Method and arrangement in a telecommunication system

A method comprising:

• establishing, in a User Equipment (UE) of an Evolved Packet System (EPS), a security key (K_eNB) for protecting Radio Resource Control/User Plane (RRC/UP) traffic exchanged with a serving eNodeB, wherein establishing is performed as part of an IDLE to ACTIVE state transition of the UE in the EPS architecture, and wherein establishing comprises:
• sending a Non-Access Stratum (NAS) Service Request to a Mobility Management Entity (MME) the request indicating a NAS uplink sequence number (NAS_U_SEQ);
• and
• deriving the K_eNB from at least said NAS_U_SEQ and from a stored Access Security Management Entity-key (K_ASME) shared with said MME.

A Mobility Management Entity (MME) adapted for an Evolved Packet System (EPS) the MME arranged to establish a security key (K_eNB), for protection of Radio Resource Control/User Plane (RRC/UP) traffic between a User Equipment (UE) and an eNodeB serving the UE, wherein the K_eNB is established as part of an IDLE to ACTIVE state transition of the UE in the EPS architecture, the MME comprising:

• a receiver configured to receive a Non-Access Stratum (NAS) Service Request from the UE, the request indicating a NAS uplink sequence number (NAS_U_SEQ);
• a processing device configured to derive a K_eNB from at least said received NAS_U_SEQ and from a stored Access Security Management Entity-key (K_ASME) shared with said UE;
• and
• a transmitter configured to send said derived K_eNB to the eNodeB serving said UE.