 has been performed, which allows the UE to be synchronized with the eNB and to deter- mine whether the next KeNB* needs to be based on the current KeNB or a fresh NH value [56,57]. AS security is used to ensure secure delivery of data between a UE and an eNB over the radio interface. It includes both integrity check and ciphering of RRC signaling messages over the control plane, and only ciphering of IP packets over the user plane. Different keys are used for integrity check/ciphering of RRC signaling messages and ciphering of IP pack- ets. Integrity verification is mandatory, but ciphering is optional. AS security keys, such as KRRCint, KRRCenc, and KUPenc, are derived from KeNB by a UE and an eNB. KRRCint and KRRCenc are used for integrity check and ciphering of control-plane information (i.e., RRC signaling messages), and KUPenc is used for ciphering of user-plane data (i.e., IP packets). Integrity verification and ciphering are performed at the PDCP sublayer. Key derivation for dual connectivity SCG bearers is depicted in Fig. 1.89, where SCG counter is a counter used as freshness input into S-Kenb derivations. For SCG bearers in dual connectivity, the user-plane keys are updated upon SCG change by conveying the value of the SCG counter to be used in key derivation to the UE via RRC signaling. When KeNB is refreshed, SCG counter is reset and S-KeNB is derived from the Kenb. The SCG bearers in dual connectivity scenarios share a common pool of radio bearer identities (DRB IDs) with the MCG bearers. When no new DRB ID can be allocated for an SCG bearer without guaranteeing COUNT reuse avoidance, the MeNB derives a new S-Kenb. The SeNB informs MeNB when the uplink or downlink PDCP COUNTs are about to wrap around. In that case, the MeNB updates the S-KeNB. To update the S-KeNB, the MeNB increases the SCG counter and uses it to derive a new S-Kenb from the currently active KeNB in the MeNB. The MeNB sends the freshly derived S-KeNB to the SeNB. The newly derived S-KeNB is then used by the SeNB in computing a new 