Method and arrangement in a telecommunication system

A method comprising:

• establishing, in a User Equipment (UE) of a Packet System, a security key for protecting Radio Resource Control/User Plane traffic exchanged with a serving radio base station, wherein establishing comprises:
• sending an integrity-protected Non-Access Stratum (NAS) Service Request to a Mobility Management Entity, the request indicating a NAS uplink sequence number;
• and
• deriving the security key from at least said NAS uplink sequence number and from a stored Access Security Management Entity-key shared with said Mobility Management Entity, using a pseudo-random function, taking the NAS uplink sequence number and the Access Security Management Entity-key as inputs.

A system comprising:

• a Mobility Management Entity adapted for a Packet System, the Mobility Management Entity arranged to establish a security key for protection of Radio Resource Control/User Plane traffic between a User Equipment and a radio base station serving the UE, the Mobility Management Entity comprising:
• a receiver configured to receive an integrity-protected Non-Access Stratum (NAS) Service Request from the UE, the NAS Service Request indicating a NAS uplink sequence number;
• processing circuitry configured to derive a security key from at least said received NAS uplink sequence number and from a stored Access Security Management Entity-key shared with said UE, using a pseudo-random function, taking the NAS uplink sequence number and the Access Security Management Entity-key as inputs;
• and
• a transmitter configured to send said derived security key to the radio base station serving said UE.

A system comprising:

• a user equipment (UE) adapted for a Packet System, the UE arranged to establish a security key for protecting Radio Resource Control/User Plane (RRC/UP) traffic exchanged with a serving radio base station, wherein the UE comprises:
• a transmitter configured to send an integrity-protected NAS Service Request to a Mobility Management Entity, the request indicating a NAS uplink sequence number;
• and
• processing circuitry configured to derive a security key from at least said NAS uplink sequence number, and from a stored Access Security Management Entity-key shared with said Mobility Management Entity, using a pseudo-random function, taking the NAS uplink sequence number and the Access Security Management Entity-key as inputs.