In order for supporting separate ciphering at an MeNB (20) and an SeNB (30), the MeNB (20) derives separate first and second keys (KUPenc-M, KUPenc-S) from a third key (KeNB). The first key (KUPenc-M) is used for confidentially protecting first traffic transmitted over U-Plane between the MeNB (20) and a UE (10). The first key (KUPenc-M) may be the same as current KUPenc or a new key. The second key (KUPenc-S) is used for confidentially protecting second traffic transmitted over the U-Plane between the UE (10) and the SeNB (30). The MeNB (20) sends the second key (KUPenc-S) to the SeNB (30). The UE (10) negotiates with the MeNB (20), and derives the second key (KUPenc-S) based on a result of the negotiation.