

As a general principle, on RRC_IDLE to RRC_CONNECTED transitions, RRC protection keys and UP protection keys are generated while keys for NAS protection as well as higher layer keys are assumed to be already available. These higher layer keys may have been established as a result of an AKA run, or as a result of a transfer from another AMF during handover or idle mode mobility see TS 23.502 [22]).

On RRC_CONNECTED to RRC_IDLE transitions, the gNBs deletes the keys it stores for that UE such that state information for idle mode UEs only has to be maintained in AMF. It is also assumed that gNB does no longer store state information about the corresponding UE and deletes the current keys from its memory. In particular, on connected to idle transitions:

• The gNB and UE delete NH, KgNB, KRRCint, KRRCenc, KUPint and KUPenc and related NCC;
• AMF and UE keeps KAMF, KNASint and KNASenc stored.

On mobility with vertical key derivation the NH is further bound to the target PCI and its frequency ARFCN-DL before it is taken into use as the KgNB in the target gNB. On mobility with horizontal key derivation the currently active KgNB is further bound to the target PCI and its frequency ARFCN-DL before it is taken into use as the KgNB in the target gNB (see clause 13.1). In both cases, ARFCN-DL is the absolute frequency of CD-SSB of the target PCell.

It is not required to change the AS security algorithms during intra-gNB-CU handover. If the UE does not receive an indication of new AS security algorithms during an intra-gNB-CU handover, the UE shall continue to use the same algorithms as before the handover (see TS 38.331 [12]).